Computer networks and internet www.it-workss.com

COMPUTER NETWORKS AND INTERNET
MBA–4th SEMESTER, M.D.U., ROHTAK
SYLLABUS
External Marks : 70
Time : 3 hrs.
Internal Marks : 30
UNIT - I
UNIT - II
UNIT - III
UNIT - IV
Computer networks : Concepts of data transmission, transmission channel, half-duplex
transmission, modems, client server computing.
Introduction to OSI,TCP/IPreference models, routing and flow control, cryptography.
Internet : ISP search engines, URL, DNS, browser, security, digital signatures, e-mail,
Ipv6, HTTP.
HTML : Build a simple HTML document, tables, frames, links, adding multimedia
documents, home page.
Lab : Web Designing in HTML, Internet Surfing.
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Q. What do you understand by data transmission? Differentiate between digital
and analog transmission.Also discuss the merits and demerits.
Ans.
Analog transmission
Data Communications is the transfer of data or information between a source and a
receiver. The source transmits the data and the receiver receives it. The actual generation of
the information is not part of Data Communications nor is the resulting action of the
information at the receiver. Data Communication is interested in the transfer of data, the
method of transfer and the preservation of the data during the transfer process.
The purpose of Data Communications is to provide the rules and regulations that allow
computers with different disk operating systems, languages, cabling and locations to share
resources. The rules and regulations are called protocols and standards in Data
Communications.
is a transmission method of conveying voice, data, image,
signal or video information using a continuous signal which varies in amplitude, phase, or
some other property in proportion to that of a variable. It could be the transfer of an analog
source signal using an analog modulation method such as FM orAM, or no modulation at all.
All telephone calls used to be transmitted in an analog format. Today they are translated to
digit
andom noise can make signal loss and distortion impossible to recover
prone to noise & interference, exactly what is transmitted is received, without being altered.
al pulses for both local and long-distance transmission. Your television at home receives
analog signals.
Analogue is a simpler technology, signals are continuous and more accurate. The
transmission of video using analog technique is very popular, especially for shorter distance
.Analogue signals uses less bandwidth compared to digital singles. when audio and video
signals are transmitted digitally an AD(Analog to Digital) converter is needed at transmitting
side and a DA(Digital to Analog) converter is again needed at receiver side. While
transmitted in analog communication these devices are not needed.
The effects of r
Advantages ofAnalogue -
Disadvantages ofAnalogue -
MBA 4th Semester (DDE)
UNIT – I
84

when the signal is weak or the equipment not perfect some reception can give some picture /
sound can be intercepted by other parties with the same technology. Incase of analog
communication it's difficult to remove the noise once added. Security is another priority of
messaging services in modern days. Digital communication provides better security to
messages than the analog communication signals.
is the physical transfer of data (a digital bit stream) over a
point-to-point or point-to-multipoint communication channel. Examples of such channels
are copper wires, optical fibers, wireless communication channels, and storage media. The
data is often represented as an electro-magnetic signal, such an electrical voltage signal, a
radio
ls are continuous electrical signals that vary in time as shown in figure .
Most of the time, the variations follow that of the non-electric (original) signal. Therefore, the
two are analogous hence the name analog.
Digital communications
wave or microwave signal or an infra-red signal.
Digital information is better transmitted in its digital form because converting the signal
to analog and sending it through an analog network can be costly. Digital data is easily
compressed; therefore it can be transmitted using a small bandwidth. Because of the nature
of devices used to boost the signal strength during transmission, error performance is much
improved when compared with analog. It is also better to transmit information in digital form
because computer components used in the transmission process are very reliable.
Information is widely available as analog. Conversion of analog signal to digital bit
stream requires special equipment. This adds cost and additional equipment may also
contribute additional points of failure. Another disadvantage is that during the conversion
process the quality of the signal is negatively affected.
1. Analog and Digital signals
2. Parallel and SerialTransmission
3. Half - Duplex and Full - Duplex transmission
4. Synchronous andAsynchronousTransmission model
The information carrying signals are divided into two broad classes;
1. Analog 2. Digital
Analog signa
Advantages
Disadvantages :
Q. Differentiate between the following
Ans.
2.1Analog vs. Digital Signals
Analog Signals
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Figure : Analog Signal
Digital Signals
Figure : Digital Signal
2.2 Parallel and Serial Transmission
Digital signals are non-continuous, they change in individual steps. They consist of pulses or
digits with discrete levels or values. The value of each pulse is constant, but there is an
abrupt change from one digit to the next. Digital signals have two amplitude levels called
nodes. The value of which are specified as one of two possibilities such as 1 or 0, HIGH or
LOW, TRUE or FALSE and so on. In reality, the values are anywhere within specific ranges
and we define values within a given range.
Parallel Transmission: Parallel data transmission involves the concurrent flow of bits
of data through separate communications lines. This pattern resembles the flow of
automobile traffic on a multilane highway. Internal transfer of binary data in a computer uses
a parallel mode. If the computer uses a 32-bit internal structure, all the 32 bits of data are
transferred simultaneously on 32 lane connections. Parallel data transmission is commonly
used for interactions between a computer and its printing unit. The printer usually located
close to the computer, because parallel cables need many wires and may not work stably in
long distance.
Intensity
Time
Intensity
Time
1 0 1 0 1
Abrupt Amplitude Variations
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Serial Data Transmission: Most data transmitted over telephone lines use a serial
pattern. That is, each individual bit of information travels along its own communications path;
the bits flow in a continuous stream along the communications channel. This pattern is
analogous to the flow of traffic down a one-lane residential street. Serial transmission is
typically slower than parallel transmission, because data are sent sequentially in a bit-by-bit
fa
ex data transmission means that data can be transmitted in both directions
on a signal carrier, but not at the same time. For example, on a local area network using a
technology that has half-duplex transmission, one workstation can send data on the line and
then immediately receive data on the line from the same direction in which data was just
transmitted. On the other hand, full-duplex transmission implies that data are transmitted in
both directions simultaneously.
For example, a walkie-talkie is a half-duplex device because only one party can talk at
a time. In contrast, a telephone is a full-duplex device because both parties can talk
simultaneously. When two computers communicate over a LAN, data typically travels in only
one direction at a time, because the baseband network used for most LANs supports only a
single signal. In other words, it is transmitted in half-duplex fashion.
shion.
Besides the previously mentioned ways in which data may travel, there are three
directional modes of travel in data transmission.
Simplex Communication: Simplex communication is a mode in which data only flows
in one direction. Because most modern communications require a two-way interchange of
data and information, this mode of transmission is not as popular as it once was. However,
one current usage of simplex communications in business involves certain point-of-sale
terminals in which sales data is entered without a corresponding reply.
Half-dupl
2.3 Direction of Data Transmission
Half Duplex Transmission
OR
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Full Duplex transmission indicates the transmission of data in two directions
simultaneously. For example, on a local area network with full-duplex transmission, one
workstation can be sending data on the line while another workstation is receiving data. On
the other hand, a half duplex transmission indicates that data transmission happens only in
one direction at a time.
The most common example of a full-duplex network is the telephone system. Both
parties can speak simultaneously during a telephone call and each party can also hear the
other at the same time. An example of a half-duplex communication system is a two-way
radio like a CB radio, in which only one party can transmit at any one time, and each party
must say "over" to signal that he or she has finished talking.
With the right equipment, full-duplex communication is possible on certain types of
LANs. The first requirement is a separate channel for traffic running in each direction.
Whether this is possible depends on the network medium. Coaxial cable, for example,
contains a single conductor and a ground, so there is no physical way that traffic could run in
both directions, unless you were to install two cable runs for each connection. Twisted pair
cable, on the other hand, contains four separate wire pairs within a single sheath, one of
which is dedicated to incoming traffic and one to outgoing. Networks that use this type of
cable can therefore theoretically operate in full-duplex mode, and some manufacturers are
making Ethernet equipment that makes this possible. Full-duplex Ethernet essentially
doubles the throughput of the existing network.
Half-duplex communications
Full-duplex communications
Full Duplex Transmission
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2.4 Synchronous andAsynchronous Transmission
Q. What is Shannon Theorem?
Ans.
Another way of classifying data communications flow is as synchronous or
asynchronous.
Synchronous Transmission: Large volumes of information can be transmitted at a
single time with synchronous transmission. This type of transmission involves the
simultaneous flow of several bytes of data. Because a large block of data being sent
synchronously cannot be interrupted, a synchronized clock is necessary to carefully
schedule the transmission of information. This special communications equipment is
expensive; but this cost can be made up in part by faster, less expensive transmission of
information.
Asynchronous Transmission: Conversely, asynchronous transmission involves the
sending and receiving of one byte of data at a time. This type of transmission is most often
used by microcomputers and other systems characterized by slow speeds.
In the 1940s, Claude Shannon developed the concept of channel capacity, based in
part on the ideas of Nyquist and Hartley, and then formulated a complete theory of
information and its transmissionShannon'sTheorem gives an upper bound to the capacity of
a link, in bits per second (bps), as a function of the available bandwidth and the signal-to-
noise ratio of the link.
TheTheorem can be stated as:
C = B * log2(1+ S/N)
where C is the achievable channel capacity, B is the bandwidth of the line, S is the average
signal power and N is the average noise power.
The signal-to-noise ratio (S/N) is usually expressed in decibels (dB) given by the formula:
10 * log10(S/N)
so for example a signal-to-noise ratio of 1000 is commonly expressed as
10 * log10(1000) = 30 dB.
The topology of a network is the geometric representation of the relationship of all the
devices in network or The Pattern of interconnection of nodes in a network is called topology
or Network topology is the study of the arrangement or mapping of the elements (links,
nodes, etc.) of a network, especially the physical (real) and logical (virtual) interconnections
between nodes.
Q. What is topology? What are its various types? Illustrate through suitable
examples?
Ans.
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Ring Mesh Star Fully Connected
Line Tree Bus
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Topology can be classified into :
1. Physical Topology :
2. Logical Topology :
Basic types of topologies :
Diagram of Different Network Topologies
Bus Topology
Physical topology describes how devices are physically cabled
together. Any given node in the LAN will have one or more links to one or more other
nodes in the network and the mapping of these links and nodes onto a graph results in
a geometrical shape that determines the physical topology of the network.
Logical topology describes how devices communicate across the
physical topology.The mapping of the flow of data between the nodes in the network
determines the logical topology of the network.
The type of network topology in which all of the nodes of the network are connected to
a common transmission medium ( Bus or Backbone) which has exactly two endpoints. A
message with unique address of the target computer is placed on the bus and is transmitted
to all nodes. The node for which the message is directed, read the message while other
ignores it.
It is easy to handle and implement.
It is best suited for small networks.
Advantages of Bus Topology

