Ece 542

Telecommunication Engineering (ECE-542). Lecturer: Engr. M. A. Ahaneku

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COURSE OUTLINES

1. Switching Networks

2. Traffic and Trunking

3. Signaling Types

4. Principles of Call Set-up

5. The Telephone Network and structure

6. Integrated Services Digital Network (ISDN)

7. Digital Transmission Hierarchy

8. Digital Switching

9. The Public switched Telephone System.

10.Mobile Communication Systems

11.GSM Technology and Systems

TEXTS:

1. Modern Communications Switching System by Hobbs

2. Telecommunication Engineering by J. Duncop & D. G. Smith

3. Communication Systems Analysis and Design

A System Approach by Richard A. Williams

4. Principles of Communication Engineering by Anokh Singh

NOTE: An important characteristic of circuit switching device is whether it

is blocking or not blocking.

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TELECOMMUNICATION ENGINEERING

Introduction: Telecommunication is the process of passing information energy

over long distances by electrical means. The information energy is passed to the

destination either over suitable insulated conduction wires called transmission

lines, or through the atmosphere without wire but via a radio link.

Telecommunication Engineering therefore can be said to be the study of the

technology involved in the design of telecommunications systems.

Definition: Telephone switching is the means by which a communication

channels, capable of carrying analog or digital information between two or

more subscribers, is established and maintained.

Any modern telecommunications switching system consists of a great

intricate equipments and components combined into an overall system

operation along certain well defined principles. A typical electromechanical

automatic switching system contains several master control circuits each of

which consists of some 1500 relays. Such circuits are able to select particular

paths and establish a desired connection in less than one second.

Present day telephone circuit switching equipments are based on either

electromechanical techniques (employing crossbar, strowger, or rotary

switches) and electronic techniques (employing either electromechanical or

solid states or computer control). The development of most of the systems has

required years of time due largely to the extreme requirement for dependability

and reliability. Even a negligible amount of down time of a network cannot be

tolerated by the telephone operating companies especially these days we have

competitive markets in the telecommunication industry.

With the introduction of switching systems, the subscribers are no longer

connected directly to another; instead, they are connected to the switching

system as shown in the figure1. When a subscriber wants to communicate with



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another person, a connection is established between the two at the switching

system. Figure 1, shows such connection between subscriber S2 and Sn-

1.signalling is now required to draw the attention of the switching system to

establish or release a connection.

Figure 1. Subscriber interconnection using a switching system

The functions performed by a switching system in establishing and releasing

connections are known as control functions. Earlier switching systems were

manual and operator oriented. Limitations of operator manned switching

systems were quickly recognized and automatic exchanges came into existence.

A simple classification of switching system is given in Figure 2

below.



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TELEPHONE SWITCHING

Switching Networks

Definition: it is an arrangement of switches whose function is to connect

inlets to outlets. In switch systems the number of cross-points is a good measure

of cost, and part of the effort involved in switch design is concerned with

reducing the number. An idea of the way in which the number of cross-points

can be reduced, and the effect of that reduction on network performance can be

obtained from the following simple approach.

If there are N lines into and N lines out of a single switch then the

number of cross-points required is N². For instance, the matrix shown in Fig 3

requires 10,000 cross-points.

Number of cross points = 10,000 =100 100

Fig 3. Full- availability switch.

This large number of cross-points allow any free inlet to be connected to

any free outlet regardless of the connections made between other inlets and

outlets. This system is called a full availability loss-less system.

However, each inlet, if it is a subscriber’s line, will carry a very small

load, probably less than 0.1 erlangs, so it will be busy for only 6mins on

average during the busy hour. This means that there will be fewer than 100 calls

in progress simultaneously and therefore the system could operate with many

fewer cross-points hence the need to reduce the number.

As a first step towards reducing the size of the network the matrix can be

split into two parts: the first part having 100 inlets and 25 outlets and the second

part having 25 inlets and 100 outlets. This reduces the number of cross-points

by 50%. See Figure 4(a)



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Fig4 (a)

A further saving can be made if the inlet and outlets switches are further

subdivided as shown in Fig4 (b). However, this last division raises serious

problems. Each inlet switch has access to only one outlet switch. So, in that

order, inlet switch 3 cannot route a call to outlet switch 5, etc.

Fig 4 (b) total number of cross-points = 1000

Note: Fig 4(a) stands for cross-point reduction by vertical partitioning.

Fig 4(b) stands for cross-point reduction by vertical and horizontal

partitioning.

One way of overcoming the difficulty of Fig 4(b) would be to

interconnect the link between the inlet and the outlet switches as shown in



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Fig. 5. But that is not all since this arrangement has its own draw back.

The problem being that only one of the calls from the inlet switch can be

connected with a particular outlet switch. When this situation is allowed,

blocking is inevitable. A better approach is to adopt the arrangement shown in

figure 6, of a three –stage link system.

Fig 5: Diagonal link connections to increase availability.

Fig 6: Three-Stage link system with a total of 1125 cross point

QUESTION:

Mention one effect of availability reduction in switching network system.

Show how the effect could be remedied.



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ANSWER:

It makes the system restrictive. To remedy the situation, a middle stage is

introduced to make the distribution paths between the inlet and outlet stages

more satisfactory.

THE GRID NETWORKS

Grid networks range from two stages of switching, in crossbar systems to

eight stages of switching in some electronic systems. A general 2-stage grid

network with its input switches designated as primary switches and its output

switches designed as secondary switches is given in Fig 7. It is a basic

requirement that each primary switch group have access via at least one link to

each secondary switch group. It is also important that the link spread between

the switch groups be laid out in an orderly fashion for ease of control and

administration. In the allocation of secondary terminations of links, the output

terminal number on the primary switch designates the secondary switch

number, and the primary switch number designates the secondary switch

terminal.

