This document provides an overview of telecommunication networks. It discusses the evolution of transmission systems from open wire carriers to current optical fiber systems. It also describes key concepts in telecommunications including the decibel unit for measuring signal power, classification of networks by geography into access, metro and core networks, and common topologies of rings and meshes. The objective is to understand the basics of telecom networks and their building blocks.

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1 OVERVIEW OF TELECOM NETWORK
STRUCTURE
1 OVERVIEW OF TELECOM NETWORK
1.1 INTRODUCTION
1.2 OBJECTIVE
1.3 BUILDING BLOCK OF COMMUNICATION SYSTEM
1.4 EVOLUTION OF TRANSMISSION SYSTEMS
1.5 DECIBEL (DB) CONCEPT
1.6 CLASSIFICATION OF TRANSPORT NETWORK BY GEOGRAPHY
1.7 TRANSPORT NETWORK AND THE ROAD ANALOGY
1.8 SUMMARY
1.9 REFERENCES AND SUGGESTED FURTHER READINGS
1.10 KEY LEARNINGS
1.11 WORKSHEET
1.1 INTRODUCTION
In earlier times, communication may have involved the use of smoke signals,
drums, semaphore, flags, homing pigeons etc. In the Middle Ages, chains of beacons
were commonly used on hilltops as a means of relaying a signal. Beacon chains
suffered the drawback that they could only pass a single bit of information, so the
meaning of the message such as "the enemy has been sighted" had to be agreed upon
in advance. One notable instance of their use was during the Spanish Armada, when a
beacon chain relayed a signal from Plymouth to London. The conventional telephone
was invented independently by Alexander Bell in 1876.
In 1792, Claude, a French engineer, built the first fixed visual telegraphy
system (or semaphore line) between Lille and Paris. However semaphore suffered
from the need for skilled operators and expensive towers at intervals of ten to thirty
kilometers (six to nineteen miles). As a result of competition from the electrical
telegraph, the last commercial line was abandoned in 1880. The first commercial
telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the
cities of New York and London.
Samuel Morse independently developed a version of the electrical telegraph
that he unsuccessfully demonstrated on 2 September 1837

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The first transatlantic telegraph cable was successfully completed on 27 July
1866, allowing transatlantic telecommunication for the first time
Fig : 1Advances in phone telephony
Fig : 2Typical diagram of a Telecom Network
Telecommunication is the assisted transmission of signals over a distance for
the purpose of communication. It is the technology of transferring information over a
distance.
Information can be of several types:
• Audio – Telephone
• Text - Telegraph, email, SMS
• Pictures – Picture attachments
• Video – Clipping over internet
• Data – ATM to bank.

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The same telecom technology/service cannot communicate all the types of
information.
Telecommunications transport networks are the largely unseen infrastructure
that provides local, regional, and international connections for voice, data, and video
signals. In fact, most “private” networks are implemented by leased connections
through the public transport network infrastructure. Telecommunications and data
communications transport networks are changing rapidly with the introduction of new
technologies that address the need for new value-added services, high availability, and
integration. Equipment vendors and network providers have made considerable effort
to bridge and unify previously dedicated networks to serve the data and
telecommunications market. With the introduction of digital transmission technology,
the most appropriate multiplexing technology was Time Division Multiplexing
(TDM). Digital TDM was used both on copper cable systems and on microwave
radio.
Fig : 3Simplified communication model
1.2 OBJECTIVE
After reading this unit, you should be able to:
 Understand the basics of telecommunication and building blocks of telecom
network.
 Understand the different multiplexing techniques
 Describe the different types of Transport Network
 Differentiate the different components of transmission network.

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1.3 BUILDING BLOCK OF COMMUNICATION SYSTEM
The most general form of communication system consists of the following
components:
Fig : 4 Communication System Generic Block Diagram
1.3.1 TRANSMITTER:
The upper portion of the channel as a whole is called transmitter.
1.3.1.1 Information Source:
The information provided to a communication system is called baseband.
One may define as “any information signal is known as baseband signal".
1.3.1.2 Processing Unit:
The baseband signal is passed through some processing unit where necessary
operations are performed. These operations may include filtering, sampling etc.
This is helpful in separating unwanted information from baseband.
1.3.1.3 Modulation:
The process of modulation is required to make the baseband signal ready for
transmission. The modulator produces a varying signal at its output which is
proportional in some way to the signal appearing across its input terminal
(baseband signal). For example, a sinusoidal modulator may vary the amplitude,
frequency or phase of a sinusoidal signal in direct proportion to the input baseband.
1.3.1.4 Antenna:
For wireless communication, antenna is used to send modulated information
into the channel or medium.
1.3.1.5 Channel:
The transmission medium or channel is vital link between the systems.
Without it there would be no communication problem. The transmission medium
may include wired transmission line, atmosphere (wireless) which may include the