Disadvantages of Bus Topology
Ring Topology
Advantage of Ring Topology
Disadvantages of Ring Topology
Advantage of Mesh Topology
Disadvantage of Mesh Topology
The cable length is limited.This limits the number of stations that can be connected.
This network topology can perform well only for a limited number of nodes. Difficult to
troubleshoot
The type of network topology in which each of the nodes of the network is connected to two
other nodes in the network and with the first and last nodes being connected to each other,
forming a ring - all data that is transmitted between nodes in the network travels from one
node to the next node in a circular manner and the data generally flows in a single direction
only.
The data being transmitted between two nodes passes through all the intermediate nodes.A
central server is not required for the management of this topology. Suitable for Optical
Fibers.
The failure of a single node of the network can cause the entire network to fail.
The movement or changes made to network nodes affects the performance of the entire
network. Difficult to diagnose fault.
In a mesh topology, every device has a dedicated point-to-point link to every other device.
Dedicated means that the link carries traffic only between the two devices it connects.Afully
connected mesh network therefore has n(n-1)/2 physical channels to link n devices.
The arrangement of the network nodes is such that it is possible to transmit data from one
node to many other nodes at the same time. It is robust.
The arrangement wherein every network node is connected to every other node of the
network, many of the connections serve no major purpose. This leads to the redundancy of
many of the network connections.
In this kind of topology all the cables run from the computers to the central location where
they are all connected by a device called hub or switch. Each computer on a star network
communicates with a central device that resends the message either to each computer or
only to the destination computer.
Mesh Topology
Star Topology
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Advantages :
Disadvantages :
Tree
Hybrid
Q. What is a Modem ? What are the functions of modem.
Ans.
It is easy to modify and add new computers without disturbing the rest of the network. The
center of the star network is a good place to diagnose the faults.Single computer failure does
not necessarily bring down the whole star network.
If the central device fails the whole network fails to operate. Star networking is expensive
because all network cables must be pulled to one central point, requires more cable than
other network topologies.
A tree topology is a variation of a star topology. In tree topology a central 'root' node (the top
level of the hierarchy) is connected to one or more other nodes that are one level lower in the
hierarchy (i.e., the second level) with a point-to-point link between each of the second level
nodes and the top level central 'root' node, while each of the second level nodes that are
connected to the top level central 'root' node will also have one or more other nodes that are
one level lower in the hierarchy (i.e., the third level) connected to it, also with a point-to-point
link, the top level central 'root' node being the only node that has no other node above it in the
hierarchy.
Hybrid networks use a combination of any two or more topologies in such a way that the user
can get benefit of booths topologies (e.g., bus, star, ring, etc.). For example, a tree network
connected to a tree network is still a tree network, but two star networks connected together
exhibit a hybrid network topology.
The modem is an acronym for "MOdulator- DEModulator." A modem converts the
digital signals into continuous analog signals (Modulation), and converts from analog to
digital (Demodulation). The modem enables digital microcomputers to communicate across
analog telephone lines. Both voice communications and data communications can be
carried over the same telephone line.
Modem
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Microcomputers require modems to send and receive messages over telephone lines.
However, not all computer communications should have converters such as modems.
Computer systems connected by coaxial or fiber- optic cables can send and receive digital
sig
ntelligent modems are also called advanced modems. These
modems can accept new instructions and then respond to the commands while transmitting
data and information. These can be done by microprocessor chips and internal read only
memory (ROM) contained in the modem. Of course, these modems are more expensive.
Client-server describes the relationship between two computer programs in which one
program, the client program, makes a service request to another, the server program.
Standard networked functions such as email exchange, web access and database access,
nals directly through these circuits. Most modems today are both data and fax modems in
that they can transfer computer data and also acting as fax machines.
The modem handles the dialing and answering of the call and controls transmission
speed. Communications speeds are expressed in bits per second (bps). A baud is
commonly and erroneously used to specify bits per second for modem speed. This is not
always true. Modems are classified by the speed at which they operate. The modems used
with microcomputers usually operate at 2400, 9600, 14400, and 28800 bits per second.
The higher the speed, the faster users can transmit a document and therefore the
cheaper your line costs. For example, transmitting a 100-page single-spaced report takes
25 minutes at 2400 bps. It takes 6 1/4 minutes at 9600 bps, about 4 1/6 minutes at 14400
bps, and 2 1/12 minutes at 28800 bps.
This is a modem separated from the system unit in the computer case. It
is connected to the serial port of the computer by means of a cable. It is connected to the
telephone wall jack by another cable.
An internal modem is a circuit board (a modem card) that can be added to
the system unit of the computer. It takes one of the expansion slots.
Most modems used today are called standard modems. These
modems are usually operated by commands entered from a microcomputer keyboard.
Users control the functions (dialing, etc.) of a modem through the keyboard. Modems may
use different command languages to control their functions, but most modems use a
popular language developed by a company "Hayes Microcomputer Products, Inc."
I
Communications Speeds
Types of Modems
External vs. Internal Modems
External Modem :
Internal Modem :
Standard vs. Intelligent Modems
Standard Modems :
Intelligent Modems :
Q. What is client server computing? Explain.
Ans.
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are based on the client-server model. For example, a web browser is a client program at the
user computer that may access information at any web server in the world. To check your
bank account from your computer, a web browser client program in your computer forwards
your request to a web server program at the bank. That program may in turn forward the
request to its own database client program that sends a request to a database server at
another bank computer to retrieve your account balance. The balance is returned to the
bank database client, which in turn serves it back to the web browser client in your personal
computer, which displays the information for you.
The client-server model has become one of the central ideas of network computing.
Most business applications being written today use the client-server model. So do the
Internet's main application protocols, such as HTTP, SMTP,Telnet, DNS, etc.
In the usual client/server model, one server, sometimes called a daemon, is activated
and awaits client requests. Typically, multiple client programs share the services of a
common server program. Both client programs and server programs are often part of a
larger program or application. Relative to the Internet, your Web browser is a client program
that requests services (the sending of Web pages or files) from a Web server (which
technically is called a Hypertext Transport Protocol or HTTP server) in another computer
somewhere on the Internet. Similarly, your computer with TCP/IP installed allows you to
make client requests for files from File Transfer Protocol (FTP) servers in other computers
on the Internet.
Each instance of the client software can send data requests to one or more connected
servers. In turn, the servers can accept these requests, process them, and return the
requested information to the client. Although this concept can be applied for a variety of
reasons to many different kinds of applications, the architecture remains fundamentally the
same.
When both the client- and server-software are running on the same computer, this is
called a single seat setup.
Specific types of clients include web browsers, email clients, and online chat clients.
Specific types of servers include web servers, ftp servers, application servers,
database servers, name servers, mail servers, file servers, print servers, and terminal
servers. Most web services are also types of servers.
Comparison to Peer-to-Peer architecture
Another type of network architecture is known as peer-to-peer, because each host or
instance of the program can simultaneously act as both a client and a server, and because
each has equivalent responsibilities and status.
1. Handle the user interface.
2. Translate the user's request into the desired protocol.
The client's responsibility is usually to :
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3. Send the request to the server.
4. Wait for the server's response.
5. Translate the response into "human-readable" results.
6. Present the results to the user.
1. Listen for a client's query.
2. Process that query.
3. Return the results back to the client.
1. The user runs client software to create a query.
2. The client connects to the server.
3. The client sends the query to the server.
4. The server analyzes the query.
5. The server computes the results of the query.
6. The server sends the results to the client.
7. The client presents the results to the user.
8. Repeat as necessary.
-server architecture enables the roles and responsibilities of a
computing system to be distributed among several independent computers that are
known to each other only through a network. This creates an additional advantage to
this architecture: greater ease of maintenance. For example, it is possible to replace,
repair, upgrade, or even relocate a server while its clients remain both unaware and
unaffected by that change. This independence from change is also referred to as
encapsulation.
The server's functions include :
Atypical client/server interaction goes like this :
A Typical Client/Server Interaction
Advantages
• In most cases, a client
Response
Client
Query
Server Data
Search
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• All the data is stored on the servers, which generally have far greater security controls
than most clients. Servers can better control access and resources, to guarantee that
only those clients with the appropriate permissions may access and change data.
• Since data storage is centralized, updates to that data are far easier to administer than
what would be possible under a P2P paradigm. Under a P2P architecture, data
updates may need to be distributed and applied to each "peer" in the network, which is
both time-consuming and error-prone, as there can be thousands or even millions of
peers.
• Many mature client-server technologies are already available which were designed to
ensure security, 'friendliness' of the user interface, and ease of use.
• It functions with multiple different clients of different capabilities.
• Reduces the total cost of ownership.
• Increases Productivity
•
Data communications can be carried from one location to another through a variety of
communications channels. These communications media include telephone lines (twisted
pairs), coaxial cable, fiber-optic cable, microwave, and satellite. In general, there can be two
ways of connecting microcomputers with each other and with other equipment: guided and
unguided media.
End User Productivity
• Developer Productivity
• Traffic congestion on the network has been an issue since the inception of the client-
server paradigm. As the number of simultaneous client requests to a given server
increases, the server can become severely overloaded. Contrast that to a P2P
network, where its bandwidth actually increases as more nodes are added, since the
P2P network's overall bandwidth can be roughly computed as the sum of the
bandwidths of every node in that network.
• The client-server paradigm lacks the robustness of a good P2P network. Under client-
server, should a critical server fail, clients' requests cannot be fulfilled. In P2P
networks, resources are usually distributed among many nodes. Even if one or more
nodes depart and abandon a downloading file, for example, the remaining nodes
should still have the data needed to complete the download
Disadvantages
Q. What is transmission channel? What are its types? Discuss.
Ans.
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Guided Transmission Media uses a "cabling" system that guides the data
signals along a specific path.The data signals are bound by the "cabling" system.
The three main types of networking cables twisted-pair cable, coaxial cable and fiber
optic cable.
Twisted-pair cable consists of two insulated strands of copper wire twisted around
each other to form a pair. One or more twisted pairs are used in a twisted-pair cable. The
purpose of twisting the wires is to eliminate electrical interference from other wires and
outside sources such as motors. Twisting the wires cancels any electrical noise from the
adjacent pair.The more twists per linear foot, the greater the effect.
There are two types of twisted pair cable.
1. ShieldedTwisted Pair (STP)
2. Unshielded Twisted Pair (UTP)
The only difference between STP and UTP is that STP has a foil or wire braid wrapped
around the individual wires of the pairs. The shielding is designed to minimize EMI radiation
and susceptibility to crosstalk. The STP cable uses a woven-copper braided jacket, which is
a higher-quality, more protective jacket than UTP.
Guided Media :
Types of Networking Cables :
Twisted-Pair Cable(telephone cable):
Shielded Twisted Pair (STP) :
Transmission
Media
Guided
(Wired)
Unguided
(Wireless)
Twisted-Pair
Cable
Coaxial
Cable
Fiber-Optic
Cable
Free
Space
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Insulating
Jackets Conductors

