
2/23

Chapter-1

OVERVIEW OF TELECOMMUNICATION NETWORKS

Introduction

The telephone is a telecommunication device that is used to transmit and receive

electronically or digitally encoded speech between two or more people conversing. It is one

of the most common household appliances in the world today. Most telephones operate

through transmission of electric signals over a complex telephone network which allows

almost any phone user to communicate with almost any other user.

Telecommunication networks carry information signals among entities, which are

geographically far apart. An entity may be a computer or human being, a facsimile

machine, a teleprinter, a data terminal and so on. The entities are involved in the process of

information transfer that may be in the form of a telephone conversation (telephony) or afile transfer between two computers or message transfer between two terminals etc.

With the rapidly growing traffic and untargeted growth of cyberspace,

telecommunication becomes a fabric of our life. The future challenges are enormous as we

anticipate rapid growth items of new services and number of users. What comes with the

challenge is a genuine need for more advanced methodology supporting analysis and

design of telecommunication architectures. Telecommunication has evaluated and growth

at an explosive rate in recent years and will undoubtedly continue to do so.

The communication switching system enables the universal connectivity. The

universal connectivity is realized when any entity in one part of the world can

communicate with any other entity in another part of the world. In many ways

telecommunication will acts as a substitute for the increasingly expensive physical

transportation.

The telecommunication links and switching were mainly designed for voice

communication. With the appropriate attachments/equipments, they can be used to transmit

data. A modern society, therefore needs new facilities including very high bandwidth

switched data networks, and large communication satellites with small, cheap earth

antennas.

Voice Signal Characteristics

Telecommunication is mainly concerned with the transmission of messages

between two distant points. The signal that contains the messages is usually converted into

electrical waves before transmission. Our voice is an analog signal, which has amplitude

and frequency characteristics.

Voice frequencies: - The range of frequencies used by a communication device

determines the communication channel, communicating devices, and bandwidth or

2


3/23

information carrying capacity. The most commonly used parameter that characterizes an

electrical signal is its bandwidth of analog signal or bit rate if it is a digital signal. In

telephone system, the frequencies it passes are restricted to between 300 to 3400 Hz.

In the field oftelecommunications, a Telephone exchange or a Telephone switch is

a system of electronic components that connects telephone calls. A central office is the

physical building used to house inside plant equipment including telephone switches,

which make telephone calls "work" in the sense of making connections and relaying the

speech information.

Switching system fundamentals

Telecommunications switching systems generally perform three basic functions:

they transmit signals over the connection or over separate channels to convey the identity

of the called (and sometimes the calling) address (for example, the telephone number), and

alert (ring) the called station; they establish connections through a switching network forconversational use during the entire call; and they process the signal information to control

and supervise the establishment and disconnection of the switching network connection.

In some data or message switching when real-time communication is not needed,

the switching network is replaced by a temporary memory for the storage of messages.

This type of switching is known as store-and-forward switching.

Signaling and control

The control of circuit switching systems is accomplished remotely by a specific

form of data communication known as signaling. Switching systems are connected withone another by telecommunication channels known as trunks. They are connected with the

served stations or terminals by lines.

In some switching systems the signals for a call directly control the switching

devices over the same path for which transmission is established. For most modern

switching systems the signals for identifying or addressing the called station are received

by a central control that processes calls on a time-shared basis. Central controls receive and

interpret signals, select and establish communication paths, and prepare signals for

transmission. These signals include addresses for use at succeeding nodes or for alerting

(ringing) the called station.

Most electronic controls are designed to process calls not only by complex logic but

also by logic tables or a program of instructions stored in bulk electronic memory. The

tabular technique is known as translator. The electronic memory is now the most accepted

technique and is known as stored program control (SPC). Either type of control may be

distributed among the switching devices rather than residing centrally. Microprocessors on

integrated circuit chips are a popular form of distributed stored program control.

3
http://en.wikipedia.org/wiki/Telecommunicationshttp://en.wikipedia.org/wiki/Inside_planthttp://en.wikipedia.org/wiki/Telephone_callhttp://www.answers.com/topic/spc-abbreviationhttp://en.wikipedia.org/wiki/Inside_planthttp://en.wikipedia.org/wiki/Telephone_callhttp://www.answers.com/topic/spc-abbreviationhttp://en.wikipedia.org/wiki/Telecommunications


4/23

Switching fabrics

Space and time division are the two basic techniques used in establishing

connections. When an individual conductor path is established through a switch for the

duration of a call, the system is known as space division. When the transmitted speech

signals are sampled and the samples multiplexed in time so that high-speed electronic

devices may be used simultaneously by several calls, the switch is known as time division.

In the early stages of development in telecommunication, manual switching

methods were deployed. But later on to overcome the limitations of manual switching;

automatic exchanges, having Electro-mechanical components, were developed. Strowger

exchange, the first automatic exchange having direct control feature, appeared in 1892 in

La Porte (Indiana). Though it improved upon the performance of a manual exchange it still

had a number of disadvantages, viz., a large number of mechanical parts, limited

availability, inflexibility, bulky in size etc. As a result of further research and development,

Crossbar exchanges,having an indirect control system, appeared in 1926 in Sweden.The Crossbar exchange improved upon many short- comings of the Strowger

system. However, much more improvement was expected and the revolutionary change in

field of electronics provided it. A large number of moving parts in Register, marker,

Translator, etc., were replaced en-block by a single computer. This made the exchange

smaller in size, volume and weight, faster and reliable, highly flexible, noise-free, easily

manageable with no preventive maintenance etc.