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ionosphere, the troposphere, free space etc.
It causes noise (unwanted addition to baseband signal), attenuation and
distortion in the form of electrical signal. This results in interference with our
error-free reception at receiving end.
1.3.2 RECEIVER:
The portion below the channel as whole called receiver.
1.3.2.1 Demodulation:
The demodulator performs the inverse process of modulator to recover the
information signal in its original form.
1.3.2.2 RF Amplifier:
The RF amplifier is used to tune the receive to frequency of the transmitted
bandwidth.
1.3.2.3 Display Unit:
This shows us the received signal in the form by which we are familiar.
1.4 EVOLUTION OF TRANSMISSION SYSTEMS
Transmission systems interconnect communication devices by guiding signal
energy in a particular direction or directions through a transmission medium such as
copper, air, or glass. Described are the key types of transmission systems used in
modern telecommunications networks. This includes multiplexed signals on twisted
wire pairs, coax cable, fiber optic cable and radio. Several technical aspects of
transmission systems are covered including: analog transmission, digital transmission,
and transmission medium limitations
1.4.1 OPEN WIRE SYSTEM
The long distance voice communication till 1950s was almost entirely
transported over Open Wire Carrier system. The voice signals for these systems were
modulated to a higher frequency and carried through open wire systems. These open
wire systems are capable of carrying traffic of three to twelve subscribers at a time.

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Fig : 5Open Wire Carrier system
1.4.2 COAXIAL SYSTEM
Coaxial Cable (often called coax for short) is high-capacity cable widely used
for high-frequency transmission of telephone, television, and digital audio signals.
The cable is very effective at carrying many analog signals at high frequencies.
Coaxial cables have become an essential component of our information
superhighway. They are found in a wide variety of residential, commercial and
industrial installations. From broadcast, community antenna television (CATV), local
area network (LAN), closed circuit television (CCTV) to many other applications,
coax has laid the foundation for a simple, cost effective communications
infrastructure.
With the introduction of symmetrical pair cable carrier system which was
followed by the Coxial Cable system, greatly enhanced, by the decade end, the
simultaneous voice channel carrying capacity to 960 voice channels. The first Coaxial
Cable System was commissioned between Agra and Delhi in the year 1959. Over the
years this system was improved and developed to carry 2,700 simultaneous voice
channels
Fig : 6Co-axial cable
1.4.3 MICROWAVE SYSTEM
Close on the heel of coaxial systems, in the mid of 60’s wireless microwave
systems were developed and inducted in the network. The first Microwave system
was installed between Calcutta and Asansole. Microwave systems with 60, 300 and
1800 voice channels capacity were inducted into the telecom network subsequently.
These systems were mostly developed and manufactured with in the country.
Fig : 7Microwave system

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1.4.4 DIGITAL TRANSMISSION SYSTEM
By mid of 1980’s Digital TAX exchanges were introduced in the network with
the aim to improve STD services. Till 1989 Coaxial cable and UHF transmission
medias were used to provide connectivity. Induction of Digital Transmission Systems
which were mainly Digital UHF, Digital Microwave, Digital Coaxial and Optical
Fiber Systems, started during 1989-90. Under ground coaxial cable was initially used
for the connectivity of large and medium cities and however, later on, it was also used
for connecting small towns. Media diversity is provided through Radio Relay (UHF
and Microwave) Systems. These Radio relay systems were very reliable and
beneficial particularly for connecting hilly and backward areas where laying and
maintenance of underground cable is extremely difficult.
1.4.5 OPTICAL FIBER SYSTEM
Introduction of Optical Fiber Cable Systems started in 1989-90. These systems
are capable of carrying large no voice channels compared to the existing technologies
that were available at that time and offer the circuit at low cost per kilometer of
circuit. Department deployed these OFC system in big way for connectivity right upto
the level of Tehsils. By the year 2000 a huge network of optical fiber cable was in
place and a large number of PDH technology (Plesiocronus Digital Hierarchy) OFC
were deployed for providing backbone connectivity to switching network.
Fig : 8Typical Optical Fiber System
1.4.6 SATELLITE SYSTEM
Work for connecting far flung, inaccessible area and island community started
in late seventies by Department of Telecommunication. The first Domestic Satellite
Network was established by connecting Port-Blair and Car-Nicobar in Andaman &
Nicobar islands, Kavaratti in Lakshadweep islands, Leh in Ladakh region and Aizwal
in North Eastern region. These station were simultaneously linked to the gateway at
Delhi and Chennai. This satellite network was commissioned in November 1980
through International Telecommunication Satellite. Satellite Communication capacity
increased with lauanch INSAT 1 and INSAT 2 series satellites.