ble is made of two conductors that share the same axis; the center is a
copper wire that is insulated by a plastic coating and then wrapped with an outer conductor
(usually a wire braid). This outer conductor around the insulation serves as electrical
shielding for the signal being carried by the inner conductor. A tough insulating plastic tube
outside the outer conductor provides physical and electrical protection. At one time, coaxial
cable was the most widely used network cabling. However, with improvements and the
lower cost of twisted-pair cables, it has lost its popularity.
Unshielded Twisted Pair (UTP) :
As the name implies, "unshielded twisted pair" (UTP) cabling is twisted pair cabling
that contains no shielding. UTPcables can be divided further into following categories:
Coaxial ca
Coaxial Cable :
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DB-9
Connector
Shielded
Twisted-Pair
Cable
IBM Data
Connector
Jacket
Shield
Two Twisted
Pairs
Plastic Jacket
Dielectric Insulator
Metallic Shield
Centre Core

There are two types of coaxial cable.
1. ThickNet
2. ThinNet
ThickNet is about .38 inches in diameter. This makes it a better conductor, and it can
carry a signal about 1640 feet (500 meters) before signal strength begins to suffer. The
disadvantage of ThickNet over ThinNet is that it is more difficult to work with. The ThickNet
version is also known as standard Ethernet cable. Thicknet uses N-connectors, which
screw on instead of using a twist-lock
ThinNet is the easiest to use. It is about .25 inches in diameter, making it flexible and
easy to work with (it is similar to the material commonly used for cable TV). ThinNet can carry
a signal about 605 feet (185 meters) before signal strength begins to suffer.
10Base2 ThinNet Coaxial 185 meters 10 Mbps
10Base5 ThickNet Coaxial 500 meters 10 Mbps
Thick Net :
Thin Net :
Name Description Type Segment Speed
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Rexible RG 58/U
50 Ohm coaxial cable
BNC Connector
on equipment
BNC 'T' Piece
To next equipment or
50 Ohm resistive terminator
N-Connector
Ethernet Back-bone
N-Connector style
Ethernet Back-bone
15-Conductor drop-cable

Fiber-Optic Cable :
This is a cable made up of super-thin filaments of glass or other transparent materials
that can carry beams of light. Because a fiber-optic cable is light-based, data can be sent
through it at the speed of light. Using a laser transmitter that encodes frequency signals into
pulses of light, ones and zeros are sent through the cable. The receiving end of the
transmission translates the light signals back into data which can be read by a computer.
Fiber optics has several advantages over traditional metal communications lines:
• Fiber optic cables are less susceptible than metal cables to interference.
• Fiber optic cables have a much greater bandwidth than metal cables. This
means that they can carry more data.
• Fiber optic cables are much thinner and lighter than metal wires.
• Data can be transmitted digitally (the natural form for computer data) rather than
analogically.
The main disadvantage of fiber optics is that the cables are expensive to install. In
addition, they are more fragile than wire and are difficult to splice.
Fiber optics is a particularly popular technology for local-area networks. In addition,
telephone companies are steadily replacing traditional telephone lines with fiber optic
cables. In the future, almost all communications will employ fiber optics.
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Core
Cladding
Coating
Strengthening
Fibers
Cable
Jacket
Outer Insulation
Fibers
Buffer
Tape
Coating
Strength
Member

Unguided Transmission Media
Radio Waves
Microwave
Unguided Transmission Media consists of a means for the data signals to travel but
nothing to guide them along a specific path The unguided media is the wireless media. It
simply transports electromagnetic waves without using any physical conductor. Signals are
normally broadcast through the air and thus are available to any one who has the device
capable of receiving them. Unguided signals can be traveled from source to the destination
in several ways. These ways include ground propagation, sky propagation and line of sight
propagation .We can divide wireless transmission into three broad groups :
Although there is no clear-cut demarcation between radio waves and microwaves,
electromagnetic waves ranging in frequencies between 3 kHz and I GHz are normally called
radio waves; waves ranging in frequencies between I and 300 GHz are called microwaves.
However, the behavior of the waves, rather than the frequencies, is a better criterion for
classification.
Radio waves, for the most part, are omnidirectional. When an antenna transmits radio
waves, they are propagated in all directions. This means that the sending and receiving
antennas do not have to be aligned.Asending antenna sends waves that can be received by
any receiving antenna. The omnidirectional property has a disadvantage, too. The radio
waves transmitted by one antenna are susceptible to interference by another antenna that
may send signals using the same frequency or band.
Radio waves, particularly those waves that propagate in the sky mode, can travel long
distances. This makes radio waves a good candidate for long-distance broadcasting such
asAM radio.
Radio waves, particularly those of low and medium frequencies, can penetrate walls.
Microwave data transmission differs from the previously mentioned communications
channels in that data is transmitted through the air instead of through cables or wires.
Microwaves are high-frequency radio waves that can only be directed in straight lines.
Radio
Wave
Microware Infrared
Wireless
Transmission
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Consequently, microwave transmission is usually limited to communications occurring
within the limits of a particular city or community. For microwave transmissions to be able to
occur over larger distances, data messages must be relayed from one location to another
us
ves are very short, measuring about 750 nm in length; longer infrared waves
which are closest to microwaves are about 1mm in length, about the size of a microbial cell.
While shorter infrared waves can be seen as colors, as infrared waves get longer they give
off radiation in the form of heat. Infrared waves are considered to be thermal. For instance,
the sun gives off infrared wave lengths, your toaster gives off infrared wave lengths and even
living organisms such as humans, dogs and cats give off thermal heat in the form of infrared
wave lengths.
To combine multiple signals (analog or digital) for transmission over a single line or
media.Acommon type of multiplexing combines several low-speed signals for transmission
over a single high-speed connection.
• Time division multiplexing (TDM)
• Frequency division multiplexing (FDM)
• Code Division Multiplexing (CDM)
Time Division Multiplexing works by the multiplexor collecting and storing the
incoming transmissions from all of the slow lines connected to it and allocating a time slice
on the fast link to each in turn. The messages are sent down the high speed link one after the
other. Each transmission when received can be separated according to the time slice
allocated.
ing antennas placed at high altitudes usually twenty to thirty miles apart.
Infrared light is a form of electromagnetic waves. Infrared can be visible light or can be
in the form of microwaves. In its visible form, infrared can be seen as red or violet. Visible
infrared wa
Infrared
Q. What is Multiplexing ?
Ans.
Types of Multiplexing
Time Division Multiplexing
102

Theoretically, the available speed of the fast link should at least be equal to the total of
all of the slow speeds coming into the multiplexor so that its maximum capacity is not
exceeded.
• SynchronousTDM
• AsynchronousTDM
Synchronous TDM works by the muliplexor giving exactly the same amount of time to
each device connected to it. This time slice is allocated even if a device has nothing to
transmit. This is wasteful in that there will be many times when allocated time slots are not
being used. Therefore, the use of Synchronous TDM does not guarantee maximum line
usage and efficiency.
SynchronousTDM is used inT1 and E1 connections.
AsynchronousTDM
Asynchronous TDM is a more flexible method of TDM. With Asynchronous TDM the
length of time allocated is not fixed for each device but time is given to devices that have data
to transmit.
This version of TDM works by tagging each frame with an identification number to note
which device it belongs to. This may require more processing by the multiplexor and take
longer, however, the time saved by efficient and effective bandwidth utilization makes it
worthwhile.
AsynchronousTDM allows more devices than there is physical bandwidth for.
This type ofTDM is used inAsynchronousTransfer Mode (ATM) networks
Two ways of implementing TDM are :
Synchronous TDM
103

