CHAP I INTRODUCTION TO TELECOMMUNICATIONS

1.1 WHAT IS TELECOMMUNICATIONS?

Telecommunications has been defined as a technology concerned with communicating from a

distance, and we can categorize it in various ways.

Figure 1.1 Telecommunications.

Figure 1.1 Possible view of the different sections of telecommunications.

It includes mechanical communication and electrical communication because

telecommunications has evolved from a mechanical to an electrical form using increasingly
more sophisticated electrical systems. This is why many authorities such as the national
post, telegraph, and telephone (PTT) companies are involved in telecommunications using both
forms.

Our main concern here is electrical and bidirectional communication, as shown in the upper
part of Figure 1.1. The share of mechanical telecommunications such as conventional mail
and press has decrease, whereas electrical, especially bidirectional, communication have
increase and taken the major share of telecommunications. Hence, today, major press
corporations are interested in electrical telecommunications as a business opportunity.

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To understand telecommunications, it is necessary to have an outline appreciation of the
basic subject of information, for which telecommunication provides a transport mechanism.
Information comes from various forms, for example: spoken words, sounds, written and
printed document and computer data. Information can be processed, stored, and
transmitted.

Telecommunications therefore involves the following:

(a) Transporting information between locations, either

(1) one –to-one (e.g. telephony)

(2) one-to-many (e.g. broadcasting);

(b) Relaying information from one telecommunication system to another;

(c) Managing this transport services. It is necessary to provide, monitor, maintain and
bill for the service;
(d) Adding communication value, e.g.
(1) Store and forward on request
(2) Versatile (e.g. time-dependent) routing
(3) Customized billing; and
(e) Adding information value, e.g. credit card verification and electronic yellow pages.

All these functions contribute to the services provided to the users of telecommunication
network. Since the must pay for them, they are known as the subscribers or customers of
the network operator. Telecommunications has been driven by powerful technological forces
and insatiable demand for communication in a large variety of forms.

1.2 SIGNIFICANCE OF TELECOMMUNICATIONS

Many different telecommunications networks have been interconnected into a continuously

changing and extremely complicated global system. We look at telecommunications from
different points of view in order to understand what a complicated system we are dealing with
and how dependent we are on it.

Telecommunications networks make up the most complicated equipment in the world. Let us
think only of the telephone network, which includes more than 2 billion fixed and cellular
telephones with universal access. When any of these telephones requests a call, the telephone
network is able to establish a connection to any other telephone in the world.

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 In addition, many other networks are interconnected with the telephone network. This
gives us a view of the complexity of the global telecommunications network—no other system
in the world exceeds the complexity of telecommunications networks.

Telecommunications services have an essential impact on the development of our various

communities. If we look at the telephone density of a country, we can estimate its level of
technical and economical development. In the developing countries the fixed telephone
density, that is, the teledensity, is fewer than 10 telephones per 1,000 inhabitants; in
developed countries in, for instance, North America and Europe, there are around 800 to
900 fixed telephones per 1,000 inhabitants. The economic development of developing
countries depends on (in addition to many other things) the availability of efficient
telecommunications services.

The operations of a modern community are highly dependent on telecommunications.

We can hardly imagine our working environment without telecommunications services. The
local area network (LAN) to which our computer is connected is interconnected with the
LANs of other sites throughout the world. This is mandatory so that the various computers
can work together efficiently. We communicate daily with people in other parts of the world
with the help of electronic mail, telephones, facsimile, and mobile telephones.

Governmental organizations that provide public services are as dependent on

telecommunications services as are private organizations. Telecommunications plays an
essential role on many areas of everyday living. Each of us uses telecommunications services
and services that rely on telecommunications daily.

Here are some examples of services that depend on telecommunications:

• Banking, automatic teller machines, telebanking;

• Aviation, booking of tickets;
• Sales, wholesale and order handling;
• Credit card payments at shops;
• Booking of hotel rooms by travel agencies;
• Material purchasing by industry;
• Government operations, such as taxation.
The various players that involve include:

(a) End users. They have an ever-increasing need for information systems of which
telecommunication plays a vital role;

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 (b) Enhanced service providers. With liberation, an ever-increasing band of competitors
are providing services beyond that of simple point-to-point communication;
(c) Network operators. Conventional public telecommunications operators (PTOs) are now
operating in an increasing competitive environments.
(d) System suppliers. The conventional telephone supply companies are now joined by wide
range of other manufacturers, particular those associated with computing.
(e) Component suppliers. The traditional electronic and electromechanical component
suppliers and their software equivalents;
(f) Regulators. Both national and international;
(g) Financial backers. National and private.

