Chapter Two.pdf

Chapter Two
Basics of Telecom
Networks
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Lecture outlines
 Telephone Networks.
 Computer Networks.
 Cable television .
 wireless networks.
 Networking principles.
 Network services
 layered architecture.
 Traffic characterization and QOS.
 Network elements.
 Network mechanisms.
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Telecommunications Networks
1.Public Networks
 Are owned and managed by telecommunications network operators.
 Any customer can be connected to the public telecommunications
network if he has the correct equipment and an agreement with the
network operator.
a. Telephone Network
 PSTN (Public Switched Telephone Network)
 Voice communication (fixed telephone service)
b. Mobile Telephone Networks
 Mobile or cellular telephone systems.
c. Telex Network
 This is a telegraph network that allows teleprinters to be
connected by means of special dedicated switches.

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d. Paging Networks
 Paging networks are unidirectional only.
 Pagers are low-cost, lightweight wireless communication systems
for contacting customers without the use of voice.
e. Internet
f. Radio and Television Networks
 Radio and television networks are usually unidirectional radio
distribution networks for mass communications.
2. Private or Dedicated Networks
 Are built and designed to serve the needs of particular
organizations.
 Voice Communication Networks
 E.g.: voice networks used by police and other emergency services,
taxi organizations and railway companies.
 Private or professional mobile radio (PMR).

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 Data Communication Networks
 Are dedicated networks especially designed for the transmission
of data between the offices of an organization.
 Banks, hotel chains, and travel agencies, for example, have their
own separate data networks to update and distribute credit and
reservation information.
3.Virtual Private Networks
 It is very expensive for an organization to set up and
maintain its own private network.
 VPN provides a dedicated network for the customer
with the help of public network equipment.

Telephone Networks
Development of Telephone
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Background
 A telephone network is a telecommunication network used for telephone calls
between two or more parties.
• World's largest machine; extends to all countries
• Huge economic and social importance.
• Specialized in voice transmission
• Basic service: full-duplex voice transmission
• Small end-to-end delays, small delay variation (more than 150ms delay disturbs)
• Call admission control, and accepted calls will complete
• Grows all the time, now mostly growth of mobile networks
• Although most traffic in the telecom networks is now data.
• Telephones are addressed by telephone numbers, that are unique
• There are special numbers or area codes that need translation.
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Basic Telephone Systems
1. Local Loop
Local loop is the telephone line that runs from the telephone
company’s central office to subscribers home or business
• The central office is the building that houses the telephone
company’s switching equipment
• It provides a local dial tone on your telephone
• Switching functionality is implemented here!
• If you place a long distance call, the central office passes your
telephone call off to a long distance provider
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2. Trunk: A special telephone line that runs between central offices and
other telephone company switching centers.
• A trunk is
• Usually digital, high speed, and carries multiple telephone circuits
• Not associated with a single telephone number like a local loop
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Telephone Numbering
 An international telephone connection from
any telephone to any other telephone is made
possible by unique identification of each
subscriber socket in the world.
 International prefix, country code, area code
and subscriber number.
 Operator Numbers:
 A subscriber will need to dial additional digits
to select a service provider (network operator).

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Example
 Calling here and UAE from Canada using LG mobile phone :

Computer Networks.
 Computer network A collection of computing devices that are
connected in various ways in order to communicate and share
resources.
 Usually, the connections between computers in a network are
made using physical wires or cables
 However, some connections are wireless, using radio waves or
infrared signals.
 The generic term node or host refers to any device on a
network
 Data transfer rate The speed with which data is moved from
one place on a network to another
 Data transfer rate is a key issue in computer networks
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Types of Networks
1. Local-area network (LAN) A network that connects a relatively
small number of machines in a relatively close geographical
area
Various configurations, called topologies, have been used
to administer LANs
Ring topology A configuration that connects all nodes in a
closed loop on which messages travel in one direction
Star topology A configuration that centers around one node
to which all others are connected and through which all
messages are sent
Bus topology All nodes are connected to a single
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communication line that carries messages in both directions