Frequency Division Multiplexing
Use of FDM
Code Division Multiplexing (CDM)
Frequency Division Multiplexing (FDM) works by transmitting all of the signals along
the same high speed link simultaneously with each signal set at a different frequency. For
FDM to work properly frequency overlap must be avoided. Therefore, the link must have
sufficient bandwidth to be able to carry the wide range of frequencies required. The
demultiplexor at the receiving end works by dividing the signals by tuning into the
appropriate frequency.
FDM operates in a similar way to radio broadcasting where a number of different
stations will broadcast simultaneously but on different frequencies. Listeners can then
"tune" their radio so that it captures the frequency or station they want.
FDM gives a total bandwidth greater than the combined bandwidth of the signals to be
transmitted. In order to prevent signal overlap there are strips of frequency that separate the
signals.These are called guard bands.
Acommon example of FDM use is Cable television (CATV). This can be achieved with
coaxial cable or fibre-optic cable.
A multiplexor is used to combine many channels to maximize the use of the available
bandwidth and a demultiplexor built into the television or set top box will separate the
channel that the viewer wants to watch
Old but now new method . Also known as code division multiple access (CDMA) An
advanced technique that allows multiple devices to transmit on the same frequencies at the
same time using different codes. Used for mobile communications. Each mobile device is
assigned a unique 64-bit code (chip spreading code).To send a binary 1, mobile device
transmits the unique code.To send a binary 0, mobile device transmits the inverse of
code.Receiver gets summed signal, multiplies it by receiver code, adds up the resulting
values.Interprets as a binary 1 if sum is near +64.Interprets as a binary 0 if sum is near -64
104

Q. Draw the Diagram and explain in detail each of the seven layers of OSI
Reference Model.
Ans.
e 0.1 :
In the late 1970s, the Open System Interconnection (OSI) reference model was
created by the International Standards Organization (ISO). The International Standards
Organization (ISO) developed the Open Systems Interconnection (OSI) Reference Model to
define functional communications standards. This reference model is widely used by
equipment manufacturers to assure their products will interoperate with products from other
vendors. The OSI model describes the functions that are performed in data
communications. It divides the process into seven groups, called layers. A layer is a
collection of conceptually similar functions that provide services to the layer above it and
receives service from the layer below it. For example, a layer that provides error-free
communications across a network provides the path needed by applications above it, while
it
OSI Reference Model
Layer Title
7 Application
Higher Layers 6 Presentation
5 Session
4 Transport
3 Network
Lower Layers 2 Data Link
1 Physical
calls the next lower layer to send and receive packets that make up the contents of the
path.
Description of OSI layers
Tabl
UNIT – II
105

PHYSICALLAYER
• Data Encoding :
• Transmission Technique :
• Physical Medium Transmission :
DATALINK LAYER
The physical layer, the lowest layer of the OSI model, is concerned with the
transmission and reception of the unstructured raw bit stream over a physical medium. It
describes the electrical/optical, mechanical, and functional interfaces to the physical
medium, and carries the signals for all of the higher layers. It provides:
modifies the simple digital signal pattern (1s and 0s) used by the PC
to better accommodate the characteristics of the physical medium, and to aid in bit and
frame synchronization. It determines:
o What signal state represents a binary 1
o How the receiving station knows when a "bit-time" starts
o How the receiving station delimits a frame
• Physical MediumAttachment, accommodating various possibilities in the medium:
o Will an external transceiver (MAU) be used to connect to the medium?
o How many pins do the connectors have and what is each pin used for?
determines whether the encoded bits will be transmitted
by baseband (digital) or broadband (analog) signaling.
transmits bits as electrical or optical signals
appropriate for the physical medium, and determines:
o What physical medium options can be used
o How many volts/db should be used to represent a given signal state, using a
given physical medium
The data link layer provides error-free transfer of data frames from one node to
another over the physical layer, allowing layers above it to assume virtually error-free
transmission over the link.To do this, the data link layer provides:
establishes and terminates the logical
link between two nodes.
tells the transmitting node to "back-off" when no frame
buffers are available.
transmits/receives frames sequentially.
provides/expects frame acknowledgments.
Detects and recovers from errors that occur in the physical layer by
retransmitting non-acknowledged frames and handling duplicate frame receipt.
creates and recognizes frame boundaries.
checks received frames for integrity.
determines when the node "has the right" to use
the physical medium.
• Link Establishment and Termination :
• Frame Traffic Control :
• Frame Sequencing :
• Frame Acknowledgment :
• Frame Delimiting :
• Frame Error Checking :
• Media Access Management :
106

NETWORK LAYER
• Routing
• Subnet Traffic Control :
• Frame Fragmentation :
• Logical-Physical Sddress Mapping :
TRANSPORT LAYER
The Network layer defines routing services that allow multiple data links to be
combined into an internetwork. The Network layer defines network-addressing schemes
that logically identify network devices. The logical network addresses are different from the
physical addresses defined at the MAC layer, and are used by routing protocols running at
this level to transfer packets from one network to another. The most common network
addressing protocols are IP, IPX, and AppleTalk. Typical routing protocols that run at this
level are RIP, OSPF, IGRP, and NLSP.It provides:
: routes frames among networks.
routers (network layer intermediate systems) can
instruct a sending station to "throttle back" its frame transmission when the
router's buffer fills up.
if it determines that a downstream router's maximum
transmission unit (MTU) size is less than the frame size, a router can fragment a
frame for transmission and re-assembly at the destination station.
translates logical addresses, or names,
into physical addresses.
The Transport layer implements reliable internetwork data transport services that are
transparent to upper-layer protocols. The services include flow control, multiplexing, and
error checking and recovery. If virtual circuits are needed for the communication to be
accomplished, they are built and maintained at this layer. Flow control is responsible for
making sure that a sending station does not transmit data faster than the receiving station
can process it. Multiplexing allows multiple applications to share a common network
interface. Error checking is implemented to discover errors on transmission and to provide a
recovery mechanism when errors are found. Typical error recovery includes retransmission
of the data.
Accepts a message from the (session) layer above it,
splits the message into smaller units (if not already small enough), and passes
the smaller units down to the network layer.The transport layer at the destination
station reassembles the message.
Provides reliable end-to-end message delivery
with acknowledgments.
Tells the transmitting station to "back-off" when no
message buffers are available.
Multiplexes several message streams, or sessions
onto one logical link and keeps track of which messages belong to which
sessions (see session layer).
The Transport Layer Provides:
• Message Segmentation :
• Message Acknowledgment :
• Message Traffic Control :
• Session Multiplexing :
107

SESSION LAYER
Session Establishment, Maintenance and Termination :
Session Support :
PRESENTATION LAYER
The Presentation Layer Provides :
• Character Code Translation :
• Data Conversion :
• Data Compression :
• Data Encryption :
APPLICATION LAYER
The session layer is responsible for creating, managing and termination sessions that are
used by entities at the presentation layer. The session layer is responsible for coordinating
the service requests and responses generated and received by a station when it is
communication with other entities on the internetwork. It provides:
• Allows two
application processes on different machines to establish, use and terminate a
connection, called a session.
• Performs the functions that allow these processes to
communicate over the network, performing security, name recognition, logging,
and so on.
The presentation layer formats the data to be presented to the application layer. It can
be viewed as the translator for the network. This layer may translate data from a format used
by the application layer into a common format at the sending station, then translate the
common format to a format known to the application layer at the receiving station.
For example,ASCII to EBCDIC.
Bit order, CR-CR/LF, integer-floating point, and so on.
Reduces the number of bits that need to be transmitted on
the network.
Encrypt data for security purposes. For example, password
encryption.
The Application layer provides the interface to the user. Any user application that
requires network communication accesses the communication resources through this layer.
This layer also is responsible for finding and determining the availability of communication
partners. Typical applications in the TCP/IP protocols are Simple Mail Transfer Protocol
(SMTP), Telnet, and File Transfer Protocol (FTP).This layer contains a variety of commonly
needed functions:
• Resource sharing and device redirection
• Remote file access
• Remote printer access
• Inter-process communication
• Network management
• Directory services
108

• Electronic messaging (such as mail)
• Network virtual terminals
7 6 5 4 3 2 1
The TCP/IP model is created in the 1970s by DARPA, an agency of the United States
Department of Defense. It evolved from ARPANET, which was the world's first wide area
network and a predecessor of the Internet. The TCP/IP Model is sometimes called the
Internet Reference Model or the DoD Model.
The TCP/IP model, or Internet Protocol Suite, describes a set of general design
guidelines and implementations of specific networking protocols to enable computers to
communicate over a network. TCP/IP provides end-to-end connectivity specifying how data
should be formatted, addressed, transmitted, routed and received at the destination.
Protocols exist for a variety of different types of communication services between
computers.
The TCP/IP model uses four layers that logically span the equivalent of the top six
layers of the OSI reference model; this is shown in Figure (The physical layer is not covered
by the TCP/IP model because the data link layer is considered the point at which the
interface occurs between the TCP/IP stack and the underlying networking hardware.) The
following are theTCP/IPmodel layers, starting from the bottom.
ALL PEOPLE SEEM TO NEED DATA PROCESSING
OSI Model TCP/IPModel
Figur
Q. Explain in detail TCP/IPreference model with help of diagram?
Ans.
7 Application
6 Presentation Application
5 Session
4 Transport (Host-to Host) Transport
3 Network Internet
2 Data Link Network Interface
1 Physical (Hardware)
e : OSI Reference Model and TCP/IP Model Layers
109