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Fig : 9A Typical Satellite system
1.5 DECIBEL (DB) CONCEPT
One tenth of the common logarithm of the ratio of relative powers, equal to
0.1 B (bel). The decibel is the conventional relative power ratio, rather than the bel,
for expressing relative powers because the decibel is smaller and therefore more
convenient than the bel. The ratio in dB is given by
X = log P2/P1 B i.e. = 10 log P2/P1 dB
where P 1 and P 2 are the actual powers. Power ratios may be expressed in terms of
voltage and impedance, E and Z, or current and impedance, I and Z. Thus dB is also
given by;
X = 20 log V2/ V1 dB. (when Z 1 = Z 2 )
Note: The dB is used rather than arithmetic ratios or percentages because when
circuits are connected in tandem, expressions of power level, in dB, may be
arithmetically added and subtracted. For example, in an optical link if a known
amount of optical power, in dBm, is launched into a fiber, and the losses, in dB, of
each component (e.g., connectors, splices, and lengths of fiber) are known, the overall
link loss may be quickly calculated with simple addition and subtraction.
Example 1
Let us look at the following network:
The input is 1W and its output 2W, therefore,
Gain = 10 log (output)/(input) dB.
Net Work
1 W 2 W

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= 10 log 2/1 dB= 10 (0.3010) dB=3.101 dB
= 3dB approximately
1.5.1 dBm
Till now decibel has referred to ratios or relative units. We cannot say that the
output of an amplifier is 33 dB. We can say that an amplifier has a gain of 33 dB or
that a certain attenuator has a 6 dB loss. These figures or units don't give any idea
whatsoever of absolute level. Whereas, several derived decibels units do.
Perhaps the dBm is the most common of these. By definition dBm is a power level
related to 1 mw. The most important relationship to remember is:
0 dBm = 1mW.
The dBm formula may then be written as:
Power (in dBm) = 10 log Power (mW)/(1mW)
Example
An amplifier has an output of 20 W; what is its output in dBm?
Power (dBm) = 10 log 20 W/1 mW = 10 log 20x103
mW/1mW = +43 dBm.
(The plus sign indicates that the quantity is above the level of reference, 0 dBm.)
1.5.2 SIGNAL-TO-NOISE RATIO
In Analogue Transmission system, the quality of communication is mainly
assessed by the value of Signal to noise ratio.
It is popularly known as SNR. SNR is the ratio of signal power to the noise
power at any point in a circuit. This ratio is usually expressed in Decibels (dB). For
satisfactory operation of a channel the value of SNR should be sufficiently high i.e.,
the signal power should be sufficiently higher than the noise power.
SNR at any point in a circuit is given as SNR = S/N = Signal Power / Noise Power
Both powers are expressed in watts.
Expressing dBs: SNR = 10 log10 (S/N) dB.
Example: Signal voltage Vs = 0.923 µV; Noise voltage Vn = 0.267 µV, then
calculate the signal-to-noise ratio.
S/N = Vs2
/ Vn2
= 0.923/0.267)2
= 11.95
In decibels, S/N = 10 log10 (11.95) = 10.77 dB.

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1.6 CLASSIFICATION OF TRANSPORT NETWORK BY
GEOGRAPHY
One traditional approach to classifying transport networks is in relation to
their geographic scope. These classifications are illustrated in Figure.
Fig : 10 Illustrations of a telecommunications network
The access network is that portion of the network that connects the end users
(subscribers) to the edge switching elements in the network.
The metropolitan (metro) transport network is the network that interconnects
central offices (COs) within an urban/suburban region. COs within a metro network
are typically directly connected to both access networks and core long distance
networks. These metro COs are typically owned by the same carrier, and in many
cases either allow the carrier to centralize specialized services (e.g. ISDN or Ethernet
routing) in just one CO, or to use different COs for back-up redundancy for each other
(e.g., to take over switching functions in the event of a failure of the primary CO for
that subscriber).