Network Architecture.

When electronic devices were introduced in the switching systems, a new conceptof switching evolved as a consequence of their extremely high operating speed compared

to their former counter-parts, i.e., the Electro-mechanical systems, where relays, the logic

elements in the electromechanical systems, have to operate and release several times which

is roughly equal to the duration of telephone signals to maintain required accuracy.

Research on electronic switching started soon after the Second World War, but

commercial fully electronic exchange began to emerge only about 30 years later. However,

electronic techniques proved economic for common control systems much earlier. In

electromechanical exchanges, common control systems mainly used switches and relays,

which were originally designed for use in switching networks. In common controls, they

are operated frequently and so wear out earlier. In contrast, the life of an electronic device

is almost independent of its frequency of operation. This gave a motivation for developing

electronic common controls and resulted in electronic replacements for registers, markers,

translators etc. having much greater reliability than their electromechanical predecessors.

In electromechanical switching, the various functions of the exchange are achieved

by the operation and release of relays and switch (rotary or crossbar) contacts, under the

direction of a Control Sub-System. These contracts are hard - wired in a predetermined

4


5/23

way. The exchange dependent data, such as subscribers class of service, translation and

routing, combination signaling characteristics are achieved by hard-ware and logic, by a of

relay sets, grouping of same type of lines, strapping on Main or Intermediate Distribution

Frame or translation fields, etc. When the data is to be modified, for introduction of a new

service, or change in services already available to a subscriber, the hardware change

ranging from inconvenient to near impossible, are involved.

In an SPC exchange, a processor similar to a general-purpose computer is used to

control the functions of the exchange. All the control functions, represented by a series of

various instructions, are stored in the memory. Therefore the processor memories hold all

exchange dependent data. such as subscriber date, translation tables, routing and charging

information and call records. For each call processing step. e.g. for taking a decision

according to class of service, the stored data is referred to, Hence, this concept of

switching. The memories are modifiable and the control program can always be rewritten if

the behavior or the use of system is to be modified. This imparts and enormous flexibility

in overall working of the exchange.Digital computers have the capability of handling many tens of thousands of

instructions every second, Hence, in addition to controlling the switching functions the

same processor can handle other functions also. The immediate effect of holding both the

control programme and the exchange data, in easily alterable memories, is that the

administration can become much more responsive to subscriber requirements. both in terms

of introducing new services and modifying general services, or in responding to the

demands of individual subscriber. For example, to restore service on payment of an

overdue bill or to permit change from a dial instrument to a multi frequency sender, simply

the appropriate entries in the subscriber data-file are to be amended. This can be done by

typing- in simple instructions from a teletypewriter or visual display unit. The ability of theadministration to respond rapidly and effectively to subscriber requirements is likely to

become increasingly important in the future.

The modifications and changes in services which were previously impossible be

achieved very simply in SPC exchange, by modifying the stored data suitably. In some

cases, the subscribers can also be given the facility to modify their own data entries for

supplementary services, such as on-demand call transfer, short code (abbreviated) dialing,

etc.

The use of a central processor also makes possible the connection of local and

remote terminals to carry out man-machine dialogue with each exchange. Thus, the

maintenance and administrative operations of all the SPC exchanges in a network can be

performed from a single centralized place. The processor sends the information on the

performance of the network, such as, traffic flow, billing information, faults, to the centre,

which carries out remedial measures with the help of commands. Similarly, other

modifications in services can also be carried out from the remote centre. This allows a

better control on the overall performance of the network.

5


6/23

As the processor is capable of performing operations at a very high speed, it has got

sufficient time to run routine test programmes to detect faults, automatically. Hence, there

is no need to carry out time consuming manual routine tests.

In an SPC exchange, all control equipment can be replaced by a single processor.

The processor must therefore be quite powerful, typically it must process hundreds of calls

per second, in addition to performing other administrative and maintenance tasks.

However, totally centralized control has drawbacks. The software for such a central

processor will be voluminous, complex, and difficult to develop reliably. Moreover, it is

not a good arrangement from the point of view of system security, as the entire system will

collapse with the failure of the processor. These difficulties can be overcome by

decentralizing the control. Some routine functions such as scanning, signal distributing,

marking, which are independent of call processing, can be delegated to auxiliary or

peripheral processors.

Stored program control (SPC)has become the principal type of control for all types

of new switching systems throughout the world, including private branch exchanges, dataand Telex systems. Two types of data are stored in the memories of electronic switching

systems. One type is the data associated with the progress of the call, such as the dialed

address of the called line.

Another type, known as the translation data, contains infrequently changing information,

such as the type of service subscribed to by the calling line and the information required for

routing calls to called numbers. These translation data, like the program, are stored in a

memory, which is easily read but protected to avoid accidental erasure. This information

may be readily changed, however, to meet service needs. The flexibility of a stored

program also aids in the administration and maintenance of the service so that system faults

may be located quickly.