NetworkAccess layer (Network Interface layer)
The Internet layer
The Internet layer contains 5 protocols:
The network access layer is the first layer of the TCP/IP stack , it offers the ability to
access whichever physical network, i.e. the resources to be implemented so as to transmit
data via a network.
So, the network access layer contains all specifications relating to the transmission of
data over a physical network, when it is a local area network (Token ring, Ethernet, FDDI),
connected by telephone line or any other type of link to a network. It deals with the following
concepts:
• Routing data over the connection
• Coordination of the data transmission (synchronisation)
• Data format
• Signal conversion (analogue/digital)
• Error detection on arrival
Fortunately all these specifications are transparent in the eyes of the user, because all
these tasks are in fact carried out by the operating system, while the hardware drivers allow
connection to the network (e.g. network card driver).
The Internet layer is the "most important" layer because it is this which defines the
datagrams and manages the IP addressing notions. It enables the routing of datagrams
(data packets) to remote machines along with the management of their division and
assembly upon receipt.
• The IPProtocol
• TheARPProtocol
• The ICMPProtocol
• The RARPProtocol
• The IGMPPSrotocol
Th
he protocols for the preceding layers make it possible to send information from one
machine to another. The transport layer enables applications running on remote machines
to communicate. The problem is identifying these applications. In fact, depending on the
machine and its operating system, the application may be a program, task, process...
Furthermore, the name of the application may vary from system to system, that is why
a numbering system has been put in place so as to be able to associate an application type
with a data type, these identifiers are called ports.
e first three protocols are the most important protocols for this layer...
T
The Transport layer
110

The transport layer contains two protocols enabling two applications to exchange data
independently of the type of network taken (i.e. independently of the lower layers), these are
the following two protocols:
• TCP, a connection orientated protocol which provides error detection
•
Service, interface and protocol Protocols in the OSI model are
Service, are not clearly defined. For better hidden and can be
interface and example, the only real services replaced relatively easily as the
protocol offered by the Internet layer are technology changes, which is
- Send IPPacket one of the main objective of
- Receive IPPacket layered protocols.
Because models were invented In this case, the protocols have
before protocols, functionalities been invented before models,
Functionalities put in each layer are not so the functionalities are
very optimized. described.
Seven layers, Network (Internet),
Numbers of Transport andApplication layers Only four layers.
layers being similar toTCP/IP
UDP, a connectionless orientated protocol where error detection is outdated
The application layer is located at the top of the TCP/IP protocol layers. This one
contains the network applications which make it possible to communicate using the lower
layers.
The software in this layer therefore communicates using one of the two protocols of
the layer below (the transport layer), i.e.TCPor UDP.
There are different types of applications for this layer, but the majority are network
services, or applications supplied to the user to provide the interface with the operating
system.They can be classed according to the services that they provide:
• File and print management services (transfer)
• Network connection services
• Remote connection services
• Various Internet utilities
TheApplication layer
Difference between TCP/IP and OSI Model
OSI Model Reference TCP/IPModel Reference
perfectly
111

Connection- Both connectionless and Only one mode in the network
less/ connection-oriented layer (connectionless) but both
Connection- communication are supported modes in the transport layer are
oriented in the network layer, but only supported, giving the users
communi- connection-oriented a choice.
cation communication in the
transport layer.
In computer networking, is the process of managing the rate of data
transmission between two nodes to prevent a fast sender from over running a slow receiver.
Flow control mechanisms can be classified by whether or not the receiving node sends
feedback to the sending node.
Flow control is important because it is possible for a sending computer to transmit
information at a faster rate than the destination computer can receive and process them.
network congestion back to the transmitter. This information is then used
by the transmitter in various ways to adapt its activity to existing network conditions.
Transmit Flow Control described above is a form of Closed-loop flow control.
Q. (a) What is Flow Control?
Ans. flow control
This can happen if the receiving computers have a heavy traffic load in comparison to the
sending computer, or if the receiving computer has less processing power than the sending
computer.
Transmit flow control may occur between data on and off terminal equipment (DTE)
and a switching center, via data circuit-terminating equipment (DCE), or between two DTEs.
The transmission rate may be controlled because of network or DTE requirements. Transmit
flow control can occur independently in the two directions of data transfer, thus permitting
the transfer rates in one direction to be different from the transfer rates in the other direction.
Transmit flow control can be either stop-and-go or use a sliding window.
The open-loop flow control mechanism is characterized by having no feedback
between the receiver and the transmitter. This simple means of control is widely used. The
allocation of resources must be a "prior reservation" or "hop-to-hop" type. The Open Loop
flow control has inherent problems with maximizing the utilization of network resources.
Resource allocation is made at connection setup using a CAC (Connection Admission
Control) and this allocation is made using information that is already "old news" during the
lifetime of the connection.
The Closed Loop flow control mechanism is characterized by the ability of the network
to report pending
Transmit Flow Control
Open-Loop Flow Control
Closed-Loop Flow Control
112

The transport layer is also responsible for data flow control and it uses two common
met
In internetworking, the process of moving a packet of data from source to destination.
Routing is usually performed by a dedicated device called a router. Routing is a key feature
of the Internet because it enables messages to pass from one computer to another and
eventually reach the target machine. Each intermediary computer performs routing by
passing along the message to the next computer. Part of this process involves analyzing a
routing table to determine the best path.
In packet switching networks, routing directs packet forwarding, the transit of logically
addressed packets from their source toward their ultimate destination through intermediate
nodes; typically hardware devices called routers, bridges, gateways, firewalls, or switches.
General-purpose computers with multiple network cards can also forward packets and
perform routing, though they are not specialized hardware and may suffer from limited
performance. The routing process usually directs forwarding on the basis of routing tables
which maintain a record of the routes to various network destinations. Thus, constructing
routing tables, which are held in the routers' memory, is very important for efficient routing.
Most routing algorithms use only one network path at a time, but multipath routing
techniques enable the use of multiple alternative paths.
Routing is often confused with bridging, which performs a similar function. The
principal difference between the two is that bridging occurs at a lower level and is therefore
more of a hardware function whereas routing occurs at a higher level where the software
component is more important. And because routing occurs at a higher level, it can perform
more complex analysis to determine the optimal path for the packet.
hods of flow control :-
Buffering When buffering flow control is used, data is temporarily stored and waits for
the destination device to become available. Buffering can cause a problem if the sending
device transmits data much faster than the receiving device is able to manage it.
Windowing In a windowing environment, data is sent in groups of segments that
require only one acknowledgment. The size of the window (that is, how many segments fit
into one acknowledgment) is defined at the time the session between the two devices is
established.As you can imagine, the need to have only one acknowledgment for every, say,
five segments can greatly reduce overhead.
Buffering
Windowing
Q. (b) What is Routing?
Ans.
Router Uses Two Types of Routing Static and Dynamic :
113

STATIC ROUTING
DYNAMIC ROUTING
RoutingAlgorithms :-
Distance vector algorithms
Static routing is not really a protocol, simply the process of manually entering routes
into the routing table via a configuration file that is loaded when the routing device starts up.
As an alternative, these routes can be entered by a network administrator who configures
the routes. Since these routes don't change after they are configured (unless a human
changes them) they are called 'static' routes.
Static routing is the simplest form of routing, but it is a manual process and does not
work well when the routing information has to be changed frequently or needs to be
configured on a large number of routing devices (routers). Static routing also does not
handle outages or down connections well because any route that is configured manually
must be reconfigured manually to fix or repair any lost connectivity.
Dynamic routing protocols are software applications that dynamically discover
network destinations and how to get to them.
A router will 'learn' routes to all directly connected networks first. It will then learn
routes from other routers that run the same routing protocol. The router will then sort through
it's list of routes and select one or more 'best' routes for each network destination it knows or
has learned.
Dynamic protocols will then distribute this 'best route' information to other routers
running the same routing protocol, thereby extending the information on what networks exist
and can be reached. This gives dynamic routing protocols the ability to adapt to logical
network topology changes, equipment failures or network outages 'on the fly'.
Distance vector algorithms use the Bellman-Ford algorithm. This approach assigns a
number, the cost, to each of the links between each node in the network. Nodes will send
information from pointAto point B via the path that results in the lowest total cost (i.e. the sum
of the costs of the links between the nodes used).
The algorithm operates in a very simple manner. When a node first starts, it only knows
of its immediate neighbours, and the direct cost involved in reaching them. Each node, on a
regular basis, sends to each neighbour its own current idea of the total cost to get to all the
destinations it knows of. The neighbouring node(s) examine this information, and compare it
to what they already 'know'; anything which represents an improvement on what they
already have, they insert in their own routing table(s). Over time, all the nodes in the network
will discover the best next hop for all destinations, and the best total cost. When one of the
nodes involved goes down, those nodes which used it as their next hop for certain
destinations discard those entries, and create new routing-table information. They then
pass this information to all adjacent nodes, which then repeat the process.
114

