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Data Communication & Networking                                                                 IV Sem BCA

                                                 Networks
The idea of networking is an old one. A network can be defined as "A collection of two or more devices
which are interconnected using common protocols to exchange data."




Networks are large distributed systems designed to send information from one location to another. An end
point is a place in a network where data transmission either originates or terminates. A node is a point in
the network where data travels through without stopping. Nodes are connected by channels, paths that data
flows down. Channels can be physical linear objects such as a wire or a fiber optic cable, or it can be less
tangible, like a wireless connection at a particular frequency.
The cellular concept of space-divided networks was first developed in AT&T in the 1940's and 1950's.
AMPS, an analog frequency division multiplexing network was first implemented in Chicago in 1983, and
was completely saturated with users the next year. The FCC, in response to overwhelming user demand,
increased the available cellular bandwidth from 40Mhz to 50Mhz.

Wireless Generations
It is often instructive to break the history of wireless networking up into several specific generations.
First Generation (1G)
The 1G wireless generation comprised of mainly analog signals for carrying voice and music. These were
one directional broadcast systems such as Television broadcast, AM/FM radio, and similar communications.
Second Generation (2G)
2G introduced concepts such as TDMA and CDMA for allowing bi-directional communications among
nodes in large networks. 2G is when some of the first cellular phones were made available, although
communications were restricted to very low bitrates.
The second generation is frequently divided into sub-sets as well. "2.5G" represented a significant increase
in throughput capacity as digital communications techniques became more refined. "2.75G" is another
common pseudo-generation that saw an additional increase in speed and capacity among digital wireless
networks.
Third Generation (3G)
3G is the current generation, and represents the combination of voice traffic with data traffic, and the advent
of high-bandwidth mobile devices such as PDAs and smartphones.
Fourth Generation (4G)
The 4G generation, which is a theoretical future generation, will see the ubiquity of broadband data
connections and universal internet access. These networks, many of which are being designed around the
WiMAX (IEEE 802.16) specification.


K. Adisesha,                                                                                                  1
Presidency College                                                                              COPY:   Jan 2009
Data Communication & Networking                                                               IV Sem BCA

Bi-directional Communications
Bi-directional communications means that data is flowing both to and from an end point. An end point can
be both a client and a server.
Point-to-Point communication
Some channels are point-to-point -- they have only a single producer (at one end), and a single consumer (at
the far end).
Many networks have "full duplex" communication between nodes, meaning they have 2 separate point-to-
point channels (one in each direction) between the nodes (on separate wires or allocated to separate
frequencies).
Some "mesh" networks are built from point-to-point channels. Since wiring every node to every other node
is prohibitively expensive, when one node needs to communicate with a distant node, the "intermediate"
nodes must pass through the information.

Multiple Access
Multiple access networks are networks where multiple clients, multiple servers, or both are attempting to
access the network simultaneously. Networks with one server and multiple clients are called "broadcast
networks", "multicast networks", or "SIMO networks". "SIMO" stands for "Single Input Multiple Output".
Networks with multiple clients and servers are known as "MIMO" or "Multiple Input Multiple Output"
networks.

Network Topologies
The shape of a network and the relationship between the nodes in that network is known as the network
topology. The network topology determines, in large part, what kinds of functions the network can perform,
and what the quality of the communication will be between nodes.
                                          Common Topologies




'Star topology' - A star topology creates a network by arranging 2 or more host machines around a central
hub. A variation of this topology, the 'star ring' topology, is in common use today.
The star topology is still regarded as one of the major network topologies of the networking world.
A star topology is typically used in a broadcast or SIMO network, where a single information source
communicates directly with multiple clients. An example of this is a radio station, where a single antenna
transmits data directly to many radios.
‘Tree topology’- A tree topology is so named because it resembles a binary tree structure from computer
science. The tree has a "root" node, which forms the base of the network. The root node then communicates

K. Adisesha,                                                                                                2
Presidency College                                                                            COPY:   Jan 2009
Data Communication & Networking                                                                    IV Sem BCA

with a number of smaller nodes, and those in turn communicate with an even greater number of smaller
nodes. An example of a tree topology network is the DNS system. DNS root servers connect to DNS
regional servers, which connect to local DNS servers which then connect with individual networks and
computers. For your personal computer to talk to the root DNS server, it needs to send a request through the
local DNS server, through the regional DNS server, and then to the root server.
'Ring topology' - A ring topology (more commonly known as a token ring topology) creates a network by
arranging 2 or more hosts in a circle. Data is passed between hosts through a 'token.' This token moves
rapidly at all times throughout the ring in one direction. If a host desires to send data to another host, it will
attach that data as well as a piece of data saying who the message is for to the token as it passes by. The
other host will then see that the token has a message for it by scanning for destination MAC addresses that
match its own. If the MAC addresses do match, the host will take the data and the message will be
delivered. A variation of this topology, the 'star ring' topology, is in common use today.
The ring topology is still regarded as one of the major network topologies of the networking world.
Mesh topology' - A mesh topology creates a network by ensuring that every host machine is connected to
more than one other host machine on the local area network. This topology's main purpose is for fault
tolerance - as opposed to a bus topology, where the entire LAN will go down if one host fails. In a mesh
topology, as long as 2 machines with a working connection are still functioning, a LAN will still exist.
The mesh topology is still regarded as one of the major network topologies of the networking world.
Line topology - This rare topology works by connecting every host to the host located to the right of it. Most
networking professionals do not even regard this as an actual topology, as it is very expensive (due to its
cabling requirements) and due to the fact that it is much more practical to connect the hosts on either end to
form a ring topology, which is much cheaper and more efficient.
'Tree topology' - A tree topology, similar to a line topology in that it is extremely rare and is generally not
regarded as one of the main network topologies, forms a network by arranging hosts in a hierarchal fashion.
A host that is a branch off from the main tree is called a 'leaf.' This topology in this respect becomes very
similar to a partial mesh topology - if a 'leaf' fails, its connection is isolated and the rest of the LAN can
continue onwards.
‘Bus topology’ - A bus topology creates a network by connecting 2 or more hosts to a length of coaxial
backbone cabling. In this topology, a terminator must be placed on the end of the backbone coaxial cabling -
in Michael Meyer's Network+ textbook, he commonly compares a network to a series of pipes that water
travels through. Think of the data as water; in this respect, the terminator must be placed in order to prevent
the water from flowing out of the network.
The bus topology is still regarded as one of the major network topologies of the networking world.
‘Hybrid topology’ - A hybrid topology, which is what most networks implement today, uses a combination
of multiple basic network topologies, usually by functioning as one topology logically while appearing as
another physically. The most common hybrid topologies include Star Bus, and Star Ring.

Network Size Designations
Personal Area Network (PAN)
       Extremely small networks, often referred to as "piconets" that encompass an area around a single
       person. These networks, such as Bluetooth, have a range of only 1-5 meters, and tend to have very
       low power requirements, but also very low datarates. personal area network (PAN) - wireless PAN
Local Area Network (LAN)
       LAN networks can encompass a building such as a house or an office, or a single floor in a multi-
       level building. Common LAN networks are IEEE 802.11x networks, such as 802.11a, 802.11g, and
       802.11n. local area network (LAN) - wireless LAN
Metropolitan Area Network (MAN)
       These networks are designed to cover large municipal areas. Data protocols such as WiMAX
       (802.16) and Cellular 3G networks are MAN networks. metropolitan area network (MAN)

K. Adisesha,                                                                                                     3
Presidency College                                                                                 COPY:   Jan 2009
Data Communication & Networking                                                            IV Sem BCA

Wide Area Network (WAN)
      Wide-Area Networks are very similar to MAN, and the two are often used interchangably. WiMAX
      is also considered a WAN protocol. Television and Radio broadcasts are frequently also considered
      MAN and WAN systems. wide area network (WAN)
Regional Area Network (RAN)
      Large regional area networks are used to communicate with nodes over very large areas. Examples
      of RAN are satellite broadcast media, and IEEE 802.22.
Sensor Area Networks
      These networks are low-datarate networks primarily used for embedded computer systems and
      wireless sensor systems. Protocols such as Zigbee (IEEE 802.15.4) and RFID fall into this category.




                                       Network Architecture

Network Types

Analog Networks
   • Circuit Switching Networks
   • Cable Television Network
   • Radio Communications
Digital Networks
   • Internet
   • Ethernet
   • Wireless Internet
Hybrid Networks
   • Analog and Digital TV
   • Analog and Digital Telephony
   • Analog and Digital Radio


K. Adisesha,                                                                                             4
Presidency College                                                                         COPY:   Jan 2009
Data Communication & Networking                                                             IV Sem BCA

Protocols
Protocols are the rules by which computers communicate. Generally a "Network Protocol" defines how
communications should begin and end properly, and the sequence of events that should occur during data
transmissions. At the transmitting computer the protocol is responsible for:
    • Breaking the data down into packets
    • Adding the address of the intended receiving computer
    • Preparing the data for transmission through the NIC and data-transmission media
    • At the receiving computer the protocol is responsible for
    • Collecting the packets off the data-transmission media through the NIC
    • Stripping off transmitting information from the packets
    • Copying only the data portion of the packet to a memory buffer
    • Reassembling the data portions of the packets in the correct order
    • Checking the data for errors

Protocol Architecture
  • Task of communication broken up into modules
  • For example file transfer could use three modules
        —File transfer application
        —Communication service module
        —Network access module

Standardized Protocol Architectures
   • Required for devices to communicate
   • Vendors have more marketable products
   • Customers can insist on standards based equipment
   • Two standards:
      —OSI Reference model
             •Never lived up to early promises
      —TCP/IP protocol suite
             •Most widely used
    • Also: IBM Systems Network Architecture (SNA), FTP

The OSI Reference Model (Open Systems Interconnection)
Developed by the ISO (International Standards Organization) in the early 1970s as a standard architecture
for the development of computer networks. It provides a structured and consistent approach for describing,
understanding, and implementing networks. The OSI Model:
    • Provides general design guidelines for data-communications systems
    • Provides a standard way to describe how portions (layers) of data-communications systems interact
    • Divides communication problems into standard layers, facilitating the development of network
        products and encouraging "mix and match" interchangeability of network components
    • Promotes the development of a global internetwork in which disparate systems can freely share
        network data and resources
    • Is a tool for learning how networks function




K. Adisesha,                                                                                              5
Presidency College                                                                          COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA

                                        OSI Reference Model




The OSI model allows for different developers to make products and software to interface with other
products, without having to worry about how the layers below are implemented. Each layer has a specified
interface with layers above and below it, so everybody can work on different areas without worrying about
compatibility.
                               The Layers and their Responsibilities
1. Application – Provides services that directly support user applications, such as the user interface, e-mail,
file transfer, terminal emulation, database access, etc... Communicates through: Gateways and Application
Interfaces
2. Presentation – Translates data between the formats the network requires and the computer expects.
Handles character encoding, bit order, and byte order issues. Encodes and decodes data. Determines the
format and structure of data. Compresses and decompresses, encrypts and decrypts data. Communicates
through: Gateways and Application Interfaces
3. Session – Allows applications on a separate computer to share a connection (called a session). Establishes
and maintains connection. Manages upper layer errors. Handles remote procedure calls. Synchronizes
communicating nodes. Communicates through: Gateways and Application Interfaces
4. Transport – Ensures that packets are delivered error free, in sequence, and without loss or duplication.
Takes action to correct faulty transmissions. Controls the flow of data. Acknowledges successful receipt of
data. Fragments and reassembles data. Communicates through: Gateway Services, Routers, and Brouters
5. Network – Makes routing decisions and forwards packets (a.k.a. datagrams) for devices that could be
farther away than a single link. Moves information to the correct address. Assembles and disassembles
packets. Addresses and routes data packets. Determines best path for moving data through the network.
Communicates through: Gateway Services, Routers, and Brouters

K. Adisesha,                                                                                                  6
Presidency College                                                                              COPY:   Jan 2009
Data Communication & Networking                                                               IV Sem BCA

6. Data Link – Provides for the flow of data over a single link from one device to another. Controls access
to communication channel. Controls flow of data. Organizes data into logical frames (logical units of
information). Identifies the specific computer on the network. Detects errors. Communicates through:
Switches, Bridges, Intelligent Hubs
The Data Link Layer contains 2 sub-layers:
        A. The LLC (Logical Link Control) – The upper sub-layer which establishes and maintains links
between communicating devices. Also responsible for frame error correction and hardware addresses.
        B. The MAC (Media Access Control) – The lower sub-layer which controls how devices share a
media channel. (Either through contention or token passing)
7. Physical – Handles the sending and receiving of bits. Provides electrical and mechanical interfaces for a
network. Specific type of medium used to connect network devices. Specifies how signals are transmitted
on network. Communicates through: Repeaters, Hubs, Switches, Cables, Connectors, Transmitters,
Receivers, Multiplexers
Layers request the services of the layers below them and provide services to the layers above them. The
point of communication between layers is called the SAP (Service Access Point).

TCP/IP Protocol Architecture
  • Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched
      network (ARPANET)
    • Used by the global Internet
    • No official model but a working one.
    • This model has five layers
      —Application layer
      —Host to host or transport layer
      —Internet layer
      —Network access layer
      —Physical layer
                                           OSI v/s TCP/IP




TCP
•Usual transport layer is Transmission Control Protocol
—Reliable connection
•Connection
—Temporary logical association between entities in different systems
•TCP PDU
—Called TCP segment

K. Adisesha,                                                                                                7
Presidency College                                                                            COPY:   Jan 2009
Data Communication & Networking                                    IV Sem BCA

—Includes source and destination port (c.f. SAP)
•Identify
        respective users (applications)
•Connection refers to pair of ports
•TCP tracks segments between entities on each connection

                                          TCP/IP Concepts




UDP
    •   Alternative to TCP is User Datagram Protocol
    •   Not guaranteed delivery
    •   No preservation of sequence
    •   No protection against duplication
    •   Minimum overhead
    •   Adds port addressing to IP

                                  Some Protocols in TCP/IP Suite




K. Adisesha,                                                                     8
Presidency College                                                 COPY:   Jan 2009
Data Communication & Networking                                                                  IV Sem BCA

Data Transmission

In a communications system, data are propagated from one point to another by means of electromagnetic
signals. Both analog and digital signals may be transmitted on suitable transmission media.
An analog signal is a continuously varying electromagnetic wave that may be propagated over a variety of
media, depending on spectrum; examples are wire media, such as twisted pair and coaxial cable; fiber optic
cable; and unguided media, such as atmosphere or space propagation.




                         Figure 1                                            Figure 2

Above figure 1, illustrates, analog signals can be used to transmit both analog data represented by an
electromagnetic signal occupying the same spectrum, and digital data using a modem
(modulator/demodulator) to modulate the digital data on some carrier frequency.
However, analog signal will become weaker (attenuate) after a certain distance. To achieve longer distances,
the analog transmission system includes amplifiers that boost the energy in the signal. Unfortunately, the
amplifier also boosts the noise components. With amplifiers cascaded to achieve long distances, the signal
becomes more and more distorted. For analog data, such as voice, quite a bit of distortion can be tolerated
and the data remain intelligible. However, for digital data, cascaded amplifiers will introduce errors.
A digital signal is a sequence of voltage pulses that may be transmitted over a wire medium; eg. a constant
positive voltage level may represent binary 0 and a constant negative voltage level may represent binary 1.
As Figure 2, illustrates, digital signals can be used to transmit both analog signals and digital data. Analog
data can converted to digital using a codec (coder-decoder), which takes an analog signal that directly
represents the voice data and approximates that signal by a bit stream. At the receiving end, the bit stream is
used to reconstruct the analog data. Digital data can be directly represented by digital signals.
A digital signal can be transmitted only a limited distance before attenuation, noise, and other impairments
endanger the integrity of the data. To achieve greater distances, repeaters are used. A repeater receives the
digital signal, recovers the pattern of 1s and 0s, and retransmits a new signal. Thus the attenuation is
overcome.
The principal advantages of digital signaling are that it is generally cheaper than analog signaling and is less
susceptible to noise interference. The principal disadvantage is that digital signals suffer more from
attenuation than do analog signals. A sequence of voltage pulses, generated by a source using two voltage
levels, and the received voltage some distance down a conducting medium. Because of the attenuation, or
reduction, of signal strength at higher frequencies, the pulses become rounded and smaller.
Which is the preferred method of transmission? The answer being supplied by the telecommunications
industry and its customers is digital. Both long-haul telecommunications facilities and intra-building
services have moved to digital transmission and, where possible, digital signaling techniques, for a range of
reasons.
The maximum rate at which data can be transmitted over a given communication channel, under given
conditions, is referred to as the channel capacity. There are four concepts here that we are trying to relate to
one another.
• Data rate, in bits per second (bps), at which data can be communicated

K. Adisesha,                                                                                                   9
Presidency College                                                                               COPY:   Jan 2009
Data Communication & Networking                                                                    IV Sem BCA

• Bandwidth, as constrained by the transmitter and the nature of the transmission medium, expressed in
cycles per second, or Hertz
• Noise, average level of noise over the communications path
• Error rate, at which errors occur, where an error is the reception of a 1 when a 0 was transmitted or the
reception of a 0 when a 1 was transmitted
All transmission channels of any practical interest are of limited bandwidth, which arise from the physical
properties of the transmission medium or from deliberate limitations at the transmitter on the bandwidth to
prevent interference from other sources. Want to make as efficient use as possible of a given bandwidth. For
digital data, this means that we would like to get as high a data rate as possible at a particular limit of error
rate for a given bandwidth. The main constraint on achieving this efficiency is noise.