2. Wide-area network (WAN) A network that connects two or
more local-area networks over a potentially large geographic
distance.
 Often one particular node on a LAN is set up to serve as a
gateway to handle all communication going between that LAN
and other networks.
wide-area network, spanning the entire globe.
3. Metropolitan-area network (MAN) The communication
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infrastructures that have been developed in and around large
cities.
 Communication between networks is called internetworking.
 The Internet, as we know it today, is essentially the ultimate

Cable Television
 a system of television reception in which signals from distant stations are
picked up by a master antenna and sent by cable to the individual
receivers of paying subscribers.
 Cable television
is a system of delivering television programming to paying
subscribers via radio frequency (RF) signals transmitted
through coaxial cables
 To receive cable television at a given location, cable distribution lines must
be available on the local utility poles or underground utility lines. Coaxial
cable brings the signal to the customer's building through a service drop.
 Cable television is mostly available in North
America, Europe, Australia and East Asia
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 Cable TV networks are one way broadcast networks, based on coax
cable. We distinguish between the download and the upload band-
width. Today the following frequencies can be used:
download: 47-450 MHz
upload: 5-30 MHz
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Advantage of Cable TV
High availability.
High band-width to low costs
Permanent connection to low costs
The costs are independent from time usage and data volumes
The phone line is not busy
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wireless networks.
 Transfer of information (i.e., voice, data, and multimedia)
over a distance without the use of electrical wires
• Distances involved may be:
• Short, e.g., blue tooth or large, e.g., satellite
Information is transmitted using electromagnetic waves
• Multiple access methods are required
• Transmissions are prone to interference
• System design is more challenging in wireless than in wired
communication
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Merits of Wireless Communication
1. Freedom from wires
• No cost of installing wires or rewiring
• No bunches of wires running here and there
• Instantaneous communications without the need for physical
connection setup.
2. Global coverage
• Communications can reach where wiring is infeasible or costly –
Rural areas, old buildings, battle fields, outer space.
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3. Stay connected
• Roaming – allows flexibility to stay connected anywhere and
anytime
• Rapidly growing market at tests to public need for mobility and
uninterrupted access.
4. Flexibility
• Services reach you wherever you go (mobility)
• You don’t have to go to the lab to check your mail
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• Connect to multiple devices simultaneously (no need for physical
connectivity)

Challenges
1.Bandwidth
• Scares spectrum and dictates low data rates.
2. Reliability
• Low data rate because of interference
3. Power
• Mobility brings about battery operation
• Need efficient hardware, e.g., low power transmitters,
receivers, and signal processing tools
4.Security
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• Shared/broadcast medium => low security

Types of Wireless Communication
1. Radio transmission
• Easily generated, Omni-directionally travel long distances, easily
penetrate buildings.
2. Microwave transmission
• Widely used for long distance communications
• Give a high S/N ratio relatively inexpensive.
3. Infrared and millimeter waves
• Widely used for millimeter waves – 30 GHz
• Unable to pass through solid objects.
4. Light-wave transmission
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• Unguided optical signal, such as laser

Types of wireless networks
1. Cellular Systems
2. Wireless Personal Area Networks (WPANs)
 Short range (10m, extendable to 100m)
• Operates in the unlicensed 2.4 GHz ISM band
3. Wireless Local Area Networks (WLANs)
Network between devices in close physical proximity (offices, homes, …),
usually stationary or moving at low speed, provide access to fixed
infrastructure.
 The term Wi-Fi is widely used
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4. Wireless Local Loop (WLL)
 Solves the “last mile” problem
• To provide high-speed services to individual subscribers.
5.Wireless Metropolitan Area Network (WMAN)
 Network covering a city, metropolitan areas
• “Last mile” application, usually at best low mobility.
6.Wide-Area Network (WAN)
Network covering country/continent/earth
• Anytime, anywhere connectivity
• Good for even highly mobile users
7. Satellite Systems
 over very large areas
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• Very useful in sparsely populated areas, rural areas, sea, mountain areas

Networking principles
1. Digitization
 There are two aspects of digitization:-
a. Any information bearing signal can be represent by a binary string with
arbitrarily high degree of accuracy.
b. Much cheaper to : - Store
- Copy
- Manipulate
- transmit
2. Economies of scale
 Communication network exhibit scale economies. that is
- the average cost per user of the network declines as the network increases in size,
which is measured by - number of user
- subscribers
- host computer
3. Network Externalities
4. Service of integration
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Network services
1.Connection oriented services.
 Connection oriented service delivers messages from the
source to destination in the correct order.
The data transfer take place over a dedicated transmission line
It involves three phases:
 A connection set up.
Data transfer phase.
Connection teardown phase.
2.Connectionless service.
 Connection less service transfers each packet of data to the
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destination one at a time independent of the other packets.