SPC exchanges can offer a wider range of facilities than earlier systems. In

addition, the facilities provided to an individual customer can be readily altered by

changing the customers class-of-service data stored in memory. Moreover, since the

processors stored data can be altered electronically,some of these facilities can be

controlled by customers. Examples include:-

1. Call barring (outgoing or incoming): The customer can prevent unauthorized calls

being made and can prevent incoming calls when wishing to be left in peace.

2. Call waiting: The Call waiting service notifies the already busy subscriber of a

third party calling him.3. Alarm calls: The exchange can be instructed to call the customer at a pre-arranged

time (e.g. morning alarm).

4. Call Forwarding: The subscriber having such a feature can enable the incoming

calls coming to his telephone to be transferred to another number during his

absence.

5. Conference calls: Subscriber can set up connections to more than one subscriber

and conduct telephone conferences under the provision of this facility.

6


7/23

6. Dynamic Barring Facility: Subscriber having STD/ISD facilities can dynamically

lock such features in their telephone to avoid misuse. Registering and dialing a

secret code will extend such such a facility.

7. Abbreviated Dialing: Most subscribers very often call only limited group of

telephone numbers. By dialing only prefix digit followed by two selection digits,

subscribers can call up to 100 predetermined subscribers connected to any

automatic exchange. This shortens the process of dialing all the digits.

8. Malicious call Identification: Malicious call identification is done immediately and

the information is obtained in the print out form either automatically or by dialing

an identification code.

9. Do Not Disturb: This facility enables the subscriber to free himself from attending

his incoming calls. Using this facility the calls coming to the subscriber can be

routed to an operator position or to an answering machine. The operator position or

the machine can inform the calling subscriber that the called subscriber is

temporarily inaccessible. Today SPC is a standard feature in all the electronicexchanges.

Implementation of Switching Network.

In an electronic exchange, the switching network is one of the largest sub-system in

terms of size of the equipment. Its main functions are Switching (setting up temporary

connection between two or more exchange terminations), Transmission of speech and

signals between these terminations, with reliable accuracy.

There are two types of electronic switching system. viz. Space division and Time

Division.

Space Division switching SystemIn a space Division Switching system, a continuous physical path is set up between

input and output terminations. This path is separate for each connection and is held for the

entire duration of the call. Path for different connections is independent of each other. Once

a continuous path has been established., Signals are interchanged between the two

terminations. Such a switching network can employ either metallic or electronic cross

points. Previously, usage of metallic cross-points using reed relays and all were favored.

They have the advantage of compatibility with the existing line and trunk signaling

conditions in the network.

Time Division Switching SystemIn Time Division Switching, a number of calls share the same path on time division

sharing basis. The path is not separate for each connection, rather, is shared sequentially for

a fraction of a time by different calls. This process is repeated periodically at a suitable

high rate. The repetition rate is 8 KHz, i.e. once every 125 microseconds for transmitting

speech on telephone network, without any appreciable distortion. These samples are time

multiplexed with staggered samples of other speech channels, to enable sharing of one path

7


8/23

by many calls. The Time Division Switching was initially accomplished by Pulse

Amplitude

Modulation (PAM) Switching. However, it still could not overcome the

performance limitations of signal distortion noise, cross-talk etc. With the advent of Pulse

Code Modulation (PCM), the PAM signals were converted into a digital format

overcoming the limitations of analog and PAM signals. PCM signals are suitable for both

transmission and switching. The PCM switching is popularly called Digital Switching.

Digital Switching Systems

A Digital switching system, in general, is one in which signals are switched in

digital form. These signals may represent speech or data. The digital signals of several

speech samples are time multiplexed on a common media before being switched through

the system.

To connect any two subscribers, it is necessary to interconnect the time-slots of the

two speech samples, which may be on same or different PCM highways. The digitalized

speech samples are switched in two modes, viz., Time Switching and Space Switching.This Time Division Multiplex Digital Switching System is popularly known as Digital

Switching System.

The ESS No.1 system was the first fully electronic switching system but not digital.

But later came ESS No.4 system which was digital for trunk portion only. When designed,

the cost of A/D conversion (CODEC) on each subscriber line was seen as prohibitive. So

the ESS No.4 system was acting as a Trunk/Tandem exchange but not as a local exchange.

So the main difficulty for implementing a digital local exchange was the implementation of

the subscriber line interface. This was solved by the introduction of Integrated Circuits,

which made the digital local exchange economically feasible. This implementation handles

the following functions:B-Battery feed

O-Over-voltage protection (from lightning and accidental power line contact)

R-Ringing

S-Supervisory Signaling

C-Coding (A/D inter conversion & low pass filtering)

H-Hybrid (2W to 4W conversion)

T-Testing the connectivity of Subscriber

Examples of digital exchanges (switching systems) include CDOT, OCB, AXE, EWSD,

5ESS etc.

The general architecture of a Digital Switching System is depicted in

fig2General architecture of Digital Switching System

8 Subs interface

N x 2Mbps

links


9/23

Figure-2

The next evolutionary step was to move the PCM codec from the exchange

end of the customers line to the customers end. This provides digital transmission over

the customers line, which can have a number of advantages. Consider data transmission.