Nyquist Signaling rate:
Consider a noise free channel where the limitation on data rate is simply the bandwidth of the signal.
Nyquist states that if the rate of signal transmission is 2B, then a signal with frequencies no greater than B is
sufficient to carry the signal rate. Conversely given a bandwidth of B, the highest signal rate that can be
carried is 2B. This limitation is due to the effect of intersymbol interference, such as is produced by delay
distortion.
If the signals to be transmitted are binary (two voltage levels), then the data rate that can be supported by B
Hz is 2B bps. However signals with more than two levels can be used; that is, each signal element can
represent more than one bit. For example, if four possible voltage levels are used as signals, then each signal
element can represent two bits. With multilevel signaling, the Nyquist formulation becomes:
                 C = 2B log2 M, where M is the number of discrete signal or voltage levels.
So, for a given bandwidth, the data rate can be increased by increasing the number of different signal
elements. However, this places an increased burden on the receiver, as it must distinguish one of M possible
signal elements. Noise and other impairments on the transmission line will limit the practical value of M.

Shannon Channel Capacity:
Consider the relationship among data rate, noise, and error rate. The presence of noise can corrupt one or
more bits. If the data rate is increased, then the bits become "shorter" so that more bits are affected by a
given pattern of noise. Mathematician Claude Shannon developed a formula relating these. For a given level
of noise, expect that a greater signal strength would improve the ability to receive data correctly in the
presence of noise. The key parameter involved is the signal-to-noise ratio (SNR, or S/N), which is the ratio
of the power in a signal to the power contained in the noise that is present at a particular point in the
transmission. Typically, this ratio is measured at a receiver, because it is at this point that an attempt is made
to process the signal and recover the data. For convenience, this ratio is often reported in decibels. This
expresses the amount, in decibels, that the intended signal exceeds the noise level. A high SNR will mean a
high-quality signal and a low number of required intermediate repeaters.
                               SNRdb=10 log10 (signal/noise)
                               Capacity C=B log2(1+SNR)
The signal-to-noise ratio is important in the transmission of digital data because it sets the upper bound on
the achievable data rate. Shannon's result is that the maximum channel capacity, in bits per second, obeys
the equation shown. C is the capacity of the channel in bits per second and B is the bandwidth of the channel
in Hertz. The Shannon formula represents the theoretical maximum that can be achieved. In practice,
however, only much lower rates are achieved, in part because formula only assumes white noise (thermal
noise).

 The successful transmission of data depends principally on two factors: the quality of the signal being
transmitted and the characteristics of the transmission medium. Data transmission occurs between
transmitter and receiver over some transmission medium. Transmission media may be classified as guided

K. Adisesha,                                                                                                    10
Presidency College                                                                                 COPY:   Jan 2009
Data Communication & Networking                                                                            IV Sem BCA

or unguided. In both cases, communication is in the form of electromagnetic waves. With guided media,
the waves are guided along a physical path; examples of guided media are twisted pair, coaxial cable, and
optical fiber. Unguided media, also called wireless, provide a means for transmitting electromagnetic
waves but do not guide them; examples are propagation through air, vacuum, and seawater.
In the case of guided media, the medium itself is more important in determining the limitations of
transmission.
        For unguided media, the bandwidth of the signal produced by the transmitting antenna is more
important than the medium in determining transmission characteristics. One key property of signals
transmitted by antenna is directionality. In general, signals at lower frequencies are omnidirectional; that is,
the signal propagates in all directions from the antenna. At higher frequencies, it is possible to focus the
signal into a directional beam. In considering the design of data transmission systems, key concerns are data
rate and distance: the greater the data rate and distance the better.

Transmission Characteristics of Guided Media

                                          Frequency         Typical         Typical Delay       Repeater
                                            Range         Attenuation                           Spacing

                 Twisted pair (with    0 to 3.5 kHz     0.2 dB/km @ 1      50 µs/km         2 km
                 loading)                               kHz


                 Twisted pairs         0 to 1 MHz       0.7 dB/km @ 1      5 µs/km          2 km
                 (multi-pair cables)                    kHz


                 Coaxial cable         0 to 500 MHz     7 dB/km @ 10       4 µs/km          1 to 9 km
                                                        MHz

                 Optical fiber         186 to 370 THz   0.2 to 0.5 dB/km   5 µs/km          40 km




                                                        Twisted Pair




By far the most common guided transmission medium for both analog and digital signals is twisted pair. It
is the most commonly used medium in the telephone network (linking residential telephones to the local
telephone exchange, or office phones to a PBX), and for communications within buildings (for LANs
running at 10-100Mbps). Twisted pair is much less expensive than the other commonly used guided
transmission media (coaxial cable, optical fiber) and is easier to work with.
        A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair
acts as a single communication link. Typically, a number of these pairs are bundled together into a cable by
wrapping them in a tough protective sheath. The twisting tends to decrease the crosstalk interference
between adjacent pairs in a cable. Neighboring pairs in a bundle typically have somewhat different twist
lengths to reduce the crosstalk interference. On long-distance links, the twist length typically varies from 5
to 15 cm. The wires in a pair have thicknesses of from 0.4 to 0.9 mm.


K. Adisesha,                                                                                                            11
Presidency College                                                                                         COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA

                                              Coaxial Cable




Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to
operate over a wider range of frequencies. It consists of a hollow outer cylindrical conductor that surrounds
a single inner wire conductor (Figure). The inner conductor is held in place by either regularly spaced
insulating rings or a solid dielectric material. The outer conductor is covered with a jacket or shield. A
single coaxial cable has a diameter of from 1 to 2.5 cm. Coaxial cable can be used over longer distances and
support more stations on a shared line than twisted pair.
        Coaxial cable is a versatile transmission medium, used in a wide variety of applications, including:
•       Television distribution - aerial to TV & CATV systems
•       Long-distance telephone transmission - traditionally used for inter-exchange links, now being
replaced by optical fiber/microwave/satellite
•       Short-run computer system links
•       Local area networks

Coaxial cable is used to transmit both analog and digital signals. It has frequency characteristics that are
superior to those of twisted pair and can hence be used effectively at higher frequencies and data rates.
Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference and
crosstalk than twisted pair. The principal constraints on performance are attenuation, thermal noise, and
intermodulation noise. The latter is present only when several channels (FDM) or frequency bands are in
use on the cable.
        For long-distance transmission of analog signals, amplifiers are needed every few kilometers, with
closer spacing required if higher frequencies are used. The usable spectrum for analog signaling extends to
about 500 MHz. For digital signaling, repeaters are needed every kilometer or so, with closer spacing
needed for higher data rates.

                                              Optical Fiber




An optical fiber is a thin (2 to 125 µm), flexible medium capable of guiding an optical ray. Various glasses
and plastics can be used to make optical fibers. An optical fiber cable has a cylindrical shape and consists of
three concentric sections: the core, the cladding, and the jacket. The core is the innermost section and
consists of one or more very thin strands, or fibers, made of glass or plastic; the core has a diameter in the
range of 8 to 50 µm. Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical
K. Adisesha,                                                                                                 12
Presidency College                                                                              COPY:   Jan 2009
Data Communication & Networking                                                                  IV Sem BCA

properties different from those of the core and a diameter of 125 µm. The interface between the core and
cladding acts as a reflector to confine light that would otherwise escape the core. The outermost layer,
surrounding one or a bundle of cladded fibers, is the jacket. The jacket is composed of plastic and other
material layered to protect against moisture, abrasion, crushing, and other environmental dangers.
        Optical fiber already enjoys considerable use in long-distance telecommunications, and its use in
military applications is growing. The continuing improvements in performance and decline in prices,
together with the inherent advantages of optical fiber, have made it increasingly attractive for local area
networking. Five basic categories of application have become important for optical fiber: Long-haul trunks,
Metropolitan trunks, Rural exchange trunks, Subscriber loops & Local area networks.
The following characteristics distinguish optical fiber from twisted pair or coaxial cable:
•       Greater capacity: The potential bandwidth, and hence data rate, of optical fiber is immense; data
rates of hundreds of Gbps over tens of kilometers have been demonstrated. Compare this to the practical
maximum of hundreds of Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or up to
100 Mbps to 10 Gbps over a few tens of meters for twisted pair.
•       Smaller size and lighter weight: Optical fibers are considerably thinner than coaxial cable or
bundled twisted-pair cable. For cramped conduits in buildings and underground along public rights-of-way,
the advantage of small size is considerable. The corresponding reduction in weight reduces structural
support requirements.
•       Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial cable or
twisted pair, and is constant over a wide range.
•       Electromagnetic isolation: Optical fiber systems are not affected by external electromagnetic fields.
Thus the system is not vulnerable to interference, impulse noise, or crosstalk. By the same token, fibers do
not radiate energy, so there is little interference with other equipment and there is a high degree of security
from eavesdropping. In addition, fiber is inherently difficult to tap.
•       Greater repeater spacing: Fewer repeaters mean lower cost and fewer sources of error. The
performance of optical fiber systems from this point of view has been steadily improving. Repeater spacing
in the tens of kilometers for optical fiber is common, and repeater spacings of hundreds of kilometers have
been demonstrated.




Figure shows the principle of optical fiber transmission. Light from a source enters the cylindrical glass or
plastic core. Rays at shallow angles are reflected and propagated along the fiber; other rays are absorbed by
the surrounding material. This form of propagation is called step-index multimode, referring to the variety
of angles that will reflect. With multimode transmission, multiple propagation paths exist, each with a
different path length and hence time to traverse the fiber. This causes signal elements (light pulses) to spread
out in time, which limits the rate at which data can be accurately received. This type of fiber is best suited
for transmission over very short distances.
        When the fiber core radius is reduced, fewer angles will reflect. By reducing the radius of the core to
the order of a wavelength, only a single angle or mode can pass: the axial ray. This single-mode
propagation provides superior performance for the following reason. Because there is a single transmission


K. Adisesha,                                                                                                  13
Presidency College                                                                               COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA

path with single-mode transmission, the distortion found in multimode cannot occur. Single-mode is
typically used for long-distance applications, including telephone and cable television.
        Finally, by varying the index of refraction of the core, a third type of transmission, known as
graded-index multimode, is possible. The higher refractive index (discussed subsequently) at the center
makes the light rays moving down the axis advance more slowly than those near the cladding. Rather than
zig-zagging off the cladding, light in the core curves helically because of the graded index, reducing its
travel distance. The shortened path and higher speed allows light at the periphery to arrive at a receiver at
about the same time as the straight rays in the core axis. Graded-index fibers are often used in local area
networks.

Unguided transmission
Unguided transmission techniques commonly used for information communications include broadcast radio,
terrestrial microwave, and satellite. Infrared transmission is used in some LAN applications. Three general
ranges of frequencies are of interest in our discussion of wireless transmission.
         Frequencies in the range of about 1 to 40 GHz are referred to as microwave frequencies. At these
frequencies, highly directional beams are possible, and microwave is quite suitable for point-to-point
transmission. Microwave is also used for satellite communications.
         Frequencies in the range of 30 MHz to 1 GHz are suitable for omni directional applications. We
refer to this range as the radio range. Another important frequency range is the infrared portion of the
spectrum, roughly from 3 × 1011 to 2 × 1014 Hz. Infrared is useful to local point-to-point and multipoint
applications within confined areas, such as a single room. For unguided media, transmission and reception
are achieved by means of an antenna. An antenna can be defined as an electrical conductor or system of
conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy.
         For transmission of a signal, radio-frequency electrical energy from the transmitter is converted into
electromagnetic energy by the antenna and radiated into the surrounding environment.
         For reception of a signal, electromagnetic energy impinging on the antenna is converted into radio-
frequency electrical energy and fed into the receiver.
         In two-way communication, the same antenna can be and often is used for both transmission and
reception. This is possible because antenna characteristics are essentially the same whether an antenna is
sending or receiving electromagnetic energy. An antenna will radiate power in all directions but, typically,
does not perform equally well in all directions. A common way to characterize the performance of an
antenna is the radiation pattern, which is a graphical representation of the radiation properties of an antenna
as a function of space coordinates.
         The simplest pattern is produced by an idealized antenna known as the isotropic antenna. An
isotropic antenna is a point in space that radiates power in all directions equally. The actual radiation
pattern for the isotropic antenna is a sphere with the antenna at the center.
An important type of antenna is the parabolic reflective antenna, which is used in terrestrial microwave
and satellite applications. A parabola is the locus of all points equidistant from a fixed line (the directrix)
and a fixed point (the focus) not on the line, as shown in Figure above. If a parabola is revolved about its
axis, the surface generated is called a paraboloid.
          Paraboloid surfaces are used in headlights, optical and radio telescopes, and microwave antennas
because: If a source of electromagnetic energy (or sound) is placed at the focus of the paraboloid, and if the
paraboloid is a reflecting surface, then the wave will bounce back in lines parallel to the axis of the
paraboloid; as shown in Figure b above. In theory, this effect creates a parallel beam without dispersion. In
practice, there will be some dispersion, because the source of energy must occupy more than one point. The
larger the diameter of the antenna, the more tightly directional is the beam. On reception, if incoming waves
are parallel to the axis of the reflecting paraboloid, the resulting signal will be concentrated at the focus.



K. Adisesha,                                                                                                 14
Presidency College                                                                              COPY:   Jan 2009
Data Communication & Networking                                                                  IV Sem BCA




                                         Parabolic Reflective Antenna
Antenna gain is a measure of the directionality of an antenna. Antenna gain is defined as the power output,
in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna
(isotropic antenna). For example, if an antenna has a gain of 3 dB, that antenna improves upon the isotropic
antenna in that direction by 3 dB, or a factor of 2. The increased power radiated in a given direction is at the
expense of other directions. In effect, increased power is radiated in one direction by reducing the power
radiated in other directions. It is important to note that antenna gain does not refer to obtaining more output
power than input power but rather to directionality.

The primary use for terrestrial microwave systems is in long haul telecommunications service, as an
alternative to coaxial cable or optical fiber. The microwave facility requires far fewer amplifiers or repeaters
than coaxial cable over the same distance, (typically every 10-100 km) but requires line-of-sight
transmission. Microwave is commonly used for both voice and television transmission. Another
increasingly common use of microwave is for short point-to-point links between buildings, for closed-circuit
TV or as a data link between local area networks.
        The most common type of microwave antenna is the parabolic "dish”, fixed rigidly to focus a narrow
beam on a receiving antenna A typical size is about 3 m in diameter. Microwave antennas are usually
located at substantial heights above ground level to extend the range between antennas and to be able to
transmit over intervening obstacles. To achieve long-distance transmission, a series of microwave relay
towers is used, and point-to-point microwave links are strung together over the desired distance.
        Microwave transmission covers a substantial portion of the electromagnetic spectrum, typically in
the range 1 to 40 GHz, with 4-6GHz and now 11GHz bands the most common. The higher the frequency
used, the higher the potential bandwidth and therefore the higher the potential data rate. As with any
transmission system, a main source of loss is attenuation, related to the square of distance. The effects of
rainfall become especially noticeable above 10 GHz. Another source of impairment is interference.
A communication satellite is, in effect, a microwave relay station. It is used to link two or more ground-
based microwave transmitter/receivers, known as earth stations, or ground stations. The satellite receives
transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another
frequency (downlink). A single orbiting satellite will operate on a number of frequency bands, called
transponder channels, or simply transponders. The optimum frequency range for satellite transmission is
in the range 1 to 10 GHz. Most satellites providing point-to-point service today use a frequency bandwidth
in the range 5.925 to 6.425 GHz for transmission from earth to satellite (uplink) and a bandwidth in the
range 3.7 to 4.2 GHz for transmission from satellite to earth (downlink). This combination is referred to as
the 4/6-GHz band, but has become saturated. So the 12/14-GHz band has been developed (uplink: 14 - 14.5
GHz; downlink: 11.7 - 12.2 GHz).