There is also a vital role played by academic institutions by providing the major source of
human expertise and the underpinning basic research.

1.3 NETWORK SERVICES

In practice there are several networks providing different services but they use common

transmission bearers, as shown below

V
I P T P
S D E P S
U N L C T
A E N
l x

Transmission-bearer network Loop

Customer
Relationship of services and bearer network.

PC- private circuit.

PDN- public data network
PSTN- public switch telephone network

(a) Public Switched telephone network (PSTN)

(b) Private network (using circuit leased from public telecommunication operator)
(c) Cellular radio network providing mobile communication.
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(d) Data network usually employing packet switching.
(e) Special service networks, introduced to meet special demand from customers.

1.4 REGULATION

The business of operating telecommunications network has tended to be a monopoly.

In fact, It is enormously expensive to dig up streets, install conduits or ducks and lay cables

throughout the country. The high cost is a barrier against competitors entering the network.

It is therefore desirable that the power of monopoly should be limited to protect customers

from exploitation.

In most countries in the world, telecommunication monopoly has been controlled by state

ownership. Typical the telecommunication operator has been a department of P&T or public

corporation.

Tariffs have often been regulated by the state. However development in technology has

greatly reduced the cost of long-distance and has enable new players to enter the field.

1.5 STANDARDIZATION

Communication networks are designed to serve a wide variety of users who are using
equipment from many different vendors. To design and build networks effectively, standards
are necessary to achieve interoperability, compatibility, and required performance in a cost-
effective manner.

Open standards are needed to enable the interconnection of systems, equipment, and
networks from different manufacturers, vendor operators.

The most important advantages and some other aspects of open telecommunications standards
are as follow.

 Standards enable competition.

Open standards are available to any telecommunications system vendor. When a new
system is standardized that is attractive from a business point of view, multiple vendors will
enter this new market. As long as a system is proprietary, specifications are the property

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 of one manufacturer and it is difficult, if not impossible, for a new manufacturer to start
to produce compatible competing systems.

 Open competition makes products more cost-effective, therefore providing low-

cost services to telecommunications users.

 Standards lead to economies of scale in manufacturing and engineering.

Standards increase the market for products adhering to the standard, which leads to mass
production and economies of scale in manufacturing and engineering, very large scale
integration (VLSI) implementations, and other benefits that decrease price and further
increase acceptance of the new technology. This supports the economic development of the
community by improving telecommunications services and decreasing their cost.

 Political interests often lead to different standards in Europe, Japan, and the
United States. Standardization is not only a technical matter. Sometimes opposing political
interests make the approval of global standards impossible, and different standards are
often adapted for Europe, the United States, and Japan. To protect local industry, Europe
does not want to accept American technology and America does not want to accept European
technology.

One example of a political decision in the 1970s was to define a different PCM coding law
for Europe instead of the existing American PCM code.

A more recent example is the American decision in the 1990s not to accept European GSM
technology as a major digital second generation cellular technology.

 International standards are threats to the local industries of large countries

but opportunities to the industries of small countries. Major manufacturers in large countries
may not support international standardization because it would open their local markets to
international competition. Manufacturers in small countries strongly support global
standardization because they are dependent on foreign markets. Their home market is not
large enough for expansion and they are looking for new markets for their technology.

 Standards make the interconnection of systems from different vendors possible.

The main technological aim of standardization is to make systems from different networks
“understand” each other. Technical specifications included in open standards make systems
compatible and support the provision of wide-area or even global services that are based on
standardized technology.
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  Standards make users and network operators vendors independent and improve
availability of the systems. A standardized interface between a terminal and its network
enables subscribers to purchase terminal equipment from multiple vendors. Standardized
interfaces among systems in the network enable network operators to use multiple competing
suppliers for systems. This improves the availability and quality of systems and reduces their
cost.

Standards make international services available. Standardization plays a key role in

the provision of international services. The standards of some systems may not have official
worldwide acceptance, but if the system becomes popular all around the world, a worldwide
service may become available. Recent examples of these services are GSM and the Internet
with WWW. Internet specifications have no official status, and GSM was originally specified
for Europe only. Their specifications have been openly available, which has supported their
expansion.