If there is an analog customers line, a modem must be added and data can only be

transmitted at relatively slow speeds. If the line is digital, data can be transmitted by

removing the codec (instead of adding a modem). Moreover, data can be transmitted at 64

kbit/s instead of at, say, 2.4 kbit/s. Indeed, any form of digital signal can be transmitted

whose rate does not exceed 64 kbit/s. This can include high-speed fax, in addition to

speech and data.This concept had led to the evolution of Integrated services digital network

(ISDN), in which the customers terminal equipment and the local digital exchange can be

used to provide many different services, all using 64 kbit/s digital streams. In simple terms,

we can say ISDN provides end-to-end digital connectivity.

Access to an ISDN is provided in two forms:

9

Other

exchanges

CONTROL

PROCESSOR

Operation &

Maintenance

Trunks interface

Other auxiliary inter facesSuch as,

(a) Tone generator

(b) Frequency receives(c) Conference call facility

(d) CCS# 7 Protocol Manager

(e) V 5.2 access manager

Digital Switch


10/23

1. Basic-Rate Access (BRA)

The customers line carries two 64 kbit/s B channels plus a 16 kbit/s D

channel (a common signaling channel) in each direction.

2. Primary Rate Access (PRA)

The line carries a complete PCM frame at 2 Mbit/s in each direction. This

gives the customer 30 circuits at 64 kbit/s plus a common signaling channel, also at 64

kbit/s.

Control of switching systems

Switching systems have evolved from being manually controlled to being

controlled by relays and then electronically. The change from the manual system to the

Strowger step-by-step system brought about a change from centralized to distributedcontrol. However, as systems developed and offered more services to customers, it became

economic to perform particular functions in specialized equipments that were associated

with connections only when required, thus, common control was introduced.

Later, the development of digital computer technology enabled different functions

to be performed by the same hardware by using different programs; thus switching system

entered the era of stored-program control (SPC).

There are basically two approaches to organizing stored program control:

centralized and distributed. Early electronic switching systems (ESS) developed during the

period 1970-75 almost invariably used centralized control. Although many present day

exchange designs continue to use centralized SPC, with the advent of low cost powerful

microprocessors and very large scale integration (VLSI) chips such as programmable logic

arrays (PLA) and programmable logic controllers (PLC), distributed SPC is gaining

popularity.

The figure below shows the evolution of electronic switching systems from the

manual switching systems. The figure also depicts the changing scenario from digital

switching to Broadband where the focus will be for high bit rate data transmissions.

Development of exchanges

10


11/23

Figure 3

Local and trunk NetworkTrunk Lines

The term Trunk Line in telecommunications refers to the high-speed connection

between telephone central offices in the Public Switched Telephone Network (PSTN).

Trunk lines are always digital. The wiring between central offices was originally just pairs

of twisted copper wire (the twists in the wiring prevented things known as crosstalk and

noise). Because it is expensive to string up (or lay trenches for buried cables), the phone

company researched ways in which to carry more data over the existing copper lines. This

was achieved by using time-division multiplexing. Later, when fiber-optic technology

became available, phone companies upgraded their trunk lines to fiber optics and used

statistical time-division multiplexing, synchronous digital heirarchy, coarse or dense wavedivision multiplexing and optical switching to further improve transmission speeds.

The signaling information exchanged between different exchanges via inter

exchange trunks for the routing of calls is termed as Inter exchange Signaling. Earlier in

band /out of band frequencies were used for transmitting signaling information. Later on,

11
http://www.inetdaemon.com/tutorials/telecom/pstn/index.shtmlhttp://www.inetdaemon.com/tutorials/telecom/t-carrier/time-division_multiplexing.shtmlhttp://www.inetdaemon.com/tutorials/telecom/sdh/index.shtmlhttp://www.inetdaemon.com/tutorials/telecom/pstn/index.shtmlhttp://www.inetdaemon.com/tutorials/telecom/t-carrier/time-division_multiplexing.shtmlhttp://www.inetdaemon.com/tutorials/telecom/sdh/index.shtml


12/23

with the emergence of PCM systems, it was possible to segregate the signaling from the

speech channel. A trunk line is a circuit connecting telephone switchboards (or other

switching equipment), as distinguished from local loop circuit which extends from

telephone exchange switching equipment to individual telephones or information

origination/termination equipment.

When dealing with a private branch exchange (PBX), trunk lines are the phone

lines coming into the PBX from the telephone provider. This differentiates these incoming

lines from extension lines that connect the PBX to (usually) individual phone sets.

Trunking saves cost, because there are usually fewer trunk lines than extension lines, since

it is unusual in most offices to have all extension lines in use for external calls at once.

Trunk lines transmit voice and data in formats such as analog, T1,E1, ISDNorPRI. The

dial tone lines for outgoing calls are called DDCO (Direct Dial Central Office) trunks.

Asignal travelling over a trunk line is not actually flowing any faster. The

electrical signal on a voice line takes the same amount of time to traverse the wire as a

similar length trunk line. What makes trunk lines faster is that the signal has been altered tocarry more data in less time using more advanced multiplexing and modulation techniques.

If you compared a voice line and a trunk line and put them side by side and observed them,

the first pieces of information arrive simultaneously on both the voice and trunk line.