K. Adisesha,                                                                                                  15
Presidency College                                                                               COPY:   Jan 2009
Data Communication & Networking                                                                   IV Sem BCA

       For a communication satellite to function effectively, it is generally required that it remain stationary
with respect to its position over the earth to be within the line of sight of its earth stations at all times. To
remain stationary, the satellite must have a period of rotation equal to the earth's period of rotation, which
occurs at a height of 35,863 km at the equator. Two satellites using the same frequency band, if close
enough together, will interfere with each other. To avoid this, current standards require a 4° spacing in the
4/6-GHz band and a 3° spacing at 12/14 GHz. Thus the number of possible satellites is quite limited.
       Among the most important applications for satellites are: Television distribution, Long-distance
telephone transmission, Private business networks, and Global positioning.

                                         Satellite Point to Point Link




Figure a, depicts in a general way two common configurations for satellite communication. In the first, the
satellite is being used to provide a point-to-point link between two distant ground-based antennas.

                                           Satellite Broadcast Link




Figure b, depicts in a general way two common configurations for satellite communication. In the second,
the satellite provides communications between one ground-based transmitter and a number of ground-based
receivers.
Radio is a general term used to encompass frequencies in the range of 3 kHz to 300 GHz. We are using the
informal term broadcast radio to cover the VHF and part of the UHF band: 30 MHz to 1 GHz. This range
covers FM radio and UHF and VHF television. This range is also used for a number of data networking
applications. The principal difference between broadcast radio and microwave is that the former is
omnidirectional and the latter is directional. Thus broadcast radio does not require dish-shaped antennas,
and the antennas need not be rigidly mounted to a precise alignment.
        The range 30 MHz to 1 GHz is an effective one for broadcast communications. Unlike the case for
lower-frequency electromagnetic waves, the ionosphere is transparent to radio waves above 30 MHz. Thus
transmission is limited to the line of sight, and distant transmitters will not interfere with each other due to
reflection from the atmosphere. Unlike the higher frequencies of the microwave region, broadcast radio

K. Adisesha,                                                                                                   16
Presidency College                                                                                COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA

waves are less sensitive to attenuation from rainfall. A prime source of impairment for broadcast radio
waves is multipath interference. Reflection from land, water, and natural or human-made objects can create
multiple paths between antennas, eg ghosting on TV pictures.
Infrared communications is achieved using transmitters/receivers (transceivers) that modulate noncoherent
infrared light. Transceivers must be within the line of sight of each other either directly or via reflection
from a light-colored surface such as the ceiling of a room.

Wireless Propagation
A signal radiated from an antenna travels along one of three routes: ground wave, sky wave, or line of sight
(LOS), as shown in Figure.

Ground Wave Propagation




Ground wave propagation more or less follows the contour of the earth and can propagate considerable
distances, well over the visual horizon. This effect is found in frequencies up to about 2 MHz. Several
factors account for the tendency of electromagnetic wave in this frequency band to follow the earth's
curvature. One factor is that the electromagnetic wave induces a current in the earth's surface, the result of
which is to slow the wavefront near the earth, causing the wavefront to tilt downward and hence follow the
earth's curvature. Another factor is diffraction, which is a phenomenon having to do with the behavior of
electromagnetic waves in the presence of obstacles. Electromagnetic waves in this frequency range are
scattered by the atmosphere in such a way that they do not penetrate the upper atmosphere. The best-known
example of ground wave communication is AM radio.

Sky Wave Propagation




Sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC and
Voice of America. With sky wave propagation, a signal from an earth-based antenna is reflected from the
ionized layer of the upper atmosphere (ionosphere) back down to earth. Although it appears the wave is
reflected from the ionosphere as if the ionosphere were a hard reflecting surface, the effect is in fact caused

K. Adisesha,                                                                                                 17
Presidency College                                                                              COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA

by refraction. Refraction is described subsequently. A sky wave signal can travel through a number of hops,
bouncing back and forth between the ionosphere and the earth's surface, as shown in figure b. With this
propagation mode, a signal can be picked up thousands of kilometers from the transmitter.

Line of Sight Propagation




Above 30 MHz, neither ground wave nor sky wave propagation modes operate, and communication must be
by line of sight. For satellite communication, a signal above 30 MHz is not reflected by the ionosphere and
therefore a signal can be transmitted between an earth station and a satellite overhead that is not beyond the
horizon. For ground-based communication, the transmitting and receiving antennas must be within an
effective line of sight of each other. The term effective is used because microwaves are bent or refracted by
the atmosphere. The amount and even the direction of the bend depends on conditions, but generally
microwaves are bent with the curvature of the earth and will therefore propagate farther than the optical line
of sight. In this book, we are almost exclusively concerned with LOS communications.




K. Adisesha,                                                                                                18
Presidency College                                                                             COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA

Digital Data Communications
Data Communications
The distance over which data moves within a computer may vary from a few thousandths of an inch, as is
the case within a single IC chip, to as much as several feet along the backplane of the main circuit board.
Over such small distances, digital data may be transmitted as direct, two-level electrical signals over simple
copper conductors. Except for the fastest computers, circuit designers are not very concerned about the
shape of the conductor or the analog characteristics of signal transmission.




Data Communications concerns the transmission of digital messages to devices external to the message
source. "External" devices are generally thought of as being independently powered circuitry that exists
beyond the chassis of a computer or other digital message source. As a rule, the maximum permissible
transmission rate of a message is directly proportional to signal power, and inversely proportional to channel
noise. It is the aim of any communications system to provide the highest possible transmission rate at the
lowest possible power and with the least possible noise.

Communications Channels
A communications channel is a pathway over which information can be conveyed. It may be defined by a
physical wire that connects communicating devices, or by a radio, laser, or other radiated energy source that
has no obvious physical presence. Information sent through a communications channel has a source from
which the information originates, and a destination to which the information is delivered. Although
information originates from a single source, there may be more than one destination, depending upon how
many receive stations are linked to the channel and how much energy the transmitted signal possesses.
In a digital communications channel, the information is represented by individual data bits, which may be
encapsulated into multibit message units. A byte, which consists of eight bits, is an example of a message
unit that may be conveyed through a digital communications channel. A collection of bytes may itself be
grouped into a frame or other higher-level message unit. Such multiple levels of encapsulation facilitate the
handling of messages in a complex data communications network.
Any communications channel has a direction associated with it:




K. Adisesha,                                                                                                19
Presidency College                                                                             COPY:   Jan 2009
Data Communication & Networking                                                                   IV Sem BCA

The message source is the transmitter, and the destination is the receiver. A channel whose direction of
transmission is unchanging is referred to as a simplex channel. For example, a radio station is a simplex
channel because it always transmits the signal to its listeners and never allows them to transmit back.
A half-duplex channel is a single physical channel in which the direction may be reversed. Messages may
flow in two directions, but never at the same time, in a half-duplex system. In a telephone call, one party
speaks while the other listens. After a pause, the other party speaks and the first party listens. Speaking
simultaneously results in garbled sound that cannot be understood.
A full-duplex channel allows simultaneous message exchange in both directions. It really consists of two
simplex channels, a forward channel and a reverse channel, linking the same points. The transmission rate
of the reverse channel may be slower if it is used only for flow control of the forward channel.

Serial Communications
Most digital messages are vastly longer than just a few bits. Because it is neither practical nor economic to
transfer all bits of a long message simultaneously, the message is broken into smaller parts and transmitted
sequentially. Bit-serial transmission conveys a message one bit at a time through a channel. Each bit
represents a part of the message. The individual bits are then reassembled at the destination to compose the
message. In general, one channel will pass only one bit at a time. Thus, bit-serial transmission is necessary
in data communications if only a single channel is available. Bit-serial transmission is normally just called
serial transmission and is the chosen communications method in many computer peripherals.
Byte-serial transmission conveys eight bits at a time through eight parallel channels. Although the raw
transfer rate is eight times faster than in bit-serial transmission, eight channels are needed, and the cost may
be as much as eight times higher to transmit the message. When distances are short, it may nonetheless be
both feasible and economic to use parallel channels in return for high data rates. This figure illustrates these
ideas:




The baud rate refers to the signalling rate at which data is sent through a channel and is measured in
electrical transitions per second. In the EIA232 serial interface standard, one signal transition, at most,
occurs per bit, and the baud rate and bit rate are identical. In this case, a rate of 9600 baud corresponds to a
transfer of 9,600 data bits per second with a bit period of 104 microseconds (1/9600 sec.). If two electrical
transitions were required for each bit, as is the case in non-return-to-zero coding, then at a rate of 9600 baud,
only 4800 bits per second could be conveyed. The channel efficiency is the number of bits of useful
information passed through the channel per second. It does not include framing, formatting, and error
detecting bits that may be added to the information bits before a message is transmitted, and will always be
less than one.




K. Adisesha,                                                                                                   20
Presidency College                                                                                COPY:   Jan 2009
Data Communication & Networking                                                                  IV Sem BCA

The data rate of a channel is often specified by its bit rate (often thought erroneously to be the same as baud
rate). However, an equivalent measure channel capacity is bandwidth. In general, the maximum data rate a
channel can support is directly proportional to the channel's bandwidth and inversely proportional to the
channel's noise level.
A communications protocol is an agreed-upon convention that defines the order and meaning of bits in a
serial transmission. It may also specify a procedure for exchanging messages. A protocol will define how
many data bits compose a message unit, the framing and formatting bits, any error-detecting bits that may
be added, and other information that governs control of the communications hardware. Channel efficiency is
determined by the protocol design rather than by digital hardware considerations. Note that there is a
tradeoff between channel efficiency and reliability - protocols that provide greater immunity to noise by
adding error-detecting and -correcting codes must necessarily become less efficient.

Digital Data Transmission
The transmission of a stream of bits from one device to another across a transmission link involves a great
deal of cooperation and agreement between the two sides. One of the most fundamental requirements is
synchronization. The receiver must know the rate at which bits are being received so that it can sample the
line at appropriate intervals to determine the value of each received bit. Two techniques are in common use
for this purpose are:
•Asynchronous transmission.
•Synchronous transmission.


The reception of digital data involves sampling the incoming signal once per bit time to determine the
binary value. This is compounded by a timing difficulty: In order for the receiver to sample the incoming
bits properly, it must know the arrival time and duration of each bit that it receives. Typically, the receiver
will attempt to sample the medium at the center of each bit time, at intervals of one bit time. If the receiver
times its samples based on its own clock, then there will be a problem if the transmitter's and receiver's
clocks are not precisely aligned. If there is a drift in the receiver's clock, then after enough samples, the
receiver may be in error because it is sampling in the wrong bit time For smaller timing differences, the
error would occur later, but eventually the receiver will be out of step with the transmitter if the transmitter
sends a sufficiently long stream of bits and if no steps are taken to synchronize the transmitter and receiver.

Asynchronous vs. Synchronous Transmission
Serialized data is not generally sent at a uniform rate through a channel. Instead, there is usually a burst of
regularly spaced binary data bits followed by a pause, after which the data flow resumes. Packets of binary
data are sent in this manner, possibly with variable-length pauses between packets, until the message has
been fully transmitted. In order for the receiving end to know the proper moment to read individual binary
bits from the channel, it must know exactly when a packet begins and how much time elapses between bits.
When this timing information is known, the receiver is said to be synchronized with the transmitter, and
accurate data transfer becomes possible. Failure to remain synchronized throughout a transmission will
cause data to be corrupted or lost.
In synchronous systems, separate channels are used to transmit data and timing information. The timing
channel transmits clock pulses to the receiver. Upon receipt of a clock pulse, the receiver reads the data
channel and latches the bit value found on the channel at that moment. The data channel is not read again
until the next clock pulse arrives. Because the transmitter originates both the data and the timing pulses, the
receiver will read the data channel only when told to do so by the transmitter (via the clock pulse), and
synchronization is guaranteed. Techniques exist to merge the timing signal with the data so that only a
single channel is required. This is especially useful when synchronous transmissions are to be sent through a
modem. Two methods in which a data signal is self-timed are nonreturn-to-zero and biphase Manchester
coding. These both refer to methods for encoding a data stream into an electrical waveform for transmission.

K. Adisesha,                                                                                                  21
Presidency College                                                                               COPY:   Jan 2009
Data Communication & Networking                                                                      IV Sem BCA




                                          Synchronous transmission

In asynchronous systems, a separate timing channel is not used. The transmitter and receiver must be preset
in advance to an agreed-upon baud rate. A very accurate local oscillator within the receiver will then
generate an internal clock signal that is equal to the transmitter's within a fraction of a percent. For the most
common serial protocol, data is sent in small packets of 10 or 11 bits, eight of which constitute message
information. When the channel is idle, the signal voltage corresponds to a continuous logic '1'. A data packet
always begins with a logic '0' (the start bit) to signal the receiver that a transmission is starting. The start bit
triggers an internal timer in the receiver that generates the needed clock pulses. Following the start bit, eight
bits of message data are sent bit by bit at the agreed upon baud rate. The packet is concluded with a parity
bit and stop bit. One complete packet is illustrated below:




The packet length is short in asynchronous systems to minimize the risk that the local oscillators in the
receiver and transmitter will drift apart. When high-quality crystal oscillators are used, synchronization can
be guaranteed over an 11-bit period. Every time a new packet is sent, the start bit resets the synchronization,
so the pause between packets can be arbitrarily long.

Parity and Checksums
 Noise and momentary electrical disturbances may cause data to be changed as it passes through a
communications channel. If the receiver fails to detect this, the received message will be incorrect, resulting
in possibly serious consequences. As a first line of defense against data errors, they must be detected. If an
error can be flagged, it might be possible to request that the faulty packet be resent, or to at least prevent the
flawed data from being taken as correct. If sufficient redundant information is sent, one- or two-bit errors
may be corrected by hardware within the receiver before the corrupted data ever reaches its destination.
A parity bit is added to a data packet for the purpose of error detection. In the even-parity convention, the
value of the parity bit is chosen so that the total number of '1' digits in the combined data plus parity packet
is an even number. Upon receipt of the packet, the parity needed for the data is recomputed by local
hardware and compared to the parity bit received with the data. If any bit has changed state, the parity will
not match, and an error will have been detected. In fact, if an odd number of bits (not just one) have been
altered, the parity will not match. If an even number of bits have been reversed, the parity will match even
though an error has occurred. However, a statistical analysis of data communication errors has shown that a
single-bit error is much more probable than a multibit error in the presence of random noise. Thus, parity is
a reliable method of error detection.

K. Adisesha,                                                                                                      22
Presidency College                                                                                   COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA




Another approach to error detection involves the computation of a checksum. In this case, the packets that
constitute a message are added arithmetically. A checksum number is appended to the packet sequence so
that the sum of data plus checksum is zero. When received, the packet sequence may be added, along with
the checksum, by a local microprocessor. If the sum is nonzero, an error has occurred. As long as the sum is
zero, it is highly unlikely (but not impossible) that any data has been corrupted during transmission.




Errors may not only be detected, but also corrected if additional code is added to a packet sequence. If the
error probability is high or if it is not possible to request retransmission, this may be worth doing. However,
including error-correcting code in a transmission lowers channel efficiency, and results in a noticeable drop
in channel throughput.

Data Compression
If a typical message were statistically analyzed, it would be found that certain characters are used much
more frequently than others. By analyzing a message before it is transmitted, short binary codes may be
assigned to frequently used characters and longer codes to rarely used characters. In doing so, it is possible
to reduce the total number of characters sent without altering the information in the message. Appropriate
decoding at the receiver will restore the message to its original form. This procedure, known as data
compression, may result in a 50 percent or greater savings in the amount of data transmitted. Even though
time is necessary to analyze the message before it is transmitted, the savings may be great enough so that the
total time for compression, transmission, and decompression will still be lower than it would be when
sending an uncompressed message.
A compression method called Huffman coding is frequently used in data communications, and particularly
in fax transmission. Clearly, most of the image data for a typical business letter represents white paper, and
only about 5 percent of the surface represents black ink. It is possible to send a single code that, for
example, represents a consecutive string of 1000 white pixels rather than a separate code for each white
pixel. Consequently, data compression will significantly reduce the total message length for a faxed
business letter. Were the letter made up of randomly distributed black ink covering 50 percent of the white
paper surface, data compression would hold no advantages.

Data Encryption
Privacy is a great concern in data communications. Faxed business letters can be intercepted at will through
tapped phone lines or intercepted microwave transmissions without the knowledge of the sender or receiver.
To increase the security of this and other data communications, including digitized telephone conversations,
the binary codes representing data may be scrambled in such a way that unauthorized interception will
produce an indecipherable sequence of characters. Authorized receive stations will be equipped with a


K. Adisesha,                                                                                                 23
Presidency College                                                                              COPY:   Jan 2009
Data Communication & Networking                                                                     IV Sem BCA

decoder that enables the message to be restored. The process of scrambling, transmitting, and descrambling
is known as encryption.

Data Storage Technology
Normally, we think of communications science as dealing with the contemporaneous exchange of
information between distant parties. However, many of the same techniques employed in data
communications are also applied to data storage to ensure that the retrieval of information from a storage
medium is accurate. We find, for example, that similar kinds of error-correcting codes used to protect digital
telephone transmissions from noise are also used to guarantee correct readback of digital data from compact
audio disks, CD-ROMs, and tape backup systems.