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The long distance core transport network provides the interconnection between
metro networks, smaller community COs, service providers (e.g., Internet), and
regional or international gateways. Higher bandwidth technologies like SDH and
DWDM are used in core network.
As shown in Figure, both metro and core transport networks can consist of ring
and mesh topologies. Rings have become increasingly popular since they provide
inherent route diversity that can be exploited for protection switching. (See City 1 and
upper portion of the core network.) Rings have also become increasingly popular in
access networks (e.g., City 3). Traffic routing on rings is also more straightforward
than in arbitrary mesh networks.
Ring topologies are not always convenient, however, due to such constraints as
geography or having to use pre-existing right of ways. Arbitrary mesh networks are
constructed in order to use convenient cable routings or, in some cases, allow more
bandwidth-efficient protection schemes. Transport networks often consist of a mix of
ring and mesh sub-networks, including interconnected rings.
Traditionally, a sharp distinction was drawn between transmission and switching
equipment. However, transmission and switching are both considered as part of the
transport network. The switches provide the automatic routing of voice (or data)
traffic, while the transmission equipment handled the multiplexing and facility
connections to carry the traffic between the switches. Typical transmission equipment
includes SONET/SDH, DWDM terminals.
1.7 TRANSPORT NETWORK AND THE ROAD ANALOGY
The transport network is analogous to any road network of a country as shown
in Figure. The national highway of a country has a greater capacity for vehicle traffic
than the state highway and the city roads. The state highway has less vehicle traffic-
carrying capacity than the city road network. Analogous to this, the access part of a
transport network has less capacity than the metro network and the metro part has less
capacity than the core part of the transport network. The transport networks are
deployed using different technologies in the different parts of the network. DSL is a
popular technology deployed in the access network today. SDH/SONET technologies
are widely being used for the deployment of the Metro network and DWDM is used
for the core network.

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Fig : 11 Transport Network and Road Analogy
1.8 SUMMARY
A communications network is a collection of transmitters, receivers, and
communications channels that send messages to one another. Some digital
communications networks contain one or more digital exchanges that work together
to transmit information to the correct user. An analog communications network
consists of one or more switches that establish a connection between two or more
users. For both types of network, repeaters may be necessary to amplify or recreate
the signal when it is being transmitted over long distances. This is to combat
attenuation that can render the signal indistinguishable from the noise. Another
advantage of digital systems over analog is that their output is easier to store in
memory i.e. two voltage states (high and low) are easier to store than a continuous
range of states.
1.9 REFERENCES AND SUGGESTED FURTHER READINGS
 ITU-T manual on OF installation
 EI of BSNL
 EI on underground OF cable laying works by BBNL
 Fiber Optics Technician's Manual
 Understanding optical communication by Dutton
 Planning Fiber Optic Networks by Bob Chomycz
 www.timbercon.com
 http://www.ofsoptics.com
 http://www.thefoa.org/
 http://www.corning.com
 http://www.fiber-optics.info
 http://www.rp-photonics.com
 http://www.occfiber.com and other websites

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1.10 KEY LEARNINGS
Qu.1 Fill in the blanks:
1. The conventional telephone was invented independently by …………………
in 1876.
2. With the introduction of digital transmission technology, the most appropriate
multiplexing technology was ………………………
3. The process of ………………….is required to make the baseband signal
ready for transmission.
4. The long distance voice communication till 1950s was almost entirely
transported over ………
5. The first Microwave system was installed between ……………………..and
Asansole in India.
Qu.2 Mark True or False:
1. Repeaters are the most commonly used devices to compensate for Loss.
2. dBm is a power level related to 1w.
3. DSL is a popular technology deployed in the access network today.
4. SDH/SONET technologies are widely being used for the deployment of the
Metro network and DWDM is used for the core network.
5. For wireline communication, antenna is used to send modulated information
into the channel or medium.
Qu. 3: Write down the different types of Information?
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Qu.4 Fill in the blank box in Generic Block Diagram of Communication System?
Qu.5 Calculate the gain of the following Network?
Qu. 6 What is Modulation?
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1.11 WORKSHEET
1. Identify the different optical fiber systems and write down their purpose?
Sl. No. Name of Equipment Purpose
Notes:
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