Link-StateAlgorithms
Path Vector Protocol
Q. What is Cryptography and what are the difference techniques of cryptography?
Ans.
Encryption and Decryption
When applying link-state algorithms, each node uses as its fundamental data a map of
the network in the form of a graph. To produce this, each node floods the entire network with
information about what other nodes it can connect to, and each node then independently
assembles this information into a map. Using this map, each router then independently
determines the least-cost path from itself to every other node using a standard shortest
paths algorithm such as Dijkstra's algorithm.
Distance vector and link state routing are both intra-domain routing protocols. They
are used inside an autonomous system, but not between autonomous systems. Both of
these routing protocols become intractable in large networks and cannot be used in Inter-
domain routing. Distance vector routing is subject to instability if there are more than a few
hops in the domain. Link state routing needs huge amount of resources to calculate routing
tables. It also creates heavy traffic because of flooding.
Path vector routing is used for inter-domain routing. It is similar to Distance vector
routing. In path vector routing we assume there is one node (there can be many) in each
autonomous system which acts on behalf of the entire autonomous system. This node is
called the speaker node. The speaker node creates a routing table and advertises it to
neighboring speaker nodes in neighboring autonomous systems. The idea is the same as
Distance vector routing except that only speaker nodes in each autonomous system can
communicate with each other.
Cryptography is the science of using mathematics to encrypt and decrypt data.
Cryptography enables you to store sensitive information or transmit it across insecure
networks (like the Internet) so that it cannot be read by anyone except the intended recipient.
While cryptography is the science of securing data, cryptanalysis is the science of
analyzing and breaking secure communication. Classical cryptanalysis involves an
interesting combination of analytical reasoning, application of mathematical tools, pattern
finding, patience, determination, and luck. Cryptanalysts are also called attackers.
Data that can be read and understood without any special measures is called plaintext
or cleartext. The method of disguising plaintext in such a way as to hide its substance is
called encryption. Encrypting plaintext results in unreadable gibberish called ciphertext. You
use encryption to ensure that information is hidden from anyone for whom it is not intended,
even those who can see the encrypted data. The process of reverting ciphertext to its
original plaintext is called decryption. Figure 1-1 illustrates this process.
115

Figure 1-1. Encryption and Decryption
How does cryptography work?
Conventional cryptography ( Secret - key or Symmetric - Key )
Figure 1-2. Conventional Encryption
Acryptographic algorithm, or cipher, is a mathematical function used in the encryption
and decryption process.Acryptographic algorithm works in combination with a key - a word,
number, or phrase - to encrypt the plaintext. The same plaintext encrypts to different
ciphertext with different keys. The security of encrypted data is entirely dependent on two
things: the strength of the cryptographic algorithm and the secrecy of the key.
Acryptographic algorithm, plus all possible keys and all the protocols that make it work
comprise a cryptosystem. PGPis a cryptosystem.
In conventional cryptography, also called secret-key or symmetric-key encryption,
one key is used both for encryption and decryption. The Data Encryption Standard (DES) is
an example of a conventional cryptosystem that is widely employed by the Federal
Government. Figure 1-2 is an illustration of the conventional encryption process.
An extremely simple example of conventional cryptography is a substitution cipher. A
substitution cipher substitutes one piece of information for another. This is most frequently
done by offsetting letters of the alphabet. Two examples are Captain Midnight's Secret
Caesar's Cipher
116
Plaintext
Encryption
Ciphertext
Decryption
Plaintext
Plaintext
Encryption
Ciphertext
Decryption
Plaintext

Decoder Ring, which you may have owned when you were a kid, and Julius Caesar's cipher.
In both cases, the algorithm is to offset the alphabet and the key is the number of characters
to offset it.
For example, if we encode the word "SECRET" using Caesar's key value of 3, we
offset the alphabet so that the 3rd letter down (D) begins the alphabet.
So starting with
ABCDEFGHIJKLMNOPQRSTUVWXYZ
and sliding everything up by 3, you get
DEFGHIJKLMNOPQRSTUVWXYZABC
where D=A, E=B, F=C, and so on.
Using this scheme, the plaintext, "SECRET" encrypts as "VHFUHW." To allow
someone else to read the ciphertext, you tell them that the key is 3.
Obviously, this is exceedingly weak cryptography by today's standards, but hey, it
worked for Caesar, and it illustrates how conventional cryptography works.
The problems of key distribution are solved by public key cryptography, the concept of
which was introduced by Whitfield Diffie and Martin Hellman in 1975.
Public key cryptography is an asymmetric scheme that uses a pair of keys for
encryption: a public key, which encrypts data, and a corresponding private, or secret key for
decryption. You publish your public key to the world while keeping your private key secret.
Anyone with a copy of your public key can then encrypt information that only you can read.
Even people you have never met.
It is computationally infeasible to deduce the private key from the public key. Anyone
who has a public key can encrypt information but cannot decrypt it. Only the person who has
the corresponding private key can decrypt the information.
Public key Cryptography
Figure 1-3. Public Key Encryption
117
Public Key Private Key
Plaintext
Encryption
Ciphertext
Decryption
Plaintext

The primary benefit of public key cryptography is that it allows people who have no
preexisting security arrangement to exchange messages securely. The need for sender and
receiver to share secret keys via some secure channel is eliminated; all communications
involve only public keys, and no private key is ever transmitted or shared.
Because conventional cryptography was once the only available means for relaying
secret information, the expense of secure channels and key distribution relegated its use
only to those who could afford it, such as governments and large banks (or small children
with secret decoder rings). Public key encryption is the technological revolution that
p
P combines some of the best features of both conventional and public key
cryptography. PGP is a hybrid cryptosystem. When a user encrypts plaintext with PGP, PGP
first compresses the plaintext. Data compression saves modem transmission time and disk
space and, more importantly, strengthens cryptographic security. Most cryptanalysis
techniques exploit patterns found in the plaintext to crack the cipher. Compression reduces
these patterns in the plaintext, thereby greatly enhancing resistance to cryptanalysis. (Files
that are too short to compress or which don't compress well aren't compressed.)
PGP then creates a session key, which is a one-time-only secret key. This key is a
random number generated from the random movements of your mouse and the keystrokes
you type. This session key works with a very secure, fast conventional encryption algorithm
to encrypt the plaintext; the result is ciphertext. Once the data is encrypted, the session key
is then encrypted to the recipient's public key. This public key-encrypted session key is
transmitted along with the ciphertext to the recipient.
rovides strong cryptography to the adult masses.
PG
How PGPworks
Figure 1-4. How PGP Encryption Works
118
Plaintext in Encrypted
with Session Key
Session Key is Encrypted
with Public Key
Ciphertext + Encrypted
Session Key

Decryption works in the reverse. The recipient's copy of PGP uses his or her private
key to recover the temporary session key, which PGP then uses to decrypt the
conventionally-encrypted ciphertext.
The combination of the two encryption methods combines the convenience of public
key encryption with the speed of conventional encryption. Conventional encryption is about
1, 000 times faster than public key encryption. Public key encryption in turn provides a
solution to key distribution and data transmission issues. Used together, performance and
key dis
ove discussion about shared secret cryptography and public key cryptography
is overly simplified - but it does give you the ideas. In modern cryptography much more
complex systems are used.
RC4 - symmetric algorithm
RC2 - symmetric algorithm
DES - symmetric algorithm
3DES - symmetric algorithm
AES - symmetric algorithm
MD5 - hash algorithm
Figure 1-5. How PGP Decryption Works
The following lists some of the algorithms commonly used in industry and
government:
tribution are improved without any sacrifice in security.
The ab
Modern CryptographicAlgorithms
119
Encrypted Message
Encrypted
Session Key
Recipient's Private Key used
to Decrypt Session Key
Session Key used to
Decrypt Ciphertext
Original
Plaintext
Ciphertext

SHA1 - hash algorithm
RSA- public key algorithm
(the information cannot be understood by anyone for whom it was
unintended)
(the information cannot be altered in storage or transit between sender and
intended receiver without the alteration being detected)
(the creator/sender of the information cannot deny at a later stage
his or her intentions in the creation or transmission of the information)
(the sender and receiver can confirm each other?s identity and the
origin/destination of the information)
Modern cryptography concerns itself with the following four objectives:
1) Confidentiality
2) Integrity
3) Non-repudiation
4) Authentication
120

Q. (a) What is Internet ? What are the hardware and software requirement for
Internet. Discuss.
Ans. Internet
The is a global system of interconnected computer networks.Acomputer that
connects to the Internet can access information from a vast number of servers and other
computers. An Internet connection also allows the computer to send information onto the
network; that information may be saved and ultimately accessed by a variety of servers and
other computers. Much of the widely accessible information on the Internet consists of the
interlinked hypertext documents and other resources of the World Wide Web (WWW). Web
users typically send and receive information using a web browser; other software for
interacting with computer networks includes specialized programs for electronic mail, online
chat, file transfer and file sharing.
Information is moved around the Internet by packet switching using the standardized
Internet Protocol Suite (TCP/IP). It is a "network of networks" that consists of millions of
private and public, academic, business, and government networks of local to global scope
that are linked by copper wires, fiber-optic cables, wireless connections, and other
technologies.
The terms Internet and World Wide Web are often used in every-day speech without
much distinction. However, the Internet and the World Wide Web are not one and the same.
The Internet is a global data communications system. It is a hardware and software
infrastructure that provides connectivity between computers. In contrast, the Web is one of
the services communicated via the Internet. It is a collection of interconnected documents
and ot
6MB of RAM (more will improve scalability).
• Adequate hard drive space and speed. 100MB of free space minimum. (20GB to
80GB are very affordable). Raided Ultra-SCSI drives are more scalable for large
organizations.We recommend server baords like Intel S845 series with dual
LAN.
her resources, linked by hyperlinks and URLs
• Celeron, Pentium II/III, Pentium 4 PC, 300MHz or higher recommended.
• 25
Hardware Requirements :
UNIT – III
121