However, the last piece of information would arrive sooner on the trunk line. No matter

what, you can't break the laws of physics. Electricity over copper or laser light over fiber

optics, you cannot break the speed of light--though that has rarely stopped uneducated IT

or IS managers from demanding that cabling perform faster instead of upgrading

equipment.

Trunk lines can contain thousands of simultaneous calls that have been combined

using time-division multiplexing. These thousands of calls are carried from one centraloffice to another where they can be connected to a de-multiplexing device and switched

through digital access cross connecting switches to reach the proper exchange and local

phone number.

Local and trunk Network

12

09 TR TR L S

s

S

L

CI

D

TR

TR

L S

S

CI

A

CT

I
http://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Telephone_switchboardhttp://en.wikipedia.org/wiki/Local_loophttp://en.wikipedia.org/wiki/Local_loophttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/Telephoneshttp://en.wikipedia.org/wiki/Telephoneshttp://en.wikipedia.org/wiki/Private_branch_exchangehttp://en.wikipedia.org/wiki/Extension_(telephone)http://en.wikipedia.org/wiki/Extension_(telephone)http://en.wikipedia.org/wiki/Digital_signal_1http://en.wikipedia.org/wiki/Digital_signal_1http://en.wikipedia.org/wiki/Digital_signal_1http://en.wikipedia.org/wiki/ISDNhttp://en.wikipedia.org/wiki/ISDNhttp://en.wikipedia.org/wiki/Primary_rate_interfacehttp://en.wikipedia.org/wiki/Primary_rate_interfacehttp://en.wikipedia.org/wiki/Dial_tonehttp://www.inetdaemon.com/tutorials/basic_concepts/communication/signals/index.shtmlhttp://www.inetdaemon.com/tutorials/basic_concepts/communication/signals/index.shtmlhttp://www.inetdaemon.com/tutorials/basic_concepts/communication/signals/index.shtmlhttp://www.inetdaemon.com/tutorials/basic_concepts/communication/modulation/index.shtmlhttp://www.inetdaemon.com/tutorials/telecom/t-carrier/time-division_multiplexing.shtmlhttp://en.wikipedia.org/wiki/Electrical_networkhttp://en.wikipedia.org/wiki/Telephone_switchboardhttp://en.wikipedia.org/wiki/Local_loophttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/Telephoneshttp://en.wikipedia.org/wiki/Private_branch_exchangehttp://en.wikipedia.org/wiki/Extension_(telephone)http://en.wikipedia.org/wiki/Digital_signal_1http://en.wikipedia.org/wiki/Digital_signal_1http://en.wikipedia.org/wiki/ISDNhttp://en.wikipedia.org/wiki/Primary_rate_interfacehttp://en.wikipedia.org/wiki/Dial_tonehttp://www.inetdaemon.com/tutorials/basic_concepts/communication/signals/index.shtmlhttp://www.inetdaemon.com/tutorials/basic_concepts/communication/signals/index.shtmlhttp://www.inetdaemon.com/tutorials/basic_concepts/communication/modulation/index.shtmlhttp://www.inetdaemon.com/tutorials/telecom/t-carrier/time-division_multiplexing.shtml


13/23

S : Remote line unit

L : Local subscriber exchange

TR : Transit exchangeCID : Outgoing international exchange

CIA : Incoming international exchange

CTI : International transit exchange

What is Trunking?

In telecommunications systems, trunking is the aggregation of multiple user circuits into a

single channel. The aggregation is achieved using some form of multiplexing. Trunking

13


14/23

theory was developed by Agner Krarup Erlang, Erlang based his studies of the statistical

nature of the arrival and the length of calls. The Erlang B formula allows for the calculation

of the number of circuits required in a trunk based on the Grade of Service and the amountof traffic in Erlangs the trunk needs cater for.

Definition

In order to provide connectivity between all users on the network one solution is to build a

full mesh network between all endpoints. A full mesh solution is however impractical, a farbetter approach is to provide a pool of resources that end points can make use of in order to

connect to foreign exchanges. The diagram below illustrates the where in a

telecommunication network trunks are used.

A Modern Telephone Network Indicating where trunks are used. SLC - Subscriber

line concentrator

14
http://en.wikiversity.org/wiki/Wikipediahttp://en.wikiversity.org/wiki/File:Trunking.jpghttp://en.wikiversity.org/wiki/Wikipedia


15/23

LE Local Exchange

TDM TAX II Level II Tax

TDM TAX I Level I Tax

Level I Taxs are connected to the Gateway.

Call routing

Routing in the PSTN is the processused to route telephone calls across the public switchedtelephone network. This process is the same whether the call is made between two phones

in the same locality, or across two different continents.

15
http://en.wikipedia.org/wiki/Telephonehttp://en.wikipedia.org/wiki/Public_switched_telephone_networkhttp://en.wikipedia.org/wiki/Public_switched_telephone_networkhttp://en.wikipedia.org/wiki/Telephonehttp://en.wikipedia.org/wiki/Public_switched_telephone_networkhttp://en.wikipedia.org/wiki/Public_switched_telephone_network


16/23

Relationship between exchanges and operators

Telephone calls must be routed across a network of multiple exchanges, potentially owned

by different telephone operators. The exchanges are all are inter-connected together usingtrunks. Each exchange has many "neighbours", some of which are also owned by the same

telephone operator, and some of which are owned by different operators. Whenneighbouring exchanges are owned by different operators, they are known as interconnectpoints.