Data Transfer in Digital Circuits
 Data is typically grouped into packets that are either 8, 16, or 32 bits long, and passed between temporary
holding units called registers. Data within a register is available in parallel because each bit exits the register
on a separate conductor. To transfer data from one register to another, the output conductors of one register
are switched onto a channel of parallel wires referred to as a bus. The input conductors of another register,
which is also connected to the bus, capture the information:




Following a data transaction, the content of the source register is reproduced in the destination register. It is
important to note that after any digital data transfer, the source and destination registers are equal; the source
register is not erased when the data is sent.
The transmit and receive switches shown above are electronic and operate in response to commands from a
central control unit. It is possible that two or more destination registers will be switched on to receive data
from a single source. However, only one source may transmit data onto the bus at any time. If multiple
sources were to attempt transmission simultaneously, an electrical conflict would occur when bits of
opposite value are driven onto a single bus conductor. Such a condition is referred to as a bus contention.
Not only will a bus contention result in the loss of information, but it also may damage the electronic
circuitry. As long as all registers in a system are linked to one central control unit, bus contentions should
never occur if the circuit has been designed properly. Note that the data buses within a typical
microprocessor are funda-mentally half-duplex channels.

Transmission over Short Distances (< 2 feet)
When the source and destination registers are part of an integrated circuit (within a microprocessor chip, for
example), they are extremely close (thousandths of an inch). Consequently, the bus signals are at very low
power levels, may traverse a distance in very little time, and are not very susceptible to external noise and
distortion. This is the ideal environment for digital communications. However, it is not yet possible to
integrate all the necessary circuitry for a computer (i.e., CPU, memory, disk control, video and display
drivers, etc.) on a single chip. When data is sent off-chip to another integrated circuit, the bus signals must
be amplified and conductors extended out of the chip through external pins. Amplifiers may be added to the
source register:



K. Adisesha,                                                                                                     24
Presidency College                                                                                  COPY:   Jan 2009
Data Communication & Networking                                                                  IV Sem BCA




Bus signals that exit microprocessor chips and other VLSI circuitry are electrically capable of traversing
about one foot of conductor on a printed circuit board, or less if many devices are connected to it. Special
buffer circuits may be added to boost the bus signals sufficiently for transmission over several additional
feet of conductor length, or for distribution to many other chips (such as memory chips).

Noise and Electrical Distortion
Because of the very high switching rate and relatively low signal strength found on data, address, and other
buses within a computer, direct extension of the buses beyond the confines of the main circuit board or
plug-in boards would pose serious problems. First, long runs of electrical conductors, either on printed
circuit boards or through cables, act like receiving antennas for electrical noise radiated by motors, switches,
and electronic circuits:




Such noise becomes progressively worse as the length increases, and may eventually impose an
unacceptable error rate on the bus signals. Just a single bit error in transferring an instruction code from
memory to a microprocessor chip may cause an invalid instruction to be introduced into the instruction
stream, in turn causing the computer to totally cease operation.
A second problem involves the distortion of electrical signals as they pass through metallic conductors.
Signals that start at the source as clean, rectangular pulses may be received as rounded pulses with ringing at
the rising and falling edges:




These effects are properties of transmission through metallic conductors, and become more pronounced as
the conductor length increases. To compensate for distortion, signal power must be increased or the
transmission rate decreased.




K. Adisesha,                                                                                                  25
Presidency College                                                                               COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA




Transmission over Medium Distances (< 20 feet)

Computer peripherals such as a printer or scanner generally include mechanisms that cannot be situated
within the computer itself. Our first thought might be just to extend the computer's internal buses with a
cable of sufficient length to reach the peripheral. Doing so, however, would expose all bus transactions to
external noise and distortion even though only a very small percentage of these transactions concern the
distant peripheral to which the bus is connected.

If a peripheral can be located within 20 feet of the computer, however, relatively simple electronics may be
added to make data transfer through a cable efficient and reliable. To accomplish this, a bus interface circuit
is installed in the computer:




It consists of a holding register for peripheral data, timing and formatting circuitry for external data
transmission, and signal amplifiers to boost the signal sufficiently for transmission through a cable. When
communication with the peripheral is necessary, data is first deposited in the holding register by the
microprocessor. This data will then be reformatted, sent with error-detecting codes, and transmitted at a
relatively slow rate by digital hardware in the bus interface circuit. In addition, the signal power is greatly
boosted before transmission through the cable. These steps ensure that the data will not be corrupted by
noise or distortion during its passage through the cable. In addition, because only data destined for the
peripheral is sent, the party-line transactions taking place on the computer's buses are not unnecessarily
exposed to noise.
Data sent in this manner may be transmitted in byte-serial format if the cable has eight parallel channels (at
least 10 conductors for half-duplex operation), or in bit-serial format if only a single channel is available.

Transmission over Long Distances (< 4000 feet)
When relatively long distances are involved in reaching a peripheral device, driver circuits must be inserted
after the bus interface unit to compensate for the electrical effects of long cables:




K. Adisesha,                                                                                                 26
Presidency College                                                                              COPY:   Jan 2009
Data Communication & Networking                                                                IV Sem BCA




This is the only change needed if a single peripheral is used. However, if many peripherals are connected, or
if other computer stations are to be linked, a local area network (LAN) is required, and it becomes necessary
to drastically change both the electrical drivers and the protocol to send messages through the cable.
Because multiconductor cable is expensive, bit-serial transmission is almost always used when the distance
exceeds 20 feet.
A great deal of technology has been developed for LAN systems to minimize the amount of cable required
and maximize the throughput. The costs of a LAN have been concentrated in the electrical interface card
that would be installed in PCs or peripherals to drive the cable, and in the communications software, not in
the cable itself (whose cost has been minimized). Thus, the cost and complexity of a LAN are not
particularly affected by the distance between stations.

Transmission over Very Long Distances (greater than 4000 feet)
Data communications through the telephone network can reach any point in the world. The volume of
overseas fax transmissions is increasing constantly, and computer networks that link thousands of
businesses, governments, and universities are pervasive. Transmissions over such distances are not
generally accomplished with a direct-wire digital link, but rather with digitally-modulated analog carrier
signals. This technique makes it possible to use existing analog telephone voice channels for digital data,
although at considerably reduced data rates compared to a direct digital link.
Transmission of data from your personal computer to a timesharing service over phone lines requires that
data signals be converted to audible tones by a modem. An audio sine wave carrier is used, and, depending
on the baud rate and protocol, will encode data by varying the frequency, phase, or amplitude of the carrier.
The receiver's modem accepts the modulated sine wave and extracts the digital data from it.

Signal Encoding Techniques




Analog and Digital information can be encoded as either analog or digital signals:
   ♦ Digital data, digital signals: simplest form of digital encoding of digital data
   ♦ Digital data, analog signal: A modem converts digital data to an analog signal so that it can be
      transmitted over an analog

K. Adisesha,                                                                                                27
Presidency College                                                                             COPY:   Jan 2009
Data Communication & Networking                                                                  IV Sem BCA

    ♦ Analog data, digital signals: Analog data, such as voice and video, are often digitized to be able to
        use digital transmission facilities
    ♦ Analog data, analog signals: Analog data are modulated by a carrier frequency to produce an
        analog signal in a different frequency band, which can be utilized on an analog transmission system
For digital signaling, a data source g(t), which may be either digital or analog, is encoded into a digital
signal x(t). The basis for analog signaling is a continuous constant-frequency fc signal known as the carrier
signal. Data may be transmitted using a carrier signal by modulation, which is the process of encoding
source data onto the carrier signal. All modulation techniques involve operation on one or more of the three
fundamental frequency domain parameters: amplitude, frequency, and phase. The input signal m(t) may be
analog or digital and is called the modulating signal, and the result of modulating the carrier signal is called
the modulated signal s(t).

Encoding - Digital data to digital signals: A digital signal is a sequence of discrete, discontinuous voltage
pulses. Each pulse is a signal element. Binary data are transmitted by encoding each data bit into signal
elements. In the simplest case, there is a one-to-one correspondence between bits and signal elements. More
complex encoding schemes are used to improve performance, by altering the spectrum of the signal and
providing synchronization capability. In general, the equipment for encoding digital data into a digital signal
is less complex and less expensive than digital-to-analog modulation equipment.
Various Encoding techniques include:
     • Nonreturn to Zero-Level (NRZ-L)
     • Nonreturn to Zero Inverted (NRZI)
     • Bipolar -AMI
     • Pseudoternary
     • Manchester
     • Differential Manchester
     • B8ZS
     • HDB3




                                            Encoding techniques

The most common, and easiest, way to transmit digital signals is to use two different voltage levels for the
two binary digits. Codes that follow this strategy share the property that the voltage level is constant during
a bit interval; there is no transition (no return to a zero voltage level). Can have absence of voltage used to

K. Adisesha,                                                                                                  28
Presidency College                                                                               COPY:   Jan 2009
Data Communication & Networking                                                                   IV Sem BCA

represent binary 0, with a constant positive voltage used to represent binary 1. More commonly a negative
voltage represents one binary value and a positive voltage represents the other. This is known as Nonreturn
to Zero-Level (NRZ-L). NRZ-L is typically the code used to generate or interpret digital data by terminals
and other devices.
A variation of NRZ is known as NRZI (Nonreturn to Zero, invert on ones). As with NRZ-L, NRZI
maintains a constant voltage pulse for the duration of a bit time. The data bits are encoded as the presence or
absence of a signal transition at the beginning of the bit time. A transition (low to high or high to low) at the
beginning of a bit time denotes a binary 1 for that bit time; no transition indicates a binary 0.
        NRZI is an example of differential encoding. In differential encoding, the information to be
transmitted is represented in terms of the changes between successive signal elements rather than the signal
elements themselves. The encoding of the current bit is determined as follows: if the current bit is a binary
0, then the current bit is encoded with the same signal as the preceding bit; if the current bit is a binary 1,
then the current bit is encoded with a different signal than the preceding bit. One benefit of differential
encoding is that it may be more reliable to detect a transition in the presence of noise than to compare a
value to a threshold. Another benefit is that with a complex transmission layout, it is easy to lose the sense
of the polarity of the signal.
A category of encoding techniques known as multilevel binary addresses some of the deficiencies of the
NRZ codes. These codes use more than two signal levels. In the bipolar-AMI scheme, a binary 0 is
represented by no line signal, and a binary 1 is represented by a positive or negative pulse. The binary 1
pulses must alternate in polarity. There are several advantages to this approach. First, there will be no loss of
synchronization if a long string of 1s occurs. Each 1 introduces a transition, and the receiver can
resynchronize on that transition. A long string of 0s would still be a problem. Second, because the 1 signals
alternate in voltage from positive to negative, there is no net dc component. Also, the bandwidth of the
resulting signal is considerably less than the bandwidth for NRZ. Finally, the pulse alternation property
provides a simple means of error detection. Any isolated error, whether it deletes a pulse or adds a pulse,
causes a violation of this property.
The comments on bipolar-AMI also apply to pseudoternary. In this case, it is the binary 1 that is
represented by the absence of a line signal, and the binary 0 by alternating positive and negative pulses.
There is no particular advantage of one technique versus the other, and each is the basis of some
applications.
There is another set of coding techniques, grouped under the term biphase, that overcomes the limitations of
NRZ codes. Two of these techniques, Manchester and differential Manchester, are in common use.
        In the Manchester code, there is a transition at the middle of each bit period. The midbit transition
serves as a clocking mechanism and also as data: a low-to-high transition represents a 1, and a high-to-low
transition represents a 0. Biphase codes are popular techniques for data transmission. The more common
Manchester code has been specified for the IEEE 802.3 (Ethernet) standard for baseband coaxial cable and
twisted-pair bus LANs.
In differential Manchester, the midbit transition is used only to provide clocking. The encoding of a 0 is
represented by the presence of a transition at the beginning of a bit period, and a 1 is represented by the
absence of a transition at the beginning of a bit period. Differential Manchester has the added advantage of
employing differential encoding.
Differential Manchester has been specified for the IEEE 802.5 token ring LAN, using shielded twisted pair.

Digital Data, Analog Signal

The most familiar use of transmitting digital data using analog signals transformation is for transmitting
digital data through the public telephone network. The telephone network was designed to receive, switch,
and transmit analog signals in the voice-frequency range of about 300 to 3400 Hz. It is not at present
suitable for handling digital signals from the subscriber locations (although this is beginning to change).

K. Adisesha,                                                                                                   29
Presidency College                                                                                COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA

Thus digital devices are attached to the network via a modem (modulator-demodulator), which converts
digital data to analog signals, and vice versa.
        Having stated that modulation involves operation on one or more of the three characteristics of a
carrier signal: amplitude, frequency, and phase. Accordingly, there are three basic encoding or modulation
techniques for transforming digital data into analog signals, as illustrated Figure: amplitude shift keying
(ASK), frequency shift keying (FSK), and phase shift keying (PSK). In all these cases, the resulting signal
occupies a bandwidth centered on the carrier frequency.

                                    Modulation Techniques




In ASK, the two binary values are represented by two different amplitudes of the carrier frequency.
Commonly, one of the amplitudes is zero; that is, one binary digit is represented by the presence, at constant
amplitude, of the carrier, the other by the absence of the carrier, ASK is susceptible to sudden gain changes
and is a rather inefficient modulation technique. On voice-grade lines, it is typically used only up to 1200
bps.
         The ASK technique is used to transmit digital data over optical fiber, where one signal element is
represented by a light pulse while the other signal element is represented by the absence of light.
The most common form of FSK is binary FSK (BFSK), in which the two binary values are represented by
two different frequencies near the carrier frequency, as shown in Figure.
         BFSK is less susceptible to error than ASK. On voice-grade lines, it is typically used up to 1200 bps.
It is also commonly used for high-frequency (3 to 30 MHz) radio transmission. It can also be used at even
higher frequencies on local area networks that use coaxial cable.
In PSK, the phase of the carrier signal is shifted to represent data. The simplest scheme uses two phases to
represent the two binary digits (Figure) and is known as binary phase shift keying.
         An alternative form of two-level PSK is differential PSK (DPSK). In this scheme, a binary 0 is
represented by sending a signal burst of the same phase as the previous signal burst sent. A binary 1 is
represented by sending a signal burst of opposite phase to the preceding one. This term differential refers to
the fact that the phase shift is with reference to the previous bit transmitted rather than to some constant
reference signal. In differential encoding, the information to be transmitted is represented in terms of the
changes between successive data symbols rather than the signal elements themselves. DPSK avoids the
requirement for an accurate local oscillator phase at the receiver that is matched with the transmitter. As
long as the preceding phase is received correctly, the phase reference is accurate.
More efficient use of bandwidth can be achieved if each signaling element represents more than one bit. For
example, instead of a phase shift of 180˚, as allowed in BPSK, a common encoding technique, known as
quadrature phase shift keying (QPSK), uses phase shifts separated by multiples of π/2 (90˚). Thus each
signal element represents two bits rather than one. The input is a stream of binary digits with a data rate of

K. Adisesha,                                                                                                 30
Presidency College                                                                              COPY:   Jan 2009
Data Communication & Networking                                                                    IV Sem BCA

R = 1/Tb, where Tb is the width of each bit. This stream is converted into two separate bit streams of R/2 bps
each, by taking alternate bits for the two streams. The two data streams are referred to as the I (in-phase) and
Q (quadrature phase) streams. The streams are modulated on a carrier of frequency fc by multiplying the bit
stream by the carrier, and the carrier shifted by 90˚. The two modulated signals are then added together and
transmitted. Thus, the combined signals have a symbol rate that is half the input bit rate.
        The use of multiple levels can be extended beyond taking bits two at a time. It is possible to transmit
bits three at a time using eight different phase angles. Further, each angle can have more than one
amplitude. For example, a standard 9600 bps modem uses 12 phase angles, four of which have two
amplitude values, for a total of 16 different signal elements.

Analog data, digital signals
In this section we examine the process of transforming analog data into digital signals. Analog data, such as
voice and video, is often digitized to be able to use digital transmission facilities. Strictly speaking, it might
be more correct to refer to this as a process of converting analog data into digital data; this process is known
as digitization. Once analog data have been converted into digital data, a number of things can happen. The
three most common are:
1.      The digital data can be transmitted using NRZ-L. In this case, we have in fact gone directly from
analog data to a digital signal.
2.      The digital data can be encoded as a digital signal using a code other than NRZ-L. Thus an extra step
is required.
3.      The digital data can be converted into an analog signal, using one of the modulation techniques.
        The device used for converting analog data into digital form for transmission, and subsequently
recovering the original analog data from the digital, is known as a codec (coder-decoder). In this section we
examine the two principal techniques used in codecs, pulse code modulation and delta modulation.