• SVGAMonitor (800x600 minimum, small fonts).
• Additional PCs on the LAN
can be e-mailed to customers, colleagues and friends as an
attachment. It can be uploaded to a website or FTP server for easy download by others. It
can be put into a "shared location" or onto a file server for instant use by colleagues. The load
of bulk downloads to many users can be eased by the use of "mirror" servers or peer-to-peer
networks.
• External and Internal Firewalls (recommended)
Aweb client program ("web browser")
Many similar products are available. Netscape Navigator is most popular, probably as
a result of its superior speed, support for special HTML tags, and flexible licensing.
Navigator needs some utility programs files to play audio and video clips. Most of these
programs are distributed for free on the internet. Microsoft's Internet Explorer is a very
popular client program.
This software is used for connecting the Internet.
Windows 95/ 98/ Me / Xp includes built-in support for TCP/IP. You need to get the correct
TCP/IPsetting from your system administrator or ISP.
If you are dialing up with a modem, your ISP will
provide you with support of SLIP and PPP. SLIP/PPP allows the use of TCP/IP across serial
connections like telephone lines.
There are following capabilities of Internet:-
The World Wide Web is a huge set of interlinked documents, images and other
resources, linked by hyperlinks and URLs. These hyperlinks and URLs allow the web
servers and other machines that store originals, and cached copies of, these resources to
deliver them as required using HTTP (Hypertext Transfer Protocol). HTTP is only one of the
communication protocols used on the Internet.
The Internet allows computer users to connect to other computers and information
stores easily, wherever they may be across the world. They may do this with or without the
use of security, authentication and encryption technologies, depending on the
requirements.
A computer file
Software Requirements :
Software for TCP/IP Networking :
Software for Dial-up Access to TCP/IP :
Q. (b)What are the capabilities of Internet ? Discuss
Ans.
The World Wide Web
RemoteAccess
File Sharing
122

E-mail,
USENET newsgroups,
File Transfer Protocol (FTP),
Gopher,
TELNET,
Internet Relay Chat (IRC),
CU-SeeMe,
Q. What is meant by Internet Browser ? Name an important browser and discuss
its importance.
Ans. web browser
The Most Popular Web Browsers
Microsoft's Internet Explorer :
• Mozilla's Firefox :
• Mac Safari :
for exchange of electronic mail messages.
for posting and responding to public "bulletin board" messages.
a system for storing and retrieving data files on large
computer systems.
a method of searching for various text-based Internet resources (largely obsolete).
a way of connecting directly to computer systems on the Internet.
a system for sending public and private messages to other users
in "real time"-that is, your message appears on the recipient's screen as soon as you type it.
a videoconferencing system that allows users to send and receive sound and
pictures simultaneously over the Internet.
A is a software application which enables a user to display and interact
with text, images, videos, music, games and other information typically located on a web
page at a website on the World Wide Web or a local area network. Text and images on a web
page can contain hyperlinks to other web pages at the same or different website. Web
browsers allow a user to quickly and easily access information provided on many web pages
at many web sites by traversing these links.
Although browsers are typically used to access the World Wide Web, they can also be used
to access information provided by web servers in private networks or content in file systems.
Browsers allow you to search for and view various kinds of information on the Web, such as
web sites, video, audio, etc.
Here are just a few of Web browsers available to you for a free download:
• Most Internet users are using Internet Explorer
because it's easy to use and most Web sites are written with Internet Explorer in
mind, meaning that they are compatible.
• Opera : Opera is another popular browser that's easy to use; however, it can
have some compatibility issues with various websites.
Firefox is rapidly gaining ground right behind Internet
Explorer because of its tabbed browsing, superior security features, and fast
load.
Specifically for Mac users, Safari is an excellent choice for a Web
browser, with fast load and good compatibility with most websites out there.
123

Breakdown of a Web Browser
• Status Bar :
• Address Bar :
• Title Bar :
• Toolbar Icons :
• Display Window :
• Scroll Bars :
Which Web Browser is the Best?
Importance of Internet Explorer 8
1. Activities :
2. Webslices :
We all know what a Web browser looks like, but it's good to have a complete
breakdown of the various parts of most Web browsers just for reference's sake. The parts of
a browser include:
This is the box at the bottom of your browser window. The status
bar displays all sorts of information, depending on what you're doing at the time,
but mostly it's for showing load speed and the URL of whatever address your
mouse is hovering over.
This is the box at the top of your browser window that displays the
entire URL, or Web site address.
The title bar is at the very top of your browser window; in both Firefox
and Internet Explorer it is the blue bar there at the top. You'll see the title of the
Web page there; for example, you should see "What Is A Web Browser?" at the
top of your browser window right now.
The toolbar and its icons are at the top of your browser window
right underneath the Title Bar. This is where you'll see the Back button, the Home
button, the Refresh button, etc.
The Display Window is just a fancy term for your browser
work space; it's the frame through which you see this website right now.
If you've ever been to a website that you had to "scroll down" to
read something, then you've used the scroll bars. They're just
navigational/directional aids.
How do you know which browser will work best for you? Well, they are all free to
download, and so it can be a good idea to test them all if you are in doubt - and especially if
you are a Web designer who needs to know what designs will work in what browser. As
already stated, it does not cost a thing to download these browsers, so go ahead and test
them out. The browser you choose to use can make all the difference in your search
experience.
This feature will allow you to do all sort of additional stuff while you are
surfing the web. You'll be able to share the link on digg, facebook, will be able to locate
an address through maps and whole lot of stuff. Internet explorer 8 becomes more
social with this features!
This has to be the best new feature of Internet Explorer 8. It will allow you
to create slices with in pages. This feature is for developers and they can specify the
regions in their webpages which can act as the webslices. It'll act as the RSS feed for
that particular area of the webpage and as soon as that area changes the user will be
notified about the change.
124

3. Automatic Crash Recovery :
4. Emulate as IE 7 :
5
6 Search Suggestions :
7 InPrivate Browsing :
Q. Discuss the following Terms
(a)
(b)
(c)
(d)
Ans. (a) HTTP Hypertext Transfer Protocol
What are "Resources"?
R URL
As the name suggests this feature will try to recover all
the work which you were doing after the browser crashes due to uncertain reasons. I'm
sure this feature will become most famous, however most of the web apps these days
support automatic saving of text and they support sessions too. Still a welcome
feature.
This feature allows you to act IE 8 as IE 7. So in case there is a
problem in accessing a particular website, you can always emulate IE 8 as IE 7. I
guess that this feature will go away after the beta period ends.
Accelerators let you map directions, translate words, email your friends, and more in
just a few mouse clicks
Search smarter with detailed suggestions from your favorite
search providers and browsing history. See visual previews and get suggested
content topics while you type in the enhanced Instant Search Box
Browse the web without saving your history with Internet
Explorer 8's InPrivate Browsing. Now you can shop for that special gift with confidence
knowing your family won't accidentally find out or use a shared computer without
leaving a trace.
HTTP
ISP
Search Engine
URL
- HTTP stands for . It's the network protocol
used to deliver virtually all files and other data (collectively called resources) on the World
Wide Web, whether they're HTML files, image files, query results, or anything else. Usually,
HTTPtakes place throughTCP/IPsockets (and this tutorial ignores other possibilities).
A browser is an HTTP client because it sends requests to an HTTP server (Web
server), which then sends responses back to the client. The standard (and default) port for
HTTPservers to listen on is 80, though they can use any port.
HTTP is used to transmit resources, not just files. A resource is some chunk of
information that can be identified by a URL (it's the in ). The most common kind of
resource is a file, but a resource may also be a dynamically-generated query result, the
output of a CGI script, a document that is available in several languages, or something else.
While learning HTTP, it may help to think of a resource as similar to a file, but more
general. As a practical matter, almost all HTTP resources are currently either files or server-
side script output.
125

Structure of HTTPTransactions
(b) ISP Internet Service Provider,
Like most network protocols, HTTP uses the client-server model: An HTTP client
opens a connection and sends a request message to an HTTP server; the server then
returns a response message, usually containing the resource that was requested. After
delivering the response, the server closes the connection (making HTTP a stateless
protocol, i.e. not maintaining any connection information between transactions).
The format of the request and response messages are similar, and English-oriented.
Both kinds of messages consist of:
• An initial line,
• Zero or more header lines,
• Ablank line (i.e. a CRLF by itself), and
• An optional message body (e.g. a file, or query data, or query output).
Put another way, the format of an HTTPmessage is:
<initial line, different for request vs. response>
Header1: value1
Header2: value2
Header3: value3
<optional message body goes here, like file contents or query data;
-An ISP, or is a company that offers internet access to
individuals and businesses for a monthly or yearly fees. In addition to internet connection,
ISPs may also provide related services like web site hosting and development, email
hosting, domain name registration etc.
Different ISPs, and sometimes even the same one, offer different types of internet
connections - dialup, cable and DSL broadband. Hardware such as dialup modem or a
wireless modem and router are usually provided by the company. When you register with an
ISP for its services, an account is created and you are provided with the login details -
username and password. You connect to the internet via your account and this way the
company keeps a watch on your online activities. All providers have strict terms of use and
ensure you read these carefully before you sign up though, I believe, the casual web surfer
would probably never be in breach of the contract. Any ISP, worth its salt, will also give you
an email address, typically, based on your username - you can choose to use this email
account or ignore it completely (not recommended).
ISPs can be both regional, confined to a geographic area, or national, servicing the
entire country and they are connected with each other through Network Access Points
(NAPs). ISPs are also referred to as InternetAccess Providers.
126