1. World Wide Web.
 It is a distributed application that enables you to navigate
through a set hyper linked document, called web page.
 each web page may contain text, picture, audio clips, video
clips and possibly links .
2. Audio or video streams.
 Streaming audio and video applications enables you to listen to
or view a program as it is being transferred.
Commonly used Network Application Services
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3. Voice over packets and video Conferences
 Inexpensive video cameras and audio devices are available
to set telephone calls or video conferences between Pcs.
4. Network game
 many networked games are played across the internet.
5. Client/server
 Many networked applications are organized according to a
client/server model to share file.
Commonly used Network Application Services . . .
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Full Network services Spectrum
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Layered Architecture
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• Layer 7: Application
– Identifies and establishes availability of communication partner(s)
• Layer 6: Presentation
– Presents data in standard formats
• Layer 5: Session
– Coordinates communication between nodes
• Layer 4: Transport
– Ensures data integrity via TCP and multiplexing applications
• Layer 3: Network (routers)
– Packet routing based on IP addressing
• Layer 2: Logical Link (switches: faster; not concerned with IP)
– Logical link control (LLC) sub-layer: error and flow control
– Media Access Control (MAC) sub-layer: MAC addressing
• Layer 1: Physical (wires)

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Telecommunication Networks by Solomon M.
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Traffic characterization and QOS
 User exchange information through application
 We will examine the characteristics of information transfer of different
application.
 These characteristics describe the traffic that the application generate as well
as the accepted delay and losses by the network in delivering that traffic.
 The information that application generated can take many forms: text, voice,
audio, data, graphics, picture, animation and videos.
 The information Transfer may be
 one way, Two way, Broad cast or Multi point
 The information exchange can be
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 Analog
 Digital
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In digital information exchange the traffic generated by the
application are called bite file or bit stream.
We classify all traffic into three types. A user application can
generate:
1. Constant bite rate (CBR) stream
2. Variable bite rate (VBR) stream.
3. Sequence of messages with different temporal characteristics.
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Types of traffic
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Characteristics of traffic for some common forms of information
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Traffic Engineering
 Traffic engineering is a key issue for telecommunications
network operators trying to keep customers (subscribers)
happy while minimizing network investments.
 The capacity of the network (e.g., number of channels
between exchanges, exchange sizes, number of radio
channels in a cellular network) should be increased.
 Therefore, the utilization of the network is continuously
measured and traffic demand in the future is estimated.
 Then, based on these estimates, the capacity of the network
can be increased before severe problems occur.
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Traffic Statistics
1. Calling rate (λ) :
 The average number of requests for connection that are made
per unit time.
 Where, n is the average number of calls to and from a terminal during a
period T seconds.
2. Holding time (h) :
 The average duration of occupancy of a traffic path by a call.
 The probability of a call lasting at least t seconds is given by
P(t) = exp(-t/h)
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3. Average occupancy (A) :
 Erlangs.
 Also referred as traffic flow or traffic intensity or carried
traffic.
4. User behavior :
 The statistical properties of the switching system are a
function of the behavior of users who encounter call
blocking.
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Busy Hour
 Busy hour :
 Continuous 60 minutes duration for which the traffic volume
or the number of call attempts is greatest.
 Peak busy hour :
 It is the busy hour each day varies from day to day, over a
number of days.
 Time consistent busy hour :
 The 1 hour period starting at the same time each day for
which the average traffic volume or the number of call
attempts is greatest over the days under consideration.
 Call Completion Rate (CCR) :
 It is the ratio of the number of successful calls to the number
of attempts.
 A CCR value of 0.75 is considered excellent and 0.70 is
usually expected.
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 Busy hour call attempts :