                                            Digitizing Analog Data

The simplest technique for transforming analog data into digital signals is pulse code modulation (PCM),
which involves sampling the analog data periodically and quantizing the samples. Pulse code modulation
(PCM) is based on the sampling theorem (quoted above). Hence if voice data is limited to frequencies below
4000 Hz (a conservative procedure for intelligibility), 8000 samples per second would be sufficient to
characterize the voice signal completely. Note, however, that these are analog samples, called pulse
amplitude modulation (PAM) samples. To convert to digital, each of these analog samples must be
assigned a binary code.




K. Adisesha,                                                                                                    31
Presidency College                                                                                 COPY:   Jan 2009
Data Communication & Networking                                                                 IV Sem BCA

                                                 PCM Example




Figure shows an example in which the original signal is assumed to be bandlimited with a bandwidth of B.
PAM samples are taken at a rate of 2B, or once every Ts = 1/2B seconds. Each PAM sample is
approximated by being quantized into one of 16 different levels. Each sample can then be represented by 4
bits. But because the quantized values are only approximations, it is impossible to recover the original signal
exactly. By using an 8-bit sample, which allows 256 quantizing levels, the quality of the recovered voice
signal is comparable with that achieved via analog transmission. Note that this implies that a data rate of
8000 samples per second × 8 bits per sample = 64 kbps is needed for a single voice signal.
                                             PCM Block Diagram




Thus, PCM starts with a continuous-time, continuous-amplitude (analog) signal, from which a digital signal
is produced, as shown in Figure. The digital signal consists of blocks of n bits, where each n-bit number is
the amplitude of a PCM pulse. On reception, the process is reversed to reproduce the analog signal. Notice,
however, that this process violates the terms of the sampling theorem. By quantizing the PAM pulse, the
original signal is now only approximated and cannot be recovered exactly. This effect is known as
quantizing error or quantizing noise. Each additional bit used for quantizing increases SNR by about 6
dB, which is a factor of 4.




                                               Non-Linear Coding
Typically, the PCM scheme is refined using a technique known as nonlinear encoding, which means, in
effect, that the quantization levels are not equally spaced. The problem with equal spacing is that the mean
absolute error for each sample is the same, regardless of signal level. Consequently, lower amplitude values
are relatively more distorted. By using a greater number of quantizing steps for signals of low amplitude,

K. Adisesha,                                                                                                 32
Presidency College                                                                              COPY:   Jan 2009
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Digital data communications