Types of InternetAccess
Most ISPs offer several types of internet access which essentially differ in connection
speeds -
Dialup internet access
Cable internet access
Internet access via DSL broadband
(C) Search Engine -
the time taken for download and upload. Many also offer different plans or
"packages" that vary in the download limit, number of email accounts on offer etc.
is probably the slowest connection and requires you to connect to
the internet via your phone line by dialing a number specified by the ISP. This means, dialup
connections are not "always on", unless you want to raise a huge phone bill, you would sever
the connection when you finish work online.
can be obtained from the local cable TV operator. However, ask
them for a demo first or check with your neighbours on the quality of service.
is indeed very fast and ISPs can offer different
download speeds - quicker the speed, higher will be the price. If you are planning for a DSL
internet connection, ask the ISP if they would also install a wireless modem and router at
your location.Awireless internet connection gives you freedom and flexibility - you need not
be confined to one place (the work table, for instance) and can access the internet from any
spot (even the bathroom) as long as your computer can catch the wireless signal. However,
ensure that the provider secures the wireless connection with a password.
In many countries, especially those in which internet is still a kind of luxury, the ISPs
may put a limit on the amount of data exchanged over a connection. For example, one may
be allowed only a few GB (gigabytes) of download and upload each month. This is usually
sufficient for routine users but if you plan to download movies and music or other large files,
you have to keep a check on the amount of the data transfer.
Search engine is a software program that searches for sites based
on the words that you designate as search terms. Search engines look through their own
databases of information in order to find what it is that you are looking for. A web search
engine helps you find information on the web based on your query. The results of your query
to the search engine can be web pages, text documents (PDF or Word files), videos, images
etc or a combination of all. Google, Yahoo!, MSN, AOL are some of the famous web search
engines.
Without search engines, it would be impossible to find something on the web.
Is a web search engine just one program? NO!There are five main ingredients to it:
• An interface through which you enter your search query
• Adatabase of the indexed information (web pages and their contents)
• A "bot" or an automated program that continuously scourges the web for new
and changed information
• Aprogram that indexes the information (contents of online web pages)
• The actual search engine program
We shall now look at each to understand web search engines better.
127

Web Search Engine Interface :
TheActual Search Engine Program :
Guidelines for Better Search :
Most people think of the interface as the actual search engine. It is not. For instance,
what you see on www.google.com is the interface - a text field in which you type in your
search query, a couple of buttons and a few links. The real Google search engine is one that
works behind the scenes - it wakes up when a query it entered in the txt field and the search
button in clicked.
Atypical web search engine interface has a text field, in which the surfer needs to enter
their query, and a submit button that passes the query to the actual search engine program.
This interface is either presented on a web page or may be a part of another program such as
the web browser or the add-on browser toolbar.
By far, the simplest web page search engine interface is that of Google and I guess it's
one of the reasons that made it so famous. Google has been very particular and "careful" of
its interface.
The web search engine program takes the query provided by the user, runs it through
the indexed database and provides the results. Note: the relevancy of the results depends
on how the information has been indexed - the actual search program simply goes through
the index and presents the results.
Most web search engines follow the same basic format in presenting the results - web
pages are listed one after another, 10 or 20 at a time. More results (if found) are displayed on
additional pages.
Keep it simple. If you're looking for a particular company, just enter its name, or as
much of its name as you can recall. If you're looking for a particular concept, place, or
product, start with its name. If you're looking for a pizza restaurant, just enter pizza and the
name of your town or your zip code.
Think how the page you are looking for will be written.Asearch engine is not a human,
it is a program that matches the words you give to pages on the web.
Use the words that are most likely to appear on the page. For example, instead of
saying [ my head hurts ], say [ headache ], because that's the term a medical page will use.
Describe what you need with as few terms as possible. The goal of each word in a
query is to focus it further. Since all words are used, each additional word limits the results. If
you limit too much, you will miss a lot of useful information.
URL stands for Uniform Resource Locator, which means it is a (same
throughout the world) way to a (file or document) on the Internet. The
(d) URL - uniform
locate resource
128

URL specifies the address of a file and every file on the Internet has a unique address.
Web software, such as your browser, use the URL to retrieve a file from the computer
on which it resides.
The actual URL is a set of four numbers separated by periods. An example of this
would be 202.147.23.8 but as these are difficult for humans to use, addresses are
represented in alphanumeric form that is more descriptive and easy to remember.
Thus, the URL of my site which is URL 209.164.80.192 can also be written as
www.simplygraphix.com. The Internet Domain Name System translates the
alp
domain name should be descriptive for easy comprehension and is
usually the name of the organization or company. There are various domain types. Some of
them are listed below:
hanumerical address to numeric.
For example, the URLof my company site is:
http://www.simplygraphix.com/
• Protocol:
• Host computer name:
• Domain name:
• Domain type:
• Path:
• File name
In addition to the http protocol (mentioned above), there are a few other protocols on
the Internet.
Enables a hyperlink to access a file on a local system.
Used to download files from remote machines.
Helps in accessing a gopher server.
Calls SMTP (the Simple Mail Transport Protocol) and enables a
hyperlink to send an addressed email message.
helps in accessing a USENETnewsgroup.
Provides the means for a hyperlink to open a telnet session on a remote
computer.
The site address consists of the host computer name, the domain name and the
domain type. The
Format of a URL:
Protocol://site address/path/filename
TheAbove URLConsist of :
http
www
simplygraphix
com
/portfolio
4.html
Protocols
• File :
• FTP:
• Gopher :
• Mailto :
• News :
• Telnet :
SiteAddress
129

com :
net :
org :
edu :
gov :
mil :
http://www.bioinfo.ernet.in
Machine name.domain name.domain type.country code.
in :
de :
ca :
jp :
uk :
ac edu. com
Q. Write a short note on the following
(1)
(2)
Ans :
(1) DNS - Domain Name System, DNS, resolve
specifies commercial entities
highlights networks or network providers
organizations (usually non-profit)
colleges and universities (education providers)
government agencies
military entities of the United States ofAmerica
For countries other than the U.S.A., the URLcan be longer as in:
The general format of such URLs is:
This represents a more localized domain name. The country code is a two-letter
extension standardized by the International Standards Organization as ISO 3166. Some
country codes are given below:
India
Germany
Canada
Japan
United Kingdom
Domain types can also be different for different countries. For example, an
educational site can have the domain name www.school.ac.uk in the United Kingdom. Thus
(academic) is used instead of Similarly is represented as co for Indian domain
names.
DNS
E-Mail
The or is used to human-readable
hostnames, such as www.dyndns.com, into machine-readable IP addresses, such as
63.208.196.66. DNS also provides other information about domain names, such as
mail services. DNS is like a phone book for the Internet. If you know a person's name,
but don't know their telephone number, you can simply look it up in a phone book. DNS
provides this same service to the Internet:
130

How does DNS work?
Step 1 : Request information
DNS query
Step 2 :Ask the Recursive DNS servers
DNS servers.
Step 3 :Ask the Root Nameservers
When you visit a domain such as www.dyndns.com, your computer follows a series of
steps to turn the human-readable web address into a machine-readable IP address. This
happens every time you use a domain name, whether you are viewing websites, sending
email, or listening to Internet radio stations.
The process begins when you ask your computer to resolve a hostname, such as
visiting http://www.dyndns.com. The first place your computer looks is its local DNS cache,
which stores information that your computer has recently retrieved. If your computer doesn't
already know the answer, it needs to perform a to find out.
If the information is not stored locally, your computer queries (contacts) your ISP's
recursive These specialized computers perform the legwork of a DNS query
on your behalf. Recursive servers have their own caches, so the process usually ends here,
and the information is returned to the user.
If the recursive servers don't have the answer, they query the A
is a computer that answers questions about domain names, such as IP
addresses. The thirteen root nameservers acting as a kind of telephone switchboard for
DNS; they don't know the answer, but they can direct our query to someone that knows
where to find it.
root nameservers.
nameserver
131

Step 4 :Ask the TLD Nameservers
com Top-Level Domain (TLD) nameservers
Step 5:Ask the authoritative DNS servers
dyndns.com
authoritative nameservers
DNS records.
Address Record (A).
Step 6: Retrieve the Record
time-to-live
Step 7: Receive theAnswer
(2) E-Mail
The root nameservers will look at the first part of our request, reading from right to left -
www.dyndns. - and direct our query to the for
.com. Each TLD, such as .com, .org, and .us, have their own set of nameservers, which act
like a receptionist for each TLD. These servers don't have the information we need, but they
can refer us directly to the servers that do have the information.
The TLD nameservers review the next part of our request - www. - and
direct our query to the nameservers responsible for this specific domain. These
are responsible for knowing all the information about a specific
domain, which are stored in There are many types of records, which each
contain a different kind of information. In this example, we want to know the IP address for
www.dyndns.com, so we ask the authoritative nameserver for the
The recursive server retrieves theArecord for www.dyndns.com from the authoritative
nameservers, and stores the record in its local cache. If anyone else requests the host
record for www.dyndns.com, the recursive servers will already have the answer, and will not
need to go through the lookup process again.All records have a value, which is
like an expiration date; after a while, the recursive server will need to ask for a new copy of
the record to make sure the information doesn't become out-of-date.
Armed with the answer, recursive server returns the A record back to your computer.
Your computer stores the record in its cache, reads the IP address from the record, then
passes this information to your browser. The browser then opens a connection to the
webserver and receives the website. This entire process, from start to finish, takes only
milliseconds to complete .
Electronic mail, often called "email," is a way to communicate with people as close as
your office or as far away as the other side of the world. You type a message and put
addresses on it; email programs use your computer and a network such as the Internet to
deliver the message. Email programs run everywhere from microcomputers to
supercomputers (of course, this book covers MH and related email programs, which run
under UNIX). The people who read your messages don't have to use the same mail system
as you do; their computers don't need to run UNIX. All online services and Internet Service
Providers (ISPs) offer e-mail, and most also support gateways so that you can exchange
mail with users of other systems. Usually, it takes only a few seconds or minutes for mail to
arrive at its destination. This is a particularly effective way to communicate with a group
because you can broadcast a message or document to everyone in the group at once.
132

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