1.6 STANDARDS ORGANIZATIONS

Successful planning and operation of the international telecommunication network depends on

co-operation between all the organizations involves. Many organizations are involved in
standardization work. We look at them from two points of view: (1) the players in the
telecommunications business involved in standardization and (2) the authorities that approve
official standards.

1.6.1 Interested Parties

Let us look at a list of these parties and their most important interests, that is, why
they are involved in standardization work.

Network operators support standardization for the following reasons:

• To improve the compatibility of telecommunications systems;

• To be able to provide wide-area or even international services;

• To be able to purchase equipment from multiple vendors.

Equipment manufacturers participate in standardization for the following reasons:

• To get information about future standards for their development activities as early
as possible;

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 • To support standards those are based on their own technologies;

• To prevent standardization if it opens their own markets.

Service users participate in standardization for these reasons:

• To support the development of standardized international services;

• To have access to alternative system vendors (multivendor networks);

• To improve the compatibility of their future network systems.

Other interested parties include governmental officials who are keen on having national
approaches adopted as international standards and academic experts who want to become
inventors of new technological approaches. The standardization which has made an effective
international network possible has been carried out through the international
Telecommunication Union (ITU).

This was originally founded in 1865 as the international Telegraph Union. The work of the

ITU is carried out through two main bodies.

(a) The ITU telecommunication sector (ITU-T), which was formerly Comité Consultatif

International Télégraphique et Téléphonique (CCITT). It duties include the study of technical

question, operating method, tariff for telephony, telegraphy and data communications.

(b) The ITU Radiocommunication sector (ITU-R) which was formerly called Comité

Consultatif International des Radiocommunications (CCIR). It study all technical and operating

question relating to radio communication including point-to-point communication, mobile

services and broadcasting.

The ITU-R and the ITU-T are composed of representative of government, operating

administration and industrial organization. Each has a large number of study groups.

Recommendations of this study groups are reported to the plenary session which meet every

few years.

In theory these bodies issues recommendations which are apply only to international

communication. However international calls must pass through parts of the national network
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of countries in addition to international circuit concerned. Consequently national standard

are affected. Hence in practice public telephone operators and manufacturers must take

account of ITU-T recommendations in planning their networks and produced equipment which

meet ITU-T specifications.

In addition, there is international standards organization which produce standard in several

fields including information technology. In particular importance to telecommunication is the

OSI reference model. They are also regional standard body such as the European

Telecommunication standards institute (ETSI). In the USA, standard are produced by the

American standards institute (AMSI) and the institute of Electrical and Electronic Engineers

(IEEE).

1.7 THE OSI MODEL

The international standard organization ISO is an international body dedicated to worldwide

agreement on international standard. An ISO standard that covers all aspect of network

communication is the Open System Interconnect (OSI).

- An open system is a model that allows any two different systems to communicate

regardless of their underlying architecture. The purpose of OSI model is to open

communication between different system without requiring changes to the logic of the

underlying hardware and software.

- OSI model is not a protocol; it is a model for understanding and designing a network

architecture that is flexible, robust and interoperable

1.7.1 The Model

The open system interconnection OSI model is a layered framework for the designing

of network systems that allow for communication across all types of computer systems. It

consists of seven separate but related layers, each of which defined a segment of the

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process of moving information across a network. Understanding the fundamental of OSI

models provides a solid base for exploration of data communication.

-The OSI model is built of 7 other layers as shown below.

Figure3: The OSI Model

1.7.2 Organization of the layers

The seven layers can be thought to 3 sub-groups. Layers 1, 2 and 3- physical, data

link, and network –are network support layers (They deal with the physical aspect of moving

data from one device to another) such as physical connections, physical addressing, transport

timing and reliability. Layers 5, 6 and 7 – Session, Presentation, and Application are thought

to be user support layers; they allow interoperability among unrelated software systems.

Layer 4, the transport layer, ensures end-to-end reliable data transmission.

The upper OSI layers are almost always implemented in software, lower layers are a

combination of hardware and software except for the physical layer which is mostly

hardware.