  • 1. Data Communication & Networking IV Sem BCA Networks The idea of networking is an old one. A network can be defined as "A collection of two or more devices which are interconnected using common protocols to exchange data." Networks are large distributed systems designed to send information from one location to another. An end point is a place in a network where data transmission either originates or terminates. A node is a point in the network where data travels through without stopping. Nodes are connected by channels, paths that data flows down. Channels can be physical linear objects such as a wire or a fiber optic cable, or it can be less tangible, like a wireless connection at a particular frequency. The cellular concept of space-divided networks was first developed in AT&T in the 1940's and 1950's. AMPS, an analog frequency division multiplexing network was first implemented in Chicago in 1983, and was completely saturated with users the next year. The FCC, in response to overwhelming user demand, increased the available cellular bandwidth from 40Mhz to 50Mhz. Wireless Generations It is often instructive to break the history of wireless networking up into several specific generations. First Generation (1G) The 1G wireless generation comprised of mainly analog signals for carrying voice and music. These were one directional broadcast systems such as Television broadcast, AM/FM radio, and similar communications. Second Generation (2G) 2G introduced concepts such as TDMA and CDMA for allowing bi-directional communications among nodes in large networks. 2G is when some of the first cellular phones were made available, although communications were restricted to very low bitrates. The second generation is frequently divided into sub-sets as well. "2.5G" represented a significant increase in throughput capacity as digital communications techniques became more refined. "2.75G" is another common pseudo-generation that saw an additional increase in speed and capacity among digital wireless networks. Third Generation (3G) 3G is the current generation, and represents the combination of voice traffic with data traffic, and the advent of high-bandwidth mobile devices such as PDAs and smartphones. Fourth Generation (4G) The 4G generation, which is a theoretical future generation, will see the ubiquity of broadband data connections and universal internet access. These networks, many of which are being designed around the WiMAX (IEEE 802.16) specification. K. Adisesha, 1 Presidency College COPY: Jan 2009
  • 2. Data Communication & Networking IV Sem BCA Bi-directional Communications Bi-directional communications means that data is flowing both to and from an end point. An end point can be both a client and a server. Point-to-Point communication Some channels are point-to-point -- they have only a single producer (at one end), and a single consumer (at the far end). Many networks have "full duplex" communication between nodes, meaning they have 2 separate point-to- point channels (one in each direction) between the nodes (on separate wires or allocated to separate frequencies). Some "mesh" networks are built from point-to-point channels. Since wiring every node to every other node is prohibitively expensive, when one node needs to communicate with a distant node, the "intermediate" nodes must pass through the information. Multiple Access Multiple access networks are networks where multiple clients, multiple servers, or both are attempting to access the network simultaneously. Networks with one server and multiple clients are called "broadcast networks", "multicast networks", or "SIMO networks". "SIMO" stands for "Single Input Multiple Output". Networks with multiple clients and servers are known as "MIMO" or "Multiple Input Multiple Output" networks. Network Topologies The shape of a network and the relationship between the nodes in that network is known as the network topology. The network topology determines, in large part, what kinds of functions the network can perform, and what the quality of the communication will be between nodes. Common Topologies 'Star topology' - A star topology creates a network by arranging 2 or more host machines around a central hub. A variation of this topology, the 'star ring' topology, is in common use today. The star topology is still regarded as one of the major network topologies of the networking world. A star topology is typically used in a broadcast or SIMO network, where a single information source communicates directly with multiple clients. An example of this is a radio station, where a single antenna transmits data directly to many radios. ‘Tree topology’- A tree topology is so named because it resembles a binary tree structure from computer science. The tree has a "root" node, which forms the base of the network. The root node then communicates K. Adisesha, 2 Presidency College COPY: Jan 2009
  • 3. Data Communication & Networking IV Sem BCA with a number of smaller nodes, and those in turn communicate with an even greater number of smaller nodes. An example of a tree topology network is the DNS system. DNS root servers connect to DNS regional servers, which connect to local DNS servers which then connect with individual networks and computers. For your personal computer to talk to the root DNS server, it needs to send a request through the local DNS server, through the regional DNS server, and then to the root server. 'Ring topology' - A ring topology (more commonly known as a token ring topology) creates a network by arranging 2 or more hosts in a circle. Data is passed between hosts through a 'token.' This token moves rapidly at all times throughout the ring in one direction. If a host desires to send data to another host, it will attach that data as well as a piece of data saying who the message is for to the token as it passes by. The other host will then see that the token has a message for it by scanning for destination MAC addresses that match its own. If the MAC addresses do match, the host will take the data and the message will be delivered. A variation of this topology, the 'star ring' topology, is in common use today. The ring topology is still regarded as one of the major network topologies of the networking world. Mesh topology' - A mesh topology creates a network by ensuring that every host machine is connected to more than one other host machine on the local area network. This topology's main purpose is for fault tolerance - as opposed to a bus topology, where the entire LAN will go down if one host fails. In a mesh topology, as long as 2 machines with a working connection are still functioning, a LAN will still exist. The mesh topology is still regarded as one of the major network topologies of the networking world. Line topology - This rare topology works by connecting every host to the host located to the right of it. Most networking professionals do not even regard this as an actual topology, as it is very expensive (due to its cabling requirements) and due to the fact that it is much more practical to connect the hosts on either end to form a ring topology, which is much cheaper and more efficient. 'Tree topology' - A tree topology, similar to a line topology in that it is extremely rare and is generally not regarded as one of the main network topologies, forms a network by arranging hosts in a hierarchal fashion. A host that is a branch off from the main tree is called a 'leaf.' This topology in this respect becomes very similar to a partial mesh topology - if a 'leaf' fails, its connection is isolated and the rest of the LAN can continue onwards. ‘Bus topology’ - A bus topology creates a network by connecting 2 or more hosts to a length of coaxial backbone cabling. In this topology, a terminator must be placed on the end of the backbone coaxial cabling - in Michael Meyer's Network+ textbook, he commonly compares a network to a series of pipes that water travels through. Think of the data as water; in this respect, the terminator must be placed in order to prevent the water from flowing out of the network. The bus topology is still regarded as one of the major network topologies of the networking world. ‘Hybrid topology’ - A hybrid topology, which is what most networks implement today, uses a combination of multiple basic network topologies, usually by functioning as one topology logically while appearing as another physically. The most common hybrid topologies include Star Bus, and Star Ring. Network Size Designations Personal Area Network (PAN) Extremely small networks, often referred to as "piconets" that encompass an area around a single person. These networks, such as Bluetooth, have a range of only 1-5 meters, and tend to have very low power requirements, but also very low datarates. personal area network (PAN) - wireless PAN Local Area Network (LAN) LAN networks can encompass a building such as a house or an office, or a single floor in a multi- level building. Common LAN networks are IEEE 802.11x networks, such as 802.11a, 802.11g, and 802.11n. local area network (LAN) - wireless LAN Metropolitan Area Network (MAN) These networks are designed to cover large municipal areas. Data protocols such as WiMAX (802.16) and Cellular 3G networks are MAN networks. metropolitan area network (MAN) K. Adisesha, 3 Presidency College COPY: Jan 2009
  • 4. Data Communication & Networking IV Sem BCA Wide Area Network (WAN) Wide-Area Networks are very similar to MAN, and the two are often used interchangably. WiMAX is also considered a WAN protocol. Television and Radio broadcasts are frequently also considered MAN and WAN systems. wide area network (WAN) Regional Area Network (RAN) Large regional area networks are used to communicate with nodes over very large areas. Examples of RAN are satellite broadcast media, and IEEE 802.22. Sensor Area Networks These networks are low-datarate networks primarily used for embedded computer systems and wireless sensor systems. Protocols such as Zigbee (IEEE 802.15.4) and RFID fall into this category. Network Architecture Network Types Analog Networks • Circuit Switching Networks • Cable Television Network • Radio Communications Digital Networks • Internet • Ethernet • Wireless Internet Hybrid Networks • Analog and Digital TV • Analog and Digital Telephony • Analog and Digital Radio K. Adisesha, 4 Presidency College COPY: Jan 2009
  • 5. Data Communication & Networking IV Sem BCA Protocols Protocols are the rules by which computers communicate. Generally a "Network Protocol" defines how communications should begin and end properly, and the sequence of events that should occur during data transmissions. At the transmitting computer the protocol is responsible for: • Breaking the data down into packets • Adding the address of the intended receiving computer • Preparing the data for transmission through the NIC and data-transmission media • At the receiving computer the protocol is responsible for • Collecting the packets off the data-transmission media through the NIC • Stripping off transmitting information from the packets • Copying only the data portion of the packet to a memory buffer • Reassembling the data portions of the packets in the correct order • Checking the data for errors Protocol Architecture • Task of communication broken up into modules • For example file transfer could use three modules —File transfer application —Communication service module —Network access module Standardized Protocol Architectures • Required for devices to communicate • Vendors have more marketable products • Customers can insist on standards based equipment • Two standards: —OSI Reference model •Never lived up to early promises —TCP/IP protocol suite •Most widely used • Also: IBM Systems Network Architecture (SNA), FTP The OSI Reference Model (Open Systems Interconnection) Developed by the ISO (International Standards Organization) in the early 1970s as a standard architecture for the development of computer networks. It provides a structured and consistent approach for describing, understanding, and implementing networks. The OSI Model: • Provides general design guidelines for data-communications systems • Provides a standard way to describe how portions (layers) of data-communications systems interact • Divides communication problems into standard layers, facilitating the development of network products and encouraging "mix and match" interchangeability of network components • Promotes the development of a global internetwork in which disparate systems can freely share network data and resources • Is a tool for learning how networks function K. Adisesha, 5 Presidency College COPY: Jan 2009
  • 6. Data Communication & Networking IV Sem BCA OSI Reference Model The OSI model allows for different developers to make products and software to interface with other products, without having to worry about how the layers below are implemented. Each layer has a specified interface with layers above and below it, so everybody can work on different areas without worrying about compatibility. The Layers and their Responsibilities 1. Application – Provides services that directly support user applications, such as the user interface, e-mail, file transfer, terminal emulation, database access, etc... Communicates through: Gateways and Application Interfaces 2. Presentation – Translates data between the formats the network requires and the computer expects. Handles character encoding, bit order, and byte order issues. Encodes and decodes data. Determines the format and structure of data. Compresses and decompresses, encrypts and decrypts data. Communicates through: Gateways and Application Interfaces 3. Session – Allows applications on a separate computer to share a connection (called a session). Establishes and maintains connection. Manages upper layer errors. Handles remote procedure calls. Synchronizes communicating nodes. Communicates through: Gateways and Application Interfaces 4. Transport – Ensures that packets are delivered error free, in sequence, and without loss or duplication. Takes action to correct faulty transmissions. Controls the flow of data. Acknowledges successful receipt of data. Fragments and reassembles data. Communicates through: Gateway Services, Routers, and Brouters 5. Network – Makes routing decisions and forwards packets (a.k.a. datagrams) for devices that could be farther away than a single link. Moves information to the correct address. Assembles and disassembles packets. Addresses and routes data packets. Determines best path for moving data through the network. Communicates through: Gateway Services, Routers, and Brouters K. Adisesha, 6 Presidency College COPY: Jan 2009
  • 7. Data Communication & Networking IV Sem BCA 6. Data Link – Provides for the flow of data over a single link from one device to another. Controls access to communication channel. Controls flow of data. Organizes data into logical frames (logical units of information). Identifies the specific computer on the network. Detects errors. Communicates through: Switches, Bridges, Intelligent Hubs The Data Link Layer contains 2 sub-layers: A. The LLC (Logical Link Control) – The upper sub-layer which establishes and maintains links between communicating devices. Also responsible for frame error correction and hardware addresses. B. The MAC (Media Access Control) – The lower sub-layer which controls how devices share a media channel. (Either through contention or token passing) 7. Physical – Handles the sending and receiving of bits. Provides electrical and mechanical interfaces for a network. Specific type of medium used to connect network devices. Specifies how signals are transmitted on network. Communicates through: Repeaters, Hubs, Switches, Cables, Connectors, Transmitters, Receivers, Multiplexers Layers request the services of the layers below them and provide services to the layers above them. The point of communication between layers is called the SAP (Service Access Point). TCP/IP Protocol Architecture • Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) • Used by the global Internet • No official model but a working one. • This model has five layers —Application layer —Host to host or transport layer —Internet layer —Network access layer —Physical layer OSI v/s TCP/IP TCP •Usual transport layer is Transmission Control Protocol —Reliable connection •Connection —Temporary logical association between entities in different systems •TCP PDU —Called TCP segment K. Adisesha, 7 Presidency College COPY: Jan 2009
  • 8. Data Communication & Networking IV Sem BCA —Includes source and destination port (c.f. SAP) •Identify respective users (applications) •Connection refers to pair of ports •TCP tracks segments between entities on each connection TCP/IP Concepts UDP • Alternative to TCP is User Datagram Protocol • Not guaranteed delivery • No preservation of sequence • No protection against duplication • Minimum overhead • Adds port addressing to IP Some Protocols in TCP/IP Suite K. Adisesha, 8 Presidency College COPY: Jan 2009
  • 9. Data Communication & Networking IV Sem BCA Data Transmission In a communications system, data are propagated from one point to another by means of electromagnetic signals. Both analog and digital signals may be transmitted on suitable transmission media. An analog signal is a continuously varying electromagnetic wave that may be propagated over a variety of media, depending on spectrum; examples are wire media, such as twisted pair and coaxial cable; fiber optic cable; and unguided media, such as atmosphere or space propagation. Figure 1 Figure 2 Above figure 1, illustrates, analog signals can be used to transmit both analog data represented by an electromagnetic signal occupying the same spectrum, and digital data using a modem (modulator/demodulator) to modulate the digital data on some carrier frequency. However, analog signal will become weaker (attenuate) after a certain distance. To achieve longer distances, the analog transmission system includes amplifiers that boost the energy in the signal. Unfortunately, the amplifier also boosts the noise components. With amplifiers cascaded to achieve long distances, the signal becomes more and more distorted. For analog data, such as voice, quite a bit of distortion can be tolerated and the data remain intelligible. However, for digital data, cascaded amplifiers will introduce errors. A digital signal is a sequence of voltage pulses that may be transmitted over a wire medium; eg. a constant positive voltage level may represent binary 0 and a constant negative voltage level may represent binary 1. As Figure 2, illustrates, digital signals can be used to transmit both analog signals and digital data. Analog data can converted to digital using a codec (coder-decoder), which takes an analog signal that directly represents the voice data and approximates that signal by a bit stream. At the receiving end, the bit stream is used to reconstruct the analog data. Digital data can be directly represented by digital signals. A digital signal can be transmitted only a limited distance before attenuation, noise, and other impairments endanger the integrity of the data. To achieve greater distances, repeaters are used. A repeater receives the digital signal, recovers the pattern of 1s and 0s, and retransmits a new signal. Thus the attenuation is overcome. The principal advantages of digital signaling are that it is generally cheaper than analog signaling and is less susceptible to noise interference. The principal disadvantage is that digital signals suffer more from attenuation than do analog signals. A sequence of voltage pulses, generated by a source using two voltage levels, and the received voltage some distance down a conducting medium. Because of the attenuation, or reduction, of signal strength at higher frequencies, the pulses become rounded and smaller. Which is the preferred method of transmission? The answer being supplied by the telecommunications industry and its customers is digital. Both long-haul telecommunications facilities and intra-building services have moved to digital transmission and, where possible, digital signaling techniques, for a range of reasons. The maximum rate at which data can be transmitted over a given communication channel, under given conditions, is referred to as the channel capacity. There are four concepts here that we are trying to relate to one another. • Data rate, in bits per second (bps), at which data can be communicated K. Adisesha, 9 Presidency College COPY: Jan 2009
  • 10. Data Communication & Networking IV Sem BCA • Bandwidth, as constrained by the transmitter and the nature of the transmission medium, expressed in cycles per second, or Hertz • Noise, average level of noise over the communications path • Error rate, at which errors occur, where an error is the reception of a 1 when a 0 was transmitted or the reception of a 0 when a 1 was transmitted All transmission channels of any practical interest are of limited bandwidth, which arise from the physical properties of the transmission medium or from deliberate limitations at the transmitter on the bandwidth to prevent interference from other sources. Want to make as efficient use as possible of a given bandwidth. For digital data, this means that we would like to get as high a data rate as possible at a particular limit of error rate for a given bandwidth. The main constraint on achieving this efficiency is noise. Nyquist Signaling rate: Consider a noise free channel where the limitation on data rate is simply the bandwidth of the signal. Nyquist states that if the rate of signal transmission is 2B, then a signal with frequencies no greater than B is sufficient to carry the signal rate. Conversely given a bandwidth of B, the highest signal rate that can be carried is 2B. This limitation is due to the effect of intersymbol interference, such as is produced by delay distortion. If the signals to be transmitted are binary (two voltage levels), then the data rate that can be supported by B Hz is 2B bps. However signals with more than two levels can be used; that is, each signal element can represent more than one bit. For example, if four possible voltage levels are used as signals, then each signal element can represent two bits. With multilevel signaling, the Nyquist formulation becomes: C = 2B log2 M, where M is the number of discrete signal or voltage levels. So, for a given bandwidth, the data rate can be increased by increasing the number of different signal elements. However, this places an increased burden on the receiver, as it must distinguish one of M possible signal elements. Noise and other impairments on the transmission line will limit the practical value of M. Shannon Channel Capacity: Consider the relationship among data rate, noise, and error rate. The presence of noise can corrupt one or more bits. If the data rate is increased, then the bits become "shorter" so that more bits are affected by a given pattern of noise. Mathematician Claude Shannon developed a formula relating these. For a given level of noise, expect that a greater signal strength would improve the ability to receive data correctly in the presence of noise. The key parameter involved is the signal-to-noise ratio (SNR, or S/N), which is the ratio of the power in a signal to the power contained in the noise that is present at a particular point in the transmission. Typically, this ratio is measured at a receiver, because it is at this point that an attempt is made to process the signal and recover the data. For convenience, this ratio is often reported in decibels. This expresses the amount, in decibels, that the intended signal exceeds the noise level. A high SNR will mean a high-quality signal and a low number of required intermediate repeaters. SNRdb=10 log10 (signal/noise) Capacity C=B log2(1+SNR) The signal-to-noise ratio is important in the transmission of digital data because it sets the upper bound on the achievable data rate. Shannon's result is that the maximum channel capacity, in bits per second, obeys the equation shown. C is the capacity of the channel in bits per second and B is the bandwidth of the channel in Hertz. The Shannon formula represents the theoretical maximum that can be achieved. In practice, however, only much lower rates are achieved, in part because formula only assumes white noise (thermal noise). The successful transmission of data depends principally on two factors: the quality of the signal being transmitted and the characteristics of the transmission medium. Data transmission occurs between transmitter and receiver over some transmission medium. Transmission media may be classified as guided K. Adisesha, 10 Presidency College COPY: Jan 2009
  • 11. Data Communication & Networking IV Sem BCA or unguided. In both cases, communication is in the form of electromagnetic waves. With guided media, the waves are guided along a physical path; examples of guided media are twisted pair, coaxial cable, and optical fiber. Unguided media, also called wireless, provide a means for transmitting electromagnetic waves but do not guide them; examples are propagation through air, vacuum, and seawater. In the case of guided media, the medium itself is more important in determining the limitations of transmission. For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important than the medium in determining transmission characteristics. One key property of signals transmitted by antenna is directionality. In general, signals at lower frequencies are omnidirectional; that is, the signal propagates in all directions from the antenna. At higher frequencies, it is possible to focus the signal into a directional beam. In considering the design of data transmission systems, key concerns are data rate and distance: the greater the data rate and distance the better. Transmission Characteristics of Guided Media Frequency Typical Typical Delay Repeater Range Attenuation Spacing Twisted pair (with 0 to 3.5 kHz 0.2 dB/km @ 1 50 µs/km 2 km loading) kHz Twisted pairs 0 to 1 MHz 0.7 dB/km @ 1 5 µs/km 2 km (multi-pair cables) kHz Coaxial cable 0 to 500 MHz 7 dB/km @ 10 4 µs/km 1 to 9 km MHz Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Twisted Pair By far the most common guided transmission medium for both analog and digital signals is twisted pair. It is the most commonly used medium in the telephone network (linking residential telephones to the local telephone exchange, or office phones to a PBX), and for communications within buildings (for LANs running at 10-100Mbps). Twisted pair is much less expensive than the other commonly used guided transmission media (coaxial cable, optical fiber) and is easier to work with. A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link. Typically, a number of these pairs are bundled together into a cable by wrapping them in a tough protective sheath. The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable. Neighboring pairs in a bundle typically have somewhat different twist lengths to reduce the crosstalk interference. On long-distance links, the twist length typically varies from 5 to 15 cm. The wires in a pair have thicknesses of from 0.4 to 0.9 mm. K. Adisesha, 11 Presidency College COPY: Jan 2009
  • 12. Data Communication & Networking IV Sem BCA Coaxial Cable Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies. It consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor (Figure). The inner conductor is held in place by either regularly spaced insulating rings or a solid dielectric material. The outer conductor is covered with a jacket or shield. A single coaxial cable has a diameter of from 1 to 2.5 cm. Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair. Coaxial cable is a versatile transmission medium, used in a wide variety of applications, including: • Television distribution - aerial to TV & CATV systems • Long-distance telephone transmission - traditionally used for inter-exchange links, now being replaced by optical fiber/microwave/satellite • Short-run computer system links • Local area networks Coaxial cable is used to transmit both analog and digital signals. It has frequency characteristics that are superior to those of twisted pair and can hence be used effectively at higher frequencies and data rates. Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference and crosstalk than twisted pair. The principal constraints on performance are attenuation, thermal noise, and intermodulation noise. The latter is present only when several channels (FDM) or frequency bands are in use on the cable. For long-distance transmission of analog signals, amplifiers are needed every few kilometers, with closer spacing required if higher frequencies are used. The usable spectrum for analog signaling extends to about 500 MHz. For digital signaling, repeaters are needed every kilometer or so, with closer spacing needed for higher data rates. Optical Fiber An optical fiber is a thin (2 to 125 µm), flexible medium capable of guiding an optical ray. Various glasses and plastics can be used to make optical fibers. An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket. The core is the innermost section and consists of one or more very thin strands, or fibers, made of glass or plastic; the core has a diameter in the range of 8 to 50 µm. Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical K. Adisesha, 12 Presidency College COPY: Jan 2009