This means that there is really only one virtual network in the world that enables any phone

to call any other phone. This virtual network comprises many interconnected operators,

each with their own exchange network. Every operator can then route calls directly to theirown customers, or pass them on to another operator if the call is not for one of their

customers.

The PSTN is not a fully meshed networkwith every operator connected to every other -

that would be both impractical and inefficient. Therefore calls may be routed throughintermediate operator networks before they reach their final destination. One of the major

problems in PSTN routing is determining how to route this call in the most cost effective

and timely manner.

Call routing

Each time a call is placed for routing, the destination number(also known as the calledparty) is entered by the calling party into their terminal. The destination number generally

has two parts, a prefix which generally identifies the geographical location of the

destination telephone, and a number unique within that prefix that determines the specific

destination terminal. Sometimes if the call is between two terminals in the same local area(that is, both terminals are on the same telephone exchange), then the prefix may be

omitted.

When a call is received by an exchange, there are two treatments that may be applied:

Either the destination terminal is directly connected to that exchange, in which case

the call is placed down that connection and the destination terminal rings.

Or the call must be placed to one of the neighbouring exchanges through a

connecting trunk for onward routing.

Each exchange in the chain uses pre-computed routing tables to determine which connectedexchange the onward call should be routed to. There may be several alternative routes to

any given destination, and the exchange can select dynamically between these in the event

of link failure orcongestion.

The routing tables are generated centrally based on the known topology of the network, the

numbering plan, and analysis oftraffic data. These are then downloaded to each exchangein the telephone operators network. Because of the hierarchical nature of the numbering

16
http://en.wikipedia.org/wiki/Telephone_operatorhttp://en.wikipedia.org/wiki/Trunkinghttp://en.wikipedia.org/wiki/Telephone_operatorhttp://en.wikipedia.org/wiki/Core_networkhttp://en.wikipedia.org/wiki/Mesh_networkinghttp://en.wikipedia.org/wiki/Telephone_numberhttp://en.wikipedia.org/wiki/Telephone_numbering_planhttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/Network_congestionhttp://en.wikipedia.org/wiki/Network_topologyhttp://en.wikipedia.org/wiki/Telephone_numbering_planhttp://en.wikipedia.org/wiki/Traffic_analysishttp://en.wikipedia.org/wiki/Telephone_operatorhttp://en.wikipedia.org/wiki/Telephone_operatorhttp://en.wikipedia.org/wiki/Trunkinghttp://en.wikipedia.org/wiki/Telephone_operatorhttp://en.wikipedia.org/wiki/Core_networkhttp://en.wikipedia.org/wiki/Mesh_networkinghttp://en.wikipedia.org/wiki/Telephone_numberhttp://en.wikipedia.org/wiki/Telephone_numbering_planhttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/Network_congestionhttp://en.wikipedia.org/wiki/Network_topologyhttp://en.wikipedia.org/wiki/Telephone_numbering_planhttp://en.wikipedia.org/wiki/Traffic_analysishttp://en.wikipedia.org/wiki/Telephone_operator


17/23

plan, and its geographical basis, most calls can be routed based only on their prefix using

these routing tables.

Some calls however cannot be routed on the basis of prefix alone, for example non-geographical numbers, such as toll-free or freephone calling. In these cases the Intelligent

Networkis used to route the call instead of using the pre-computed routing tables.

In determining routing plans, special attention is paid for example to ensure that two routes

do not mutually overflow to each other, otherwise congestion will cause a destination to becompletely blocked.

According to Braess' paradox, the addition of a new, shorter, and lower cost route can lead

to an increase overall congestion[. The network planner must take this into account when

designing routing paths.

One approach to routing involves the use of Dynamic Alternative Routing (DAR). DAR

makes use of the distributed nature of a telecommunications network and its inherentrandomness to dynamically determine optimal routing paths. This method generates a

distributed, random, parallel computing platform that minimises congestion across thenetwork, and is able to adapt to take changing traffic patterns and demands into account.

Routing can be loosely described as the process of getting from here to there. Routing may

be discussed in the context of telephone networks or computer networks. In telephone

networks, routing is facilitated by switches in the network, whereby in computer networksrouting is performed by routers in the network.

Definition: Routing in telephone networks

Routing in the context oftelephone networks is the selection of a specific circiut group, for

a given call or traffic stream, at an exchange in the network . "The objective of routing is toestablish a successful connection between any two exchangesin the network" . By selecting

routes that meet the constraints set by the user traffic and the network, routing determines

which network resources (circuit group) should be used to transport which user traffic.

Different networks employ different routing techniques, but all communication networks

share a basic routing functionality based on three core routing functions

Assembling and distributing information on the state of the network and user traffic that is

used to generate and select routes.

Generating and selecting feasible and optimal routes based on network and user traffic state

information.

Forwarding user traffic along the selected routes.

Thepublic switched telephone network (PSTN)architecture is made up of a hierarchy ofexchanges (e.g local and regoinal exchanges) with each level of the hierarchy performing

different functions . Two adjacent exchanges in the network may be connected by several

direct routes consisting of one or more circuits .