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 1.7.3 Functions of layers

1) Physical Layer

The physical layer coordinates the function required to transmit a bit stream over a physical

medium. It deals with mechanical and electrical specification of the interface and

transmission medium. These are the connections, wire and cards which you can see.

2) Data link layer

The data link layer transform the physical layer, a raw transmission facility, to a reliable

link and is responsible for node-to-node delivery, it makes the physical layer appears error

free.

 Flow control: if the rate at which data absorbed by the receiver is less than

the rate produced in the sender. The data link imposes a flow control mechanism

to prevent overwhelming the receiver.

 Error control: it adds reliability to the physical layer by adding mechanisms to

detect and retransmit damage frame.

 Access control: when two or more device is connected to the same link, the data

link protocols are necessary to determine which device accesses the link at any

given time.

3) Network Layer

Routing – when independent network are connected together to create a large network,

the connecting devices called routers or gateways route the packet to their final destination.

The network layer provides that mechanism.

Logical addressing: The physical addressing implemented by the data link layer handles

addressing problem locally. But if a packet passes the network boundary, it needs another

addressing system to help distinguish the source and the destination system. The network

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layer adds a header to the packet coming from the upper layer that includes logical addresses

of the sender and receiver.

(4) Transport layer

This layer is responsible for end to end delivery of the entire message whereas the network

layer oversees end-to-end delivery of individual packets, it doesn’t recognize any relationship

between the packets. The transport layer ensures the whole message arrives intact and in

order overseeing both error-control and flows control at the source-to-destination level.

 Segmentation and Reassembly: A message is divided into transmittable segment, each

segment containing a sequence number. These numbers enable the transport layer to

reassemble correctly upon arriving the destination, and to identify and replace packet

that were lost in transmission.

 Connection Control: The transport layer can be connectionless or connection-oriented.

A connectionless transport layer treats each segment as independent packet and

delivers it to the transport layer of the destination machine. A connection-oriented

transport layer makes a connection with the transport layer of the destination machine

before delivering the packets. After all data has been transferred, the layer, the

connection is terminated.

 Flow Control: Like the data link layer, the transport layer is responsible for flow

control. The flow control here is performed end to end rather than access a single

link.

 Error control: Like the data link layer. The transport layer is responsible for error

control. Error control here is performed end-to-end rather than access a link. The

transport layer makes sure the entire message arrive the receiving transport layer

without error. Error correction is achieved through retransmission.

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 5) Session Layer

The session Layer is the network dialog controller. It establishes, maintains, and

synchronizes the interaction between communicating systems.

 Dialog control: The session layer allows two systems to enter into dialog. It allows the

communication between two processes to take place either in half duplex or full duplex

(two -way at a time)

 Synchronization: The session lawyer allows a process to add synchronization points

into the stream of data. For example if a system is sending a file of about 2000

pages, it is advisable to insert a checkpoint at every 100 pages to ensure that every

100 page unit is received and acknowledge independently. In case if a crash happens

during the transmission of page 523, retransmission begins at page 501. Pages 1 to

500 need not be retransmitted.

6) Presentation Layer

This layer is concerned with syntax and semantics of the information exchange between two

systems.

 Translation: The processes (running programs) in two systems are usually exchanging

information in the form of character strings, numbers, and so on. This information

should be change to bit stream before being transmitted. Because different computers

use different encoding system, the presentation layer is responsible for the

interoperability between these different encoding methods.

 Encryption: To carry sensitive information, the system must be able to assure privacy.

Encryption means that the sender transforms the original information to another form

and sends the resulting message out over the network. Decryption reverses the original

process to transform the message back to its original form.

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  Compression: Data compression reduces the number of bits to be transmitted. It

becomes important in the transmission of multimedia such as audio, text and video.

7) Application layer

Application layer enables the user, whether human or software to access the network. It

provides user interfaces and support for services such as electronic mail, remote file access

and transfer, shared database management.

 Network future terminal: It is a software version of a physical terminal and allows a

user to logon on a remote host. To do so, the application creates a software emulation

of a terminal at the remote host. The user’s computer talks to the software terminal,

which, in turn, talks to the host, and vice versa. The remote host believes it is

communicating with one of its own terminals and allows you to log on.

 File Transfer Access and Management (FTAM): The application allows a user to access

file in a remote computer to retrieve files from a remote computer.

 Mail services: The application provides the basis for e-mail forwarding and storage.

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