  • 13. Data Communication & Networking IV Sem BCA properties different from those of the core and a diameter of 125 µm. The interface between the core and cladding acts as a reflector to confine light that would otherwise escape the core. The outermost layer, surrounding one or a bundle of cladded fibers, is the jacket. The jacket is composed of plastic and other material layered to protect against moisture, abrasion, crushing, and other environmental dangers. Optical fiber already enjoys considerable use in long-distance telecommunications, and its use in military applications is growing. The continuing improvements in performance and decline in prices, together with the inherent advantages of optical fiber, have made it increasingly attractive for local area networking. Five basic categories of application have become important for optical fiber: Long-haul trunks, Metropolitan trunks, Rural exchange trunks, Subscriber loops & Local area networks. The following characteristics distinguish optical fiber from twisted pair or coaxial cable: • Greater capacity: The potential bandwidth, and hence data rate, of optical fiber is immense; data rates of hundreds of Gbps over tens of kilometers have been demonstrated. Compare this to the practical maximum of hundreds of Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or up to 100 Mbps to 10 Gbps over a few tens of meters for twisted pair. • Smaller size and lighter weight: Optical fibers are considerably thinner than coaxial cable or bundled twisted-pair cable. For cramped conduits in buildings and underground along public rights-of-way, the advantage of small size is considerable. The corresponding reduction in weight reduces structural support requirements. • Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial cable or twisted pair, and is constant over a wide range. • Electromagnetic isolation: Optical fiber systems are not affected by external electromagnetic fields. Thus the system is not vulnerable to interference, impulse noise, or crosstalk. By the same token, fibers do not radiate energy, so there is little interference with other equipment and there is a high degree of security from eavesdropping. In addition, fiber is inherently difficult to tap. • Greater repeater spacing: Fewer repeaters mean lower cost and fewer sources of error. The performance of optical fiber systems from this point of view has been steadily improving. Repeater spacing in the tens of kilometers for optical fiber is common, and repeater spacings of hundreds of kilometers have been demonstrated. Figure shows the principle of optical fiber transmission. Light from a source enters the cylindrical glass or plastic core. Rays at shallow angles are reflected and propagated along the fiber; other rays are absorbed by the surrounding material. This form of propagation is called step-index multimode, referring to the variety of angles that will reflect. With multimode transmission, multiple propagation paths exist, each with a different path length and hence time to traverse the fiber. This causes signal elements (light pulses) to spread out in time, which limits the rate at which data can be accurately received. This type of fiber is best suited for transmission over very short distances. When the fiber core radius is reduced, fewer angles will reflect. By reducing the radius of the core to the order of a wavelength, only a single angle or mode can pass: the axial ray. This single-mode propagation provides superior performance for the following reason. Because there is a single transmission K. Adisesha, 13 Presidency College COPY: Jan 2009
  • 14. Data Communication & Networking IV Sem BCA path with single-mode transmission, the distortion found in multimode cannot occur. Single-mode is typically used for long-distance applications, including telephone and cable television. Finally, by varying the index of refraction of the core, a third type of transmission, known as graded-index multimode, is possible. The higher refractive index (discussed subsequently) at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Rather than zig-zagging off the cladding, light in the core curves helically because of the graded index, reducing its travel distance. The shortened path and higher speed allows light at the periphery to arrive at a receiver at about the same time as the straight rays in the core axis. Graded-index fibers are often used in local area networks. Unguided transmission Unguided transmission techniques commonly used for information communications include broadcast radio, terrestrial microwave, and satellite. Infrared transmission is used in some LAN applications. Three general ranges of frequencies are of interest in our discussion of wireless transmission. Frequencies in the range of about 1 to 40 GHz are referred to as microwave frequencies. At these frequencies, highly directional beams are possible, and microwave is quite suitable for point-to-point transmission. Microwave is also used for satellite communications. Frequencies in the range of 30 MHz to 1 GHz are suitable for omni directional applications. We refer to this range as the radio range. Another important frequency range is the infrared portion of the spectrum, roughly from 3 × 1011 to 2 × 1014 Hz. Infrared is useful to local point-to-point and multipoint applications within confined areas, such as a single room. For unguided media, transmission and reception are achieved by means of an antenna. An antenna can be defined as an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy. For transmission of a signal, radio-frequency electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment. For reception of a signal, electromagnetic energy impinging on the antenna is converted into radio- frequency electrical energy and fed into the receiver. In two-way communication, the same antenna can be and often is used for both transmission and reception. This is possible because antenna characteristics are essentially the same whether an antenna is sending or receiving electromagnetic energy. An antenna will radiate power in all directions but, typically, does not perform equally well in all directions. A common way to characterize the performance of an antenna is the radiation pattern, which is a graphical representation of the radiation properties of an antenna as a function of space coordinates. The simplest pattern is produced by an idealized antenna known as the isotropic antenna. An isotropic antenna is a point in space that radiates power in all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center. An important type of antenna is the parabolic reflective antenna, which is used in terrestrial microwave and satellite applications. A parabola is the locus of all points equidistant from a fixed line (the directrix) and a fixed point (the focus) not on the line, as shown in Figure above. If a parabola is revolved about its axis, the surface generated is called a paraboloid. Paraboloid surfaces are used in headlights, optical and radio telescopes, and microwave antennas because: If a source of electromagnetic energy (or sound) is placed at the focus of the paraboloid, and if the paraboloid is a reflecting surface, then the wave will bounce back in lines parallel to the axis of the paraboloid; as shown in Figure b above. In theory, this effect creates a parallel beam without dispersion. In practice, there will be some dispersion, because the source of energy must occupy more than one point. The larger the diameter of the antenna, the more tightly directional is the beam. On reception, if incoming waves are parallel to the axis of the reflecting paraboloid, the resulting signal will be concentrated at the focus. K. Adisesha, 14 Presidency College COPY: Jan 2009
  • 15. Data Communication & Networking IV Sem BCA Parabolic Reflective Antenna Antenna gain is a measure of the directionality of an antenna. Antenna gain is defined as the power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna). For example, if an antenna has a gain of 3 dB, that antenna improves upon the isotropic antenna in that direction by 3 dB, or a factor of 2. The increased power radiated in a given direction is at the expense of other directions. In effect, increased power is radiated in one direction by reducing the power radiated in other directions. It is important to note that antenna gain does not refer to obtaining more output power than input power but rather to directionality. The primary use for terrestrial microwave systems is in long haul telecommunications service, as an alternative to coaxial cable or optical fiber. The microwave facility requires far fewer amplifiers or repeaters than coaxial cable over the same distance, (typically every 10-100 km) but requires line-of-sight transmission. Microwave is commonly used for both voice and television transmission. Another increasingly common use of microwave is for short point-to-point links between buildings, for closed-circuit TV or as a data link between local area networks. The most common type of microwave antenna is the parabolic "dish”, fixed rigidly to focus a narrow beam on a receiving antenna A typical size is about 3 m in diameter. Microwave antennas are usually located at substantial heights above ground level to extend the range between antennas and to be able to transmit over intervening obstacles. To achieve long-distance transmission, a series of microwave relay towers is used, and point-to-point microwave links are strung together over the desired distance. Microwave transmission covers a substantial portion of the electromagnetic spectrum, typically in the range 1 to 40 GHz, with 4-6GHz and now 11GHz bands the most common. The higher the frequency used, the higher the potential bandwidth and therefore the higher the potential data rate. As with any transmission system, a main source of loss is attenuation, related to the square of distance. The effects of rainfall become especially noticeable above 10 GHz. Another source of impairment is interference. A communication satellite is, in effect, a microwave relay station. It is used to link two or more ground- based microwave transmitter/receivers, known as earth stations, or ground stations. The satellite receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink). A single orbiting satellite will operate on a number of frequency bands, called transponder channels, or simply transponders. The optimum frequency range for satellite transmission is in the range 1 to 10 GHz. Most satellites providing point-to-point service today use a frequency bandwidth in the range 5.925 to 6.425 GHz for transmission from earth to satellite (uplink) and a bandwidth in the range 3.7 to 4.2 GHz for transmission from satellite to earth (downlink). This combination is referred to as the 4/6-GHz band, but has become saturated. So the 12/14-GHz band has been developed (uplink: 14 - 14.5 GHz; downlink: 11.7 - 12.2 GHz). K. Adisesha, 15 Presidency College COPY: Jan 2009
  • 16. Data Communication & Networking IV Sem BCA For a communication satellite to function effectively, it is generally required that it remain stationary with respect to its position over the earth to be within the line of sight of its earth stations at all times. To remain stationary, the satellite must have a period of rotation equal to the earth's period of rotation, which occurs at a height of 35,863 km at the equator. Two satellites using the same frequency band, if close enough together, will interfere with each other. To avoid this, current standards require a 4° spacing in the 4/6-GHz band and a 3° spacing at 12/14 GHz. Thus the number of possible satellites is quite limited. Among the most important applications for satellites are: Television distribution, Long-distance telephone transmission, Private business networks, and Global positioning. Satellite Point to Point Link Figure a, depicts in a general way two common configurations for satellite communication. In the first, the satellite is being used to provide a point-to-point link between two distant ground-based antennas. Satellite Broadcast Link Figure b, depicts in a general way two common configurations for satellite communication. In the second, the satellite provides communications between one ground-based transmitter and a number of ground-based receivers. Radio is a general term used to encompass frequencies in the range of 3 kHz to 300 GHz. We are using the informal term broadcast radio to cover the VHF and part of the UHF band: 30 MHz to 1 GHz. This range covers FM radio and UHF and VHF television. This range is also used for a number of data networking applications. The principal difference between broadcast radio and microwave is that the former is omnidirectional and the latter is directional. Thus broadcast radio does not require dish-shaped antennas, and the antennas need not be rigidly mounted to a precise alignment. The range 30 MHz to 1 GHz is an effective one for broadcast communications. Unlike the case for lower-frequency electromagnetic waves, the ionosphere is transparent to radio waves above 30 MHz. Thus transmission is limited to the line of sight, and distant transmitters will not interfere with each other due to reflection from the atmosphere. Unlike the higher frequencies of the microwave region, broadcast radio K. Adisesha, 16 Presidency College COPY: Jan 2009
  • 17. Data Communication & Networking IV Sem BCA waves are less sensitive to attenuation from rainfall. A prime source of impairment for broadcast radio waves is multipath interference. Reflection from land, water, and natural or human-made objects can create multiple paths between antennas, eg ghosting on TV pictures. Infrared communications is achieved using transmitters/receivers (transceivers) that modulate noncoherent infrared light. Transceivers must be within the line of sight of each other either directly or via reflection from a light-colored surface such as the ceiling of a room. Wireless Propagation A signal radiated from an antenna travels along one of three routes: ground wave, sky wave, or line of sight (LOS), as shown in Figure. Ground Wave Propagation Ground wave propagation more or less follows the contour of the earth and can propagate considerable distances, well over the visual horizon. This effect is found in frequencies up to about 2 MHz. Several factors account for the tendency of electromagnetic wave in this frequency band to follow the earth's curvature. One factor is that the electromagnetic wave induces a current in the earth's surface, the result of which is to slow the wavefront near the earth, causing the wavefront to tilt downward and hence follow the earth's curvature. Another factor is diffraction, which is a phenomenon having to do with the behavior of electromagnetic waves in the presence of obstacles. Electromagnetic waves in this frequency range are scattered by the atmosphere in such a way that they do not penetrate the upper atmosphere. The best-known example of ground wave communication is AM radio. Sky Wave Propagation Sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC and Voice of America. With sky wave propagation, a signal from an earth-based antenna is reflected from the ionized layer of the upper atmosphere (ionosphere) back down to earth. Although it appears the wave is reflected from the ionosphere as if the ionosphere were a hard reflecting surface, the effect is in fact caused K. Adisesha, 17 Presidency College COPY: Jan 2009
  • 18. Data Communication & Networking IV Sem BCA by refraction. Refraction is described subsequently. A sky wave signal can travel through a number of hops, bouncing back and forth between the ionosphere and the earth's surface, as shown in figure b. With this propagation mode, a signal can be picked up thousands of kilometers from the transmitter. Line of Sight Propagation Above 30 MHz, neither ground wave nor sky wave propagation modes operate, and communication must be by line of sight. For satellite communication, a signal above 30 MHz is not reflected by the ionosphere and therefore a signal can be transmitted between an earth station and a satellite overhead that is not beyond the horizon. For ground-based communication, the transmitting and receiving antennas must be within an effective line of sight of each other. The term effective is used because microwaves are bent or refracted by the atmosphere. The amount and even the direction of the bend depends on conditions, but generally microwaves are bent with the curvature of the earth and will therefore propagate farther than the optical line of sight. In this book, we are almost exclusively concerned with LOS communications. K. Adisesha, 18 Presidency College COPY: Jan 2009
  • 19. Data Communication & Networking IV Sem BCA Digital Data Communications Data Communications The distance over which data moves within a computer may vary from a few thousandths of an inch, as is the case within a single IC chip, to as much as several feet along the backplane of the main circuit board. Over such small distances, digital data may be transmitted as direct, two-level electrical signals over simple copper conductors. Except for the fastest computers, circuit designers are not very concerned about the shape of the conductor or the analog characteristics of signal transmission. Data Communications concerns the transmission of digital messages to devices external to the message source. "External" devices are generally thought of as being independently powered circuitry that exists beyond the chassis of a computer or other digital message source. As a rule, the maximum permissible transmission rate of a message is directly proportional to signal power, and inversely proportional to channel noise. It is the aim of any communications system to provide the highest possible transmission rate at the lowest possible power and with the least possible noise. Communications Channels A communications channel is a pathway over which information can be conveyed. It may be defined by a physical wire that connects communicating devices, or by a radio, laser, or other radiated energy source that has no obvious physical presence. Information sent through a communications channel has a source from which the information originates, and a destination to which the information is delivered. Although information originates from a single source, there may be more than one destination, depending upon how many receive stations are linked to the channel and how much energy the transmitted signal possesses. In a digital communications channel, the information is represented by individual data bits, which may be encapsulated into multibit message units. A byte, which consists of eight bits, is an example of a message unit that may be conveyed through a digital communications channel. A collection of bytes may itself be grouped into a frame or other higher-level message unit. Such multiple levels of encapsulation facilitate the handling of messages in a complex data communications network. Any communications channel has a direction associated with it: K. Adisesha, 19 Presidency College COPY: Jan 2009
  • 20. Data Communication & Networking IV Sem BCA The message source is the transmitter, and the destination is the receiver. A channel whose direction of transmission is unchanging is referred to as a simplex channel. For example, a radio station is a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back. A half-duplex channel is a single physical channel in which the direction may be reversed. Messages may flow in two directions, but never at the same time, in a half-duplex system. In a telephone call, one party speaks while the other listens. After a pause, the other party speaks and the first party listens. Speaking simultaneously results in garbled sound that cannot be understood. A full-duplex channel allows simultaneous message exchange in both directions. It really consists of two simplex channels, a forward channel and a reverse channel, linking the same points. The transmission rate of the reverse channel may be slower if it is used only for flow control of the forward channel. Serial Communications Most digital messages are vastly longer than just a few bits. Because it is neither practical nor economic to transfer all bits of a long message simultaneously, the message is broken into smaller parts and transmitted sequentially. Bit-serial transmission conveys a message one bit at a time through a channel. Each bit represents a part of the message. The individual bits are then reassembled at the destination to compose the message. In general, one channel will pass only one bit at a time. Thus, bit-serial transmission is necessary in data communications if only a single channel is available. Bit-serial transmission is normally just called serial transmission and is the chosen communications method in many computer peripherals. Byte-serial transmission conveys eight bits at a time through eight parallel channels. Although the raw transfer rate is eight times faster than in bit-serial transmission, eight channels are needed, and the cost may be as much as eight times higher to transmit the message. When distances are short, it may nonetheless be both feasible and economic to use parallel channels in return for high data rates. This figure illustrates these ideas: The baud rate refers to the signalling rate at which data is sent through a channel and is measured in electrical transitions per second. In the EIA232 serial interface standard, one signal transition, at most, occurs per bit, and the baud rate and bit rate are identical. In this case, a rate of 9600 baud corresponds to a transfer of 9,600 data bits per second with a bit period of 104 microseconds (1/9600 sec.). If two electrical transitions were required for each bit, as is the case in non-return-to-zero coding, then at a rate of 9600 baud, only 4800 bits per second could be conveyed. The channel efficiency is the number of bits of useful information passed through the channel per second. It does not include framing, formatting, and error detecting bits that may be added to the information bits before a message is transmitted, and will always be less than one. K. Adisesha, 20 Presidency College COPY: Jan 2009
  • 21. Data Communication & Networking IV Sem BCA The data rate of a channel is often specified by its bit rate (often thought erroneously to be the same as baud rate). However, an equivalent measure channel capacity is bandwidth. In general, the maximum data rate a channel can support is directly proportional to the channel's bandwidth and inversely proportional to the channel's noise level. A communications protocol is an agreed-upon convention that defines the order and meaning of bits in a serial transmission. It may also specify a procedure for exchanging messages. A protocol will define how many data bits compose a message unit, the framing and formatting bits, any error-detecting bits that may be added, and other information that governs control of the communications hardware. Channel efficiency is determined by the protocol design rather than by digital hardware considerations. Note that there is a tradeoff between channel efficiency and reliability - protocols that provide greater immunity to noise by adding error-detecting and -correcting codes must necessarily become less efficient. Digital Data Transmission The transmission of a stream of bits from one device to another across a transmission link involves a great deal of cooperation and agreement between the two sides. One of the most fundamental requirements is synchronization. The receiver must know the rate at which bits are being received so that it can sample the line at appropriate intervals to determine the value of each received bit. Two techniques are in common use for this purpose are: •Asynchronous transmission. •Synchronous transmission. The reception of digital data involves sampling the incoming signal once per bit time to determine the binary value. This is compounded by a timing difficulty: In order for the receiver to sample the incoming bits properly, it must know the arrival time and duration of each bit that it receives. Typically, the receiver will attempt to sample the medium at the center of each bit time, at intervals of one bit time. If the receiver times its samples based on its own clock, then there will be a problem if the transmitter's and receiver's clocks are not precisely aligned. If there is a drift in the receiver's clock, then after enough samples, the receiver may be in error because it is sampling in the wrong bit time For smaller timing differences, the error would occur later, but eventually the receiver will be out of step with the transmitter if the transmitter sends a sufficiently long stream of bits and if no steps are taken to synchronize the transmitter and receiver. Asynchronous vs. Synchronous Transmission Serialized data is not generally sent at a uniform rate through a channel. Instead, there is usually a burst of regularly spaced binary data bits followed by a pause, after which the data flow resumes. Packets of binary data are sent in this manner, possibly with variable-length pauses between packets, until the message has been fully transmitted. In order for the receiving end to know the proper moment to read individual binary bits from the channel, it must know exactly when a packet begins and how much time elapses between bits. When this timing information is known, the receiver is said to be synchronized with the transmitter, and accurate data transfer becomes possible. Failure to remain synchronized throughout a transmission will cause data to be corrupted or lost. In synchronous systems, separate channels are used to transmit data and timing information. The timing channel transmits clock pulses to the receiver. Upon receipt of a clock pulse, the receiver reads the data channel and latches the bit value found on the channel at that moment. The data channel is not read again until the next clock pulse arrives. Because the transmitter originates both the data and the timing pulses, the receiver will read the data channel only when told to do so by the transmitter (via the clock pulse), and synchronization is guaranteed. Techniques exist to merge the timing signal with the data so that only a single channel is required. This is especially useful when synchronous transmissions are to be sent through a modem. Two methods in which a data signal is self-timed are nonreturn-to-zero and biphase Manchester coding. These both refer to methods for encoding a data stream into an electrical waveform for transmission. K. Adisesha, 21 Presidency College COPY: Jan 2009
  • 22. Data Communication & Networking IV Sem BCA Synchronous transmission In asynchronous systems, a separate timing channel is not used. The transmitter and receiver must be preset in advance to an agreed-upon baud rate. A very accurate local oscillator within the receiver will then generate an internal clock signal that is equal to the transmitter's within a fraction of a percent. For the most common serial protocol, data is sent in small packets of 10 or 11 bits, eight of which constitute message information. When the channel is idle, the signal voltage corresponds to a continuous logic '1'. A data packet always begins with a logic '0' (the start bit) to signal the receiver that a transmission is starting. The start bit triggers an internal timer in the receiver that generates the needed clock pulses. Following the start bit, eight bits of message data are sent bit by bit at the agreed upon baud rate. The packet is concluded with a parity bit and stop bit. One complete packet is illustrated below: The packet length is short in asynchronous systems to minimize the risk that the local oscillators in the receiver and transmitter will drift apart. When high-quality crystal oscillators are used, synchronization can be guaranteed over an 11-bit period. Every time a new packet is sent, the start bit resets the synchronization, so the pause between packets can be arbitrarily long. Parity and Checksums Noise and momentary electrical disturbances may cause data to be changed as it passes through a communications channel. If the receiver fails to detect this, the received message will be incorrect, resulting in possibly serious consequences. As a first line of defense against data errors, they must be detected. If an error can be flagged, it might be possible to request that the faulty packet be resent, or to at least prevent the flawed data from being taken as correct. If sufficient redundant information is sent, one- or two-bit errors may be corrected by hardware within the receiver before the corrupted data ever reaches its destination. A parity bit is added to a data packet for the purpose of error detection. In the even-parity convention, the value of the parity bit is chosen so that the total number of '1' digits in the combined data plus parity packet is an even number. Upon receipt of the packet, the parity needed for the data is recomputed by local hardware and compared to the parity bit received with the data. If any bit has changed state, the parity will not match, and an error will have been detected. In fact, if an odd number of bits (not just one) have been altered, the parity will not match. If an even number of bits have been reversed, the parity will match even though an error has occurred. However, a statistical analysis of data communication errors has shown that a single-bit error is much more probable than a multibit error in the presence of random noise. Thus, parity is a reliable method of error detection. K. Adisesha, 22 Presidency College COPY: Jan 2009