17
http://en.wikipedia.org/wiki/Non_Geographical_Numbershttp://en.wikipedia.org/wiki/Non_Geographical_Numbershttp://en.wikipedia.org/wiki/Toll-free_telephone_numberhttp://en.wikipedia.org/wiki/Intelligent_Networkhttp://en.wikipedia.org/wiki/Intelligent_Networkhttp://en.wikipedia.org/wiki/Braess'_paradoxhttp://en.wikipedia.org/wiki/Telephone_networkhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Routerhttp://en.wikipedia.org/wiki/Telephone_networkhttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/PSTNhttp://en.wikipedia.org/wiki/PSTNhttp://en.wikipedia.org/wiki/PSTNhttp://en.wikipedia.org/wiki/Non_Geographical_Numbershttp://en.wikipedia.org/wiki/Non_Geographical_Numbershttp://en.wikipedia.org/wiki/Toll-free_telephone_numberhttp://en.wikipedia.org/wiki/Intelligent_Networkhttp://en.wikipedia.org/wiki/Intelligent_Networkhttp://en.wikipedia.org/wiki/Braess'_paradoxhttp://en.wikipedia.org/wiki/Telephone_networkhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Routerhttp://en.wikipedia.org/wiki/Telephone_networkhttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/PSTN


18/23

In circuit-switched networks, such as the PSTN, switching and transmission resources are

dedicated to a call along the path from source to destination for the complete duration of

the call. Routing decisions are imperative in facilitating this process as they determine themost efficient links to use to connect users for a call . Routing in the PSTN is done using a

hop-by-hop approach . When a user wants to make a call, they dial the destination number

to which the call should be routed. This destination number is made up of a prefix (areacode or national destination network), which identifies the geographical location of the

called party, and a unique number (the subscriber number) linked to the prefix that

identifies the exact destination to which the call should be routed The end exchange towhich the calling party is connected (the originating exchange) uses the area code to

identify the outgoing circuit group connecting to the first choice adjacent exchange en-

route This circuit group is called the first choice route and is obtained using a routing table

at the originating switch . The function of the switch at the originating end exchange is toconnect the switch input port to which the calling user is connected to a free outgoing

circuit group in the first choice group . If all the circuits along the first choice route are

fully occupied, the switch then attempts to use an alternative route circuit group to route the

call to the destination exchange . The originating exchange then forwards the address to theadjacent exchange (first choice or alternate route), and the procedure is repeated at the

adjacent exchange in order to reach the destination end exchange to which the called partyis connected . When the address reaches the destination exchange, it only needsto process

the last part of the address to identify the switch input port that the called party is

connected .Routing directs forwarding . Forwarding of traffic can be done using connection-oriented

or connectionless approaches . In connection-oriented forwrding, forwarding instructions

are installed in all the switches along a designated route before the route can be used to

transport traffic . Traffic forwarded using the connectionless approach carries its ownforwarding information either as precise routing commands for each switch along a route

or as hints that may be autonomously interpreted by any switch in the network .

InPSTN, forwarding of traffic is based on the connection-oriented approach. Call routing

is achieved using pre-computed routing tables, containing all the possible pre-definedroutes for a connection, at each switch .The pre-defined routes specified in the routing table

include information of a direct route (or routes) to be used under normal traffic and

network conditions (e.g no link failure ornetwork congestion) as well as alternative routesthat should be used in the event that all circuits along the direct route are fully occupied .

An alternative route may be an indirect route consisting of several circuit groups

connecting two exchanges via other exchanges . The following example illustrates the useof an alternative route to connect two exchanges in the event of the direct route being

congetsed.

A Typical Telephone Exchange -OCB-283FUNCTIONAL ARCHITURE

The Alcatel E10 system is located at the heart of the telecommunication networks

concerned. It is made up of three independent functional units:

- The Subscriber Access Subsystem which carries out connection of analogue anddigital subscriber lines,

18
http://en.wikipedia.org/wiki/PSTNhttp://en.wikipedia.org/wiki/Routing_tableshttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/PSTNhttp://en.wikipedia.org/wiki/PSTNhttp://en.wikipedia.org/wiki/PSTNhttp://en.wikipedia.org/wiki/Routing_tableshttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Routing_tableshttp://en.wikipedia.org/wiki/Network_congestionhttp://en.wikipedia.org/wiki/PSTNhttp://en.wikipedia.org/wiki/Routing_tableshttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/PSTNhttp://en.wikipedia.org/wiki/Routing_tableshttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Routing_tableshttp://en.wikipedia.org/wiki/Network_congestion


19/23

- Connection and Control which carries out connections and processing of calls,

- Operation and Maintenance which is responsible for all functions needed by the

network operating authority.

Each functional unit is equipped with softwares which are appropriate for handling the

functions for which it is responsible.Synchronization and Time Base Station STS

Time base (BT)

The BT ensures times distribution for LR and PCM to provide the synchronization, and

also for working out the exchange clock.Time distribution is tripled.

Time generation can be either autonomous or slaved to an external rhythm with a view tosynchronise the system with the network

Auxiliary Equipment Control Station SMA

Auxiliary equipment manager (ETA)

The ETA Supports:

- The tone generators (GT).