 It is the number of call attempts in a busy hour.
 Busy hour calling rate :
 It is the average number of calls originated by a
subscriber during the busy hour.
 Day-to-day hour traffic ratio :
 It is the ratio of busy hour calling rate to the average
calling rate for that day.
 It is normally 6 or 7 for rural areas and over 20 for city
exchanges.
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Units of Telephone Traffic
Erlangs :
 It is named after the Danish Mathematician, Agner Krarup
Erlang .
 The erlang unit is defined as
(1) A unit of telephone traffic specifying the percentage of
average use of a line or circuit (one channel)
or
(2) The ratio of time during which a circuit is occupied and the
time for which the circuit is available to be occupied. Traffic
that occupies a circuit for 1 hour during a busy hour is equal to
1 erlang.
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• If the traffic intensity of a subscriber line is 1
erlang, the line is occupied for 60 minutes in an
hour.
• If a subscriber line is in use 6 minutes out of an
hour (on average), the traffic intensity is 6
minutes/60 minutes or 100 mErl.
• The maximum traffic intensity of a 2-Mbps (30
PCM channels) line system is 30 erlangs, that is, all
channels are in use 60 minutes during the busy
hour.
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Probability of Blocking
 The problem in traffic engineering is determining the capacity if
the average offered traffic (including the traffic that is blocked)
intensity is known (or estimated).
 Clearly, the capacity should (at least usually) be higher than
offered traffic.
How much higher should the capacity be for the subscribers to
feel that the grade of service is acceptable ?
 The starting point is how often subscribers are allowed to be
blocked and receive a busy tone.
 Probability of blockage for an acceptable GoS is usually set
to be in the range of 0.2% to 5% which means that every
500th to 20th call is blocked during a busy hour.
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 Probability of occurrence of x events when the
average number of events is A, is given by:
 Average number of occupied channels is A erlangs, P(x) gives
the probability that x number of channels is occupied at a time
when a subscriber makes a call.
 Blocking occurs if all n channels are occupied or there may
even be a need for a larger number of channels. This probability
is given by:
where,
e = 2.71828
x ! = 1*2*3 … x
Poisson Distribution
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 On the other hand, one number of channels is always in use,
giving the probability for
 Therefore, P(x>n) can be given as:
 Blocking probability :
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A = 1 erlangs, number of available channels, n = 3.
 Blocking probability (the probability that the number
of occupied channels x > 3)
= 1- ( P(0)+P(1)+P(2) )
= 1- (0.37+0.37+0.18)
= 0.08 = 8 %
 P(0) = 0.37, probability that all channels are free.
 P(1) = 0.37, probability that one channel is occupied.
 P(2) = 0.18, probability that two channels are occupied (one is
free).
 Every twelfth call the user makes is blocked and a busy
signal is received.
Example :
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• Networking requirements:
Bandwidth
Bursty
Symmetry (uplink /downlink rates)
Bit errors and blocking
Delay
Security
• These define QOS (Quality of Service)
QOS (Quality of Service)
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Network elements
1. Transmission
 Transmission is the process of transporting information between end
points of a system or a network.
 Transmission systems use four basic media for information transfer
from one point to another:
1. Copper cables, such as those used in LANs and telephone subscriber lines;
2. Optical fiber cables, such as high-data-rate transmission in
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telecommunications networks;
3. Radio waves, such as cellular telephones and satellite transmission;
4. Free-space optics, such as infrared remote controllers.
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2. Switching
 Circuit switching
 Packet Switching
3. Signaling
Signaling is the mechanism that allows network entities
(customer premises or network switches) to establish,
maintain, and terminate sessions in a network.
Signaling is carried out with the help of specific signals or
messages that indicate to the other end what is requested of it
by this connection.
Network elements . . .
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Basic Telecommunications Network.
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Network mechanisms
 A network service comprise the end to end transport of bit
streams, in specific format , over a set of routes.
 This services are differentiated by quality: speed, delay, error.
 All this are produced using five basic network mechanisms.
1. Multiplexing
2. Switching
3. Error Control
4. Flow control.
5. congestion control and resource allocation
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