  • 23. Data Communication & Networking IV Sem BCA Another approach to error detection involves the computation of a checksum. In this case, the packets that constitute a message are added arithmetically. A checksum number is appended to the packet sequence so that the sum of data plus checksum is zero. When received, the packet sequence may be added, along with the checksum, by a local microprocessor. If the sum is nonzero, an error has occurred. As long as the sum is zero, it is highly unlikely (but not impossible) that any data has been corrupted during transmission. Errors may not only be detected, but also corrected if additional code is added to a packet sequence. If the error probability is high or if it is not possible to request retransmission, this may be worth doing. However, including error-correcting code in a transmission lowers channel efficiency, and results in a noticeable drop in channel throughput. Data Compression If a typical message were statistically analyzed, it would be found that certain characters are used much more frequently than others. By analyzing a message before it is transmitted, short binary codes may be assigned to frequently used characters and longer codes to rarely used characters. In doing so, it is possible to reduce the total number of characters sent without altering the information in the message. Appropriate decoding at the receiver will restore the message to its original form. This procedure, known as data compression, may result in a 50 percent or greater savings in the amount of data transmitted. Even though time is necessary to analyze the message before it is transmitted, the savings may be great enough so that the total time for compression, transmission, and decompression will still be lower than it would be when sending an uncompressed message. A compression method called Huffman coding is frequently used in data communications, and particularly in fax transmission. Clearly, most of the image data for a typical business letter represents white paper, and only about 5 percent of the surface represents black ink. It is possible to send a single code that, for example, represents a consecutive string of 1000 white pixels rather than a separate code for each white pixel. Consequently, data compression will significantly reduce the total message length for a faxed business letter. Were the letter made up of randomly distributed black ink covering 50 percent of the white paper surface, data compression would hold no advantages. Data Encryption Privacy is a great concern in data communications. Faxed business letters can be intercepted at will through tapped phone lines or intercepted microwave transmissions without the knowledge of the sender or receiver. To increase the security of this and other data communications, including digitized telephone conversations, the binary codes representing data may be scrambled in such a way that unauthorized interception will produce an indecipherable sequence of characters. Authorized receive stations will be equipped with a K. Adisesha, 23 Presidency College COPY: Jan 2009
  • 24. Data Communication & Networking IV Sem BCA decoder that enables the message to be restored. The process of scrambling, transmitting, and descrambling is known as encryption. Data Storage Technology Normally, we think of communications science as dealing with the contemporaneous exchange of information between distant parties. However, many of the same techniques employed in data communications are also applied to data storage to ensure that the retrieval of information from a storage medium is accurate. We find, for example, that similar kinds of error-correcting codes used to protect digital telephone transmissions from noise are also used to guarantee correct readback of digital data from compact audio disks, CD-ROMs, and tape backup systems. Data Transfer in Digital Circuits Data is typically grouped into packets that are either 8, 16, or 32 bits long, and passed between temporary holding units called registers. Data within a register is available in parallel because each bit exits the register on a separate conductor. To transfer data from one register to another, the output conductors of one register are switched onto a channel of parallel wires referred to as a bus. The input conductors of another register, which is also connected to the bus, capture the information: Following a data transaction, the content of the source register is reproduced in the destination register. It is important to note that after any digital data transfer, the source and destination registers are equal; the source register is not erased when the data is sent. The transmit and receive switches shown above are electronic and operate in response to commands from a central control unit. It is possible that two or more destination registers will be switched on to receive data from a single source. However, only one source may transmit data onto the bus at any time. If multiple sources were to attempt transmission simultaneously, an electrical conflict would occur when bits of opposite value are driven onto a single bus conductor. Such a condition is referred to as a bus contention. Not only will a bus contention result in the loss of information, but it also may damage the electronic circuitry. As long as all registers in a system are linked to one central control unit, bus contentions should never occur if the circuit has been designed properly. Note that the data buses within a typical microprocessor are funda-mentally half-duplex channels. Transmission over Short Distances (< 2 feet) When the source and destination registers are part of an integrated circuit (within a microprocessor chip, for example), they are extremely close (thousandths of an inch). Consequently, the bus signals are at very low power levels, may traverse a distance in very little time, and are not very susceptible to external noise and distortion. This is the ideal environment for digital communications. However, it is not yet possible to integrate all the necessary circuitry for a computer (i.e., CPU, memory, disk control, video and display drivers, etc.) on a single chip. When data is sent off-chip to another integrated circuit, the bus signals must be amplified and conductors extended out of the chip through external pins. Amplifiers may be added to the source register: K. Adisesha, 24 Presidency College COPY: Jan 2009
  • 25. Data Communication & Networking IV Sem BCA Bus signals that exit microprocessor chips and other VLSI circuitry are electrically capable of traversing about one foot of conductor on a printed circuit board, or less if many devices are connected to it. Special buffer circuits may be added to boost the bus signals sufficiently for transmission over several additional feet of conductor length, or for distribution to many other chips (such as memory chips). Noise and Electrical Distortion Because of the very high switching rate and relatively low signal strength found on data, address, and other buses within a computer, direct extension of the buses beyond the confines of the main circuit board or plug-in boards would pose serious problems. First, long runs of electrical conductors, either on printed circuit boards or through cables, act like receiving antennas for electrical noise radiated by motors, switches, and electronic circuits: Such noise becomes progressively worse as the length increases, and may eventually impose an unacceptable error rate on the bus signals. Just a single bit error in transferring an instruction code from memory to a microprocessor chip may cause an invalid instruction to be introduced into the instruction stream, in turn causing the computer to totally cease operation. A second problem involves the distortion of electrical signals as they pass through metallic conductors. Signals that start at the source as clean, rectangular pulses may be received as rounded pulses with ringing at the rising and falling edges: These effects are properties of transmission through metallic conductors, and become more pronounced as the conductor length increases. To compensate for distortion, signal power must be increased or the transmission rate decreased. K. Adisesha, 25 Presidency College COPY: Jan 2009
  • 26. Data Communication & Networking IV Sem BCA Transmission over Medium Distances (< 20 feet) Computer peripherals such as a printer or scanner generally include mechanisms that cannot be situated within the computer itself. Our first thought might be just to extend the computer's internal buses with a cable of sufficient length to reach the peripheral. Doing so, however, would expose all bus transactions to external noise and distortion even though only a very small percentage of these transactions concern the distant peripheral to which the bus is connected. If a peripheral can be located within 20 feet of the computer, however, relatively simple electronics may be added to make data transfer through a cable efficient and reliable. To accomplish this, a bus interface circuit is installed in the computer: It consists of a holding register for peripheral data, timing and formatting circuitry for external data transmission, and signal amplifiers to boost the signal sufficiently for transmission through a cable. When communication with the peripheral is necessary, data is first deposited in the holding register by the microprocessor. This data will then be reformatted, sent with error-detecting codes, and transmitted at a relatively slow rate by digital hardware in the bus interface circuit. In addition, the signal power is greatly boosted before transmission through the cable. These steps ensure that the data will not be corrupted by noise or distortion during its passage through the cable. In addition, because only data destined for the peripheral is sent, the party-line transactions taking place on the computer's buses are not unnecessarily exposed to noise. Data sent in this manner may be transmitted in byte-serial format if the cable has eight parallel channels (at least 10 conductors for half-duplex operation), or in bit-serial format if only a single channel is available. Transmission over Long Distances (< 4000 feet) When relatively long distances are involved in reaching a peripheral device, driver circuits must be inserted after the bus interface unit to compensate for the electrical effects of long cables: K. Adisesha, 26 Presidency College COPY: Jan 2009
  • 27. Data Communication & Networking IV Sem BCA This is the only change needed if a single peripheral is used. However, if many peripherals are connected, or if other computer stations are to be linked, a local area network (LAN) is required, and it becomes necessary to drastically change both the electrical drivers and the protocol to send messages through the cable. Because multiconductor cable is expensive, bit-serial transmission is almost always used when the distance exceeds 20 feet. A great deal of technology has been developed for LAN systems to minimize the amount of cable required and maximize the throughput. The costs of a LAN have been concentrated in the electrical interface card that would be installed in PCs or peripherals to drive the cable, and in the communications software, not in the cable itself (whose cost has been minimized). Thus, the cost and complexity of a LAN are not particularly affected by the distance between stations. Transmission over Very Long Distances (greater than 4000 feet) Data communications through the telephone network can reach any point in the world. The volume of overseas fax transmissions is increasing constantly, and computer networks that link thousands of businesses, governments, and universities are pervasive. Transmissions over such distances are not generally accomplished with a direct-wire digital link, but rather with digitally-modulated analog carrier signals. This technique makes it possible to use existing analog telephone voice channels for digital data, although at considerably reduced data rates compared to a direct digital link. Transmission of data from your personal computer to a timesharing service over phone lines requires that data signals be converted to audible tones by a modem. An audio sine wave carrier is used, and, depending on the baud rate and protocol, will encode data by varying the frequency, phase, or amplitude of the carrier. The receiver's modem accepts the modulated sine wave and extracts the digital data from it. Signal Encoding Techniques Analog and Digital information can be encoded as either analog or digital signals: ♦ Digital data, digital signals: simplest form of digital encoding of digital data ♦ Digital data, analog signal: A modem converts digital data to an analog signal so that it can be transmitted over an analog K. Adisesha, 27 Presidency College COPY: Jan 2009
  • 28. Data Communication & Networking IV Sem BCA ♦ Analog data, digital signals: Analog data, such as voice and video, are often digitized to be able to use digital transmission facilities ♦ Analog data, analog signals: Analog data are modulated by a carrier frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission system For digital signaling, a data source g(t), which may be either digital or analog, is encoded into a digital signal x(t). The basis for analog signaling is a continuous constant-frequency fc signal known as the carrier signal. Data may be transmitted using a carrier signal by modulation, which is the process of encoding source data onto the carrier signal. All modulation techniques involve operation on one or more of the three fundamental frequency domain parameters: amplitude, frequency, and phase. The input signal m(t) may be analog or digital and is called the modulating signal, and the result of modulating the carrier signal is called the modulated signal s(t). Encoding - Digital data to digital signals: A digital signal is a sequence of discrete, discontinuous voltage pulses. Each pulse is a signal element. Binary data are transmitted by encoding each data bit into signal elements. In the simplest case, there is a one-to-one correspondence between bits and signal elements. More complex encoding schemes are used to improve performance, by altering the spectrum of the signal and providing synchronization capability. In general, the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog modulation equipment. Various Encoding techniques include: • Nonreturn to Zero-Level (NRZ-L) • Nonreturn to Zero Inverted (NRZI) • Bipolar -AMI • Pseudoternary • Manchester • Differential Manchester • B8ZS • HDB3 Encoding techniques The most common, and easiest, way to transmit digital signals is to use two different voltage levels for the two binary digits. Codes that follow this strategy share the property that the voltage level is constant during a bit interval; there is no transition (no return to a zero voltage level). Can have absence of voltage used to K. Adisesha, 28 Presidency College COPY: Jan 2009
  • 29. Data Communication & Networking IV Sem BCA represent binary 0, with a constant positive voltage used to represent binary 1. More commonly a negative voltage represents one binary value and a positive voltage represents the other. This is known as Nonreturn to Zero-Level (NRZ-L). NRZ-L is typically the code used to generate or interpret digital data by terminals and other devices. A variation of NRZ is known as NRZI (Nonreturn to Zero, invert on ones). As with NRZ-L, NRZI maintains a constant voltage pulse for the duration of a bit time. The data bits are encoded as the presence or absence of a signal transition at the beginning of the bit time. A transition (low to high or high to low) at the beginning of a bit time denotes a binary 1 for that bit time; no transition indicates a binary 0. NRZI is an example of differential encoding. In differential encoding, the information to be transmitted is represented in terms of the changes between successive signal elements rather than the signal elements themselves. The encoding of the current bit is determined as follows: if the current bit is a binary 0, then the current bit is encoded with the same signal as the preceding bit; if the current bit is a binary 1, then the current bit is encoded with a different signal than the preceding bit. One benefit of differential encoding is that it may be more reliable to detect a transition in the presence of noise than to compare a value to a threshold. Another benefit is that with a complex transmission layout, it is easy to lose the sense of the polarity of the signal. A category of encoding techniques known as multilevel binary addresses some of the deficiencies of the NRZ codes. These codes use more than two signal levels. In the bipolar-AMI scheme, a binary 0 is represented by no line signal, and a binary 1 is represented by a positive or negative pulse. The binary 1 pulses must alternate in polarity. There are several advantages to this approach. First, there will be no loss of synchronization if a long string of 1s occurs. Each 1 introduces a transition, and the receiver can resynchronize on that transition. A long string of 0s would still be a problem. Second, because the 1 signals alternate in voltage from positive to negative, there is no net dc component. Also, the bandwidth of the resulting signal is considerably less than the bandwidth for NRZ. Finally, the pulse alternation property provides a simple means of error detection. Any isolated error, whether it deletes a pulse or adds a pulse, causes a violation of this property. The comments on bipolar-AMI also apply to pseudoternary. In this case, it is the binary 1 that is represented by the absence of a line signal, and the binary 0 by alternating positive and negative pulses. There is no particular advantage of one technique versus the other, and each is the basis of some applications. There is another set of coding techniques, grouped under the term biphase, that overcomes the limitations of NRZ codes. Two of these techniques, Manchester and differential Manchester, are in common use. In the Manchester code, there is a transition at the middle of each bit period. The midbit transition serves as a clocking mechanism and also as data: a low-to-high transition represents a 1, and a high-to-low transition represents a 0. Biphase codes are popular techniques for data transmission. The more common Manchester code has been specified for the IEEE 802.3 (Ethernet) standard for baseband coaxial cable and twisted-pair bus LANs. In differential Manchester, the midbit transition is used only to provide clocking. The encoding of a 0 is represented by the presence of a transition at the beginning of a bit period, and a 1 is represented by the absence of a transition at the beginning of a bit period. Differential Manchester has the added advantage of employing differential encoding. Differential Manchester has been specified for the IEEE 802.5 token ring LAN, using shielded twisted pair. Digital Data, Analog Signal The most familiar use of transmitting digital data using analog signals transformation is for transmitting digital data through the public telephone network. The telephone network was designed to receive, switch, and transmit analog signals in the voice-frequency range of about 300 to 3400 Hz. It is not at present suitable for handling digital signals from the subscriber locations (although this is beginning to change). K. Adisesha, 29 Presidency College COPY: Jan 2009
  • 30. Data Communication & Networking IV Sem BCA Thus digital devices are attached to the network via a modem (modulator-demodulator), which converts digital data to analog signals, and vice versa. Having stated that modulation involves operation on one or more of the three characteristics of a carrier signal: amplitude, frequency, and phase. Accordingly, there are three basic encoding or modulation techniques for transforming digital data into analog signals, as illustrated Figure: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). In all these cases, the resulting signal occupies a bandwidth centered on the carrier frequency. Modulation Techniques In ASK, the two binary values are represented by two different amplitudes of the carrier frequency. Commonly, one of the amplitudes is zero; that is, one binary digit is represented by the presence, at constant amplitude, of the carrier, the other by the absence of the carrier, ASK is susceptible to sudden gain changes and is a rather inefficient modulation technique. On voice-grade lines, it is typically used only up to 1200 bps. The ASK technique is used to transmit digital data over optical fiber, where one signal element is represented by a light pulse while the other signal element is represented by the absence of light. The most common form of FSK is binary FSK (BFSK), in which the two binary values are represented by two different frequencies near the carrier frequency, as shown in Figure. BFSK is less susceptible to error than ASK. On voice-grade lines, it is typically used up to 1200 bps. It is also commonly used for high-frequency (3 to 30 MHz) radio transmission. It can also be used at even higher frequencies on local area networks that use coaxial cable. In PSK, the phase of the carrier signal is shifted to represent data. The simplest scheme uses two phases to represent the two binary digits (Figure) and is known as binary phase shift keying. An alternative form of two-level PSK is differential PSK (DPSK). In this scheme, a binary 0 is represented by sending a signal burst of the same phase as the previous signal burst sent. A binary 1 is represented by sending a signal burst of opposite phase to the preceding one. This term differential refers to the fact that the phase shift is with reference to the previous bit transmitted rather than to some constant reference signal. In differential encoding, the information to be transmitted is represented in terms of the changes between successive data symbols rather than the signal elements themselves. DPSK avoids the requirement for an accurate local oscillator phase at the receiver that is matched with the transmitter. As long as the preceding phase is received correctly, the phase reference is accurate. More efficient use of bandwidth can be achieved if each signaling element represents more than one bit. For example, instead of a phase shift of 180˚, as allowed in BPSK, a common encoding technique, known as quadrature phase shift keying (QPSK), uses phase shifts separated by multiples of π/2 (90˚). Thus each signal element represents two bits rather than one. The input is a stream of binary digits with a data rate of K. Adisesha, 30 Presidency College COPY: Jan 2009
  • 31. Data Communication & Networking IV Sem BCA R = 1/Tb, where Tb is the width of each bit. This stream is converted into two separate bit streams of R/2 bps each, by taking alternate bits for the two streams. The two data streams are referred to as the I (in-phase) and Q (quadrature phase) streams. The streams are modulated on a carrier of frequency fc by multiplying the bit stream by the carrier, and the carrier shifted by 90˚. The two modulated signals are then added together and transmitted. Thus, the combined signals have a symbol rate that is half the input bit rate. The use of multiple levels can be extended beyond taking bits two at a time. It is possible to transmit bits three at a time using eight different phase angles. Further, each angle can have more than one amplitude. For example, a standard 9600 bps modem uses 12 phase angles, four of which have two amplitude values, for a total of 16 different signal elements. Analog data, digital signals In this section we examine the process of transforming analog data into digital signals. Analog data, such as voice and video, is often digitized to be able to use digital transmission facilities. Strictly speaking, it might be more correct to refer to this as a process of converting analog data into digital data; this process is known as digitization. Once analog data have been converted into digital data, a number of things can happen. The three most common are: 1. The digital data can be transmitted using NRZ-L. In this case, we have in fact gone directly from analog data to a digital signal. 2. The digital data can be encoded as a digital signal using a code other than NRZ-L. Thus an extra step is required. 3. The digital data can be converted into an analog signal, using one of the modulation techniques. The device used for converting analog data into digital form for transmission, and subsequently recovering the original analog data from the digital, is known as a codec (coder-decoder). In this section we examine the two principal techniques used in codecs, pulse code modulation and delta modulation. Digitizing Analog Data The simplest technique for transforming analog data into digital signals is pulse code modulation (PCM), which involves sampling the analog data periodically and quantizing the samples. Pulse code modulation (PCM) is based on the sampling theorem (quoted above). Hence if voice data is limited to frequencies below 4000 Hz (a conservative procedure for intelligibility), 8000 samples per second would be sufficient to characterize the voice signal completely. Note, however, that these are analog samples, called pulse amplitude modulation (PAM) samples. To convert to digital, each of these analog samples must be assigned a binary code. K. Adisesha, 31 Presidency College COPY: Jan 2009
  • 32. Data Communication & Networking IV Sem BCA PCM Example Figure shows an example in which the original signal is assumed to be bandlimited with a bandwidth of B. PAM samples are taken at a rate of 2B, or once every Ts = 1/2B seconds. Each PAM sample is approximated by being quantized into one of 16 different levels. Each sample can then be represented by 4 bits. But because the quantized values are only approximations, it is impossible to recover the original signal exactly. By using an 8-bit sample, which allows 256 quantizing levels, the quality of the recovered voice signal is comparable with that achieved via analog transmission. Note that this implies that a data rate of 8000 samples per second × 8 bits per sample = 64 kbps is needed for a single voice signal. PCM Block Diagram Thus, PCM starts with a continuous-time, continuous-amplitude (analog) signal, from which a digital signal is produced, as shown in Figure. The digital signal consists of blocks of n bits, where each n-bit number is the amplitude of a PCM pulse. On reception, the process is reversed to reproduce the analog signal. Notice, however, that this process violates the terms of the sampling theorem. By quantizing the PAM pulse, the original signal is now only approximated and cannot be recovered exactly. This effect is known as quantizing error or quantizing noise. Each additional bit used for quantizing increases SNR by about 6 dB, which is a factor of 4. Non-Linear Coding Typically, the PCM scheme is refined using a technique known as nonlinear encoding, which means, in effect, that the quantization levels are not equally spaced. The problem with equal spacing is that the mean absolute error for each sample is the same, regardless of signal level. Consequently, lower amplitude values are relatively more distorted. By using a greater number of quantizing steps for signals of low amplitude, K. Adisesha, 32 Presidency College COPY: Jan 2009