- The frequency receiving and generation (RGF) devices,

- Conference circuits (CCF),

- The exchange clock

CCS7 protocol handler (PUPE) and CCS7 controller (PC): CCITT No. 7 protocol

processing

For connection of 64 kbit/s signaling channels, semi- permanent connections areestablished via the connection matrix, to the PUPE which processes the CCITT No. 7

protocol.

More precisely, the PUPE function carries out the following:

- signaling channel Level 2 processing,

- the message routing function

(Part of Level 3). The PC carries out:

- the network management function (part of Level 3),

- PUPE defence,

- Various observation tasks which are not directly linked to CCITT No. 7.

19

OCB 283

SUBCRIBER

ACCESS

SUBSYSTEM

CONNECTION

AND

CONTROL

OPERATION

AND

MAINTENANCE

DATA

NETWORK

TELEPHONE

NETWORK

VALUE ADDED

NETWORK

CCITT N07

SIGNALLING

NETWORK

NT

PABXALCATEL 1000 E10

OCB 283

OPERATION ANDMAINTENANCE

NETWORK


20/23

Host switching matrix (SMX)

The SMX is a square connection matrix with a single time stage, T, duplicated in full,which enables up to 2048 matrix links (LR) to be connected.

20


21/23

A matrix link LR is an internal PCM, with 16 bits per channel (32 channels). The MCX can

execute the following:

1) an unidirectional connection between any incoming channel and any out goingchannel. There can be as many simultaneous connections as there are outgoing

channels. It should be remembered that a connection consists of allocating the

information contained within an incoming channel to an outgoing channel,

2) connection between any incoming channel and any M outgoing channels,

3) connection of N incoming channels belonging to one frame structure of anymultiplex onto N outgoing channels which belong to the same frame structure,

abiding to the integrity and sequencing of the frame received. This function is

referred to as connection with N x 64 kbit/s.

The MCX is controlled by the COM function (matrix switch controller) to ensure the:

- set up and breakdown of the connections by access to the matrix command memory.

This access is used to write at the output T.S. address the incoming T.S. address

-defense of the connections. Security of the connections in order to assure a good data

switching.

Truck Control Station SMT

PCM controller (URM)

The URM provides the interface between external PCMs and the OCB283. These PCMcome from either:

- a remote subscriber digital access unit (CSN) or from a remote electronic satellite

concentrator CSE,

-

another switching centre, on channel-associated signalling or CCITT No.7,- the digital recorded announcement equipment

In particular, the URM carries out the following functions:

- HDB3 conversion to binary (PCM matrix link),

- binary conversion to HDB3 (matrix linkPCM),

- extraction and pre-processing of the channel-associated signalling of T.S.16 (PCM command),

- transmission of channel-associated signalling in T.S.16 (command PCM).

Main Control Station SMC

Call handler (MR)

The MR is responsible for the establishment and breaking off of communications.

The call handler takes the decisions necessary for processing of communications in terms

of the signaling received, after consultation of the subscriber and analysis database

manager (TR) if necessary. The call handler processes new calls and handling-upoperations, releases equipment, commands switching on and switching off etc.

21


22/23

In addition, the call handler is responsible for different management tasks (control of tests

of circuits, sundry observations).

Operation and maintenance function (OM) SMM

The functions of the operation and maintenance subsystem are carried out by the operation

and maintenance software OM).The operating authority accesses all hardware and software equipment of the Alcatel 1000

E10 system via computer terminals belonging to the operation and maintenance subsystem:

consoles, magnetic media, intelligent terminal. These functions can be grouped into 2categories:

- operation of the telephone application,

- operation and maintenance of the system.

In addition, the operation and maintenance subsystem carries out:

- loading of softwares and of data for connection and command and for the

subscriber digital access units,- temporary backup of detailed billing information,

- centralisation of alarm data coming from connection and control stations, via

alarm rings,

- central defence of the system.

Finally, the operation and maintenance subsystem permits two-way communication with

operation and maintenance networks, at regional or national level (TMN).

CSN - digital satellite center

The digital satellite center [CSN center satellite numerique) is a subscriber connection

unit on which both analogue and digital subscribers can be connected.

Its design and composition enable the CSN to fit into an existing network and can be

connected to time-based systems using the CCITT N 7 type of semaphore signalling.

The CSN is a connection unit designed to adapt to a variety of geographical situation: it canbe either local [CSNL] or distant [CSND] with respect to the connecting switch.

A Typical Telephone Exchange -OCB-283

22


23/23

CSN : Digital satellite center

SMC : Main Control Station

SMA : Auxiliary Equipment Control Station

SMT : Truck Control Station

SMX : Matrix Control Station

SMM : Maintenance Station

STS : Synchronization and Time Base Station

23

SMX

STS

1 x 3

CSNN

NN

LCS

NN

DCS

EDCircuits and

announcemen

t machine

L

R

SMT

( 1 TO 28) X 2

SMA

( 2 TO 37)

SMC

2 TO 14

1 TO 4 MAS

1

MIS

SMM1 x 2

L

R

L

R

A

LT

M

N

Documents

Telecom Network I