Fig 7: 2- stage switching network



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To extend the 2 stage and for connection to a particular output, it is only

necessary to add a third stage, the links to which will duplicate the link spread

between the first two stages, such as arrangement is shown in figure 8.

Assuming each stage consists of ‘n’ switch blocks, any network input can be

connected to any network output over ‘n’ matching pairs of paths. To determine

the set of ‘n’ paths that can be used, you need to know only the input and output

switch blocks involved. However because of the relatively limited number of

paths it provides the 3 stage grid network is generally useful only for small

exchanges. Note that the interconnections between grids is necessary that at

least one junctor per secondary switch of each input grid, which provides a

minimum of one junctor to match any pair of originating and terminating links.

Traffic balance must be maintained carefully on the gird inputs and

outputs, because the junctors from each input grid are divided equally among all

output grids. If excessive traffic reached particular output grid, the probability

of failure to match due to busy junctors might be too great. Generally an

attempt is made to obtain as wide a distribution as possible, over both switches

and grids, to minimize the effect of switch or circuit troubles and to handle

traffic with an optimum grade of services.

Fig 8: 3-Stage grid network



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CROSS-POINT REDUCTION USING 3-STAGE GRID NETWORK

A middle stage can be included to make the distribution of paths between

the inlet and outlet stages more satisfactory. The distributor stage is called the

secondary stage or stage 2. in the same way as was done with the inlet and

outlet stages, the distributor stage can be sub-divided into five 5x5 switches and

the link pattern between first and second stages, and second and third stages

arranged to provide five paths through the network from any first

stage(inlet)switch to any third stage (outlet) switch. See Fig 9.

Fig. 9:

Number of Crosspoints: 500 125 500

Total Number of Crosspoints: 1125

Clearly, there has been a very large reduction in the number of cross-

points. The chance that a call is lost because of the reduced availability is a

question that can only be answered by calculation of internal blocking of the

network at given traffic levels, using the jacobaeous equation, or some other

switch-able method.

ASSIGNMENT 1

State the Jacobaeous equation. Assume any network hence or otherwise

calculate the internal blocking using the Jacobaeous equation.



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For obvious reasons the stages of Fig 9 are named:

First stage- concentrator

Reason: many more inlets than outlets.

Second stage- distributor

Reason: same number of inlets and outlets.

Third stage- expander

Reason: many more outlets than inlets.

Fig 10: Functional diagram of three-stage link system.

PSTN-BASIC ANALOGUE SWITCHING SYSTEM

Three methods are in common use

(i) Strowger or Step-by-step, (ii) stored program controlled (SPC)

otherwise known as electronic switching system (ESS) or Centralized

control, (iii) Crossbar

Recall:

The function of a telephone exchange is to interconnect four-wire lines,

so as to permit a call to be established correctly. When we are talking about

switching, we are referring to telephone exchanges. There have been basically

three generation of exchanges the first was the step-by-step or strowger type,

which has incredible number of relays that made interconnections step-by-step.



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i.e. after each digit was received. The second generation was the cross bar

exchange, which had even more relays, but miniaturized and arranged so that

up to 20 connections are made simultaneously by the cross bar switch after all

the digits were received. The processor- controller or centralized control or

common control represents the third generation. Here all the interconnections

are made by the exchange processor or computer and as a result the space

occupies is very much smaller. When compared with other generations.

(i) strowger/Step-by-step:

In step-by-step systems, the control path and speech path are the same.

As each digit is dialed, the connection is made to one further stage, until the

final selection when two digits are required

Fig 11: Diagrammatic representation of a step-by-step system.

The step-by-step systems operate in this way. As an example, consider a

local exchange call on a four-digit number system. See Fig 11.



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When the caller goes off-hook, the line circuit connects his line to a free

first numerical selector (group selector). This connection may take some time,

and not until the first selector is seized that a dial tone is returned to the caller.

The dial tone delay is a measure of system capacity from the subscriber’s view

point. Having seized the first selector, the control waits for the first digit to be

dialed: on its receipt, the first selector wipers are racked up to the corresponding

level and hunt round for a free outlet to a second selector. The second digit

caused the second selector to go through a similar process and seizes final

selector. The final selector has subscribers line circuits attached to it, and so

each of its hundred contacts represents a different subscriber: consequently, it

needs two digits for its operation.

If at some stage during the setting up of the call, after the dial tone has

been received a free selector is not available, a busy tone is returned to the

caller: the wipers having hunted over all the outlets to find a free selector at the

next stage, without success, automatically return to their home position.

Common control:

The step-by-step system described earlier operates on the principle that

the control of the call follows the same path as the speech circuit. This means

that all the control equipment is provided on a per-call basis. However, some of

it need not be, and savings can be made by separating some of the control from

the speech path. It can then be used only as required by a call, and can then

become available for other calls, for example, the register-translator, which is

only required during the routing, or path selection process, could be placed in a

common-control area, used to set up a path between caller and called, and then

released for use in setting up another call. Fig 10 shows a diagram of a common

control system.



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Fig 12: Use of common control

The switching block is not of the step-by-step (strowger) type but either

of a cross-bar switch or a reed-relay system. A crossbar switch as we know

consists of a matrix of horizontal and vertical conductors that can be made to

interconnect at any required cross-point by the vertical bar trapping a metal

finger attached to the horizontal bar when the letter is tilted by a relay operated

by the marker-circuit in the common control. The reed-relay switch is an

electronic cross-point consisting of two contacts inside an evacuated envelope

and surrounded by an inductive coil. When the coil is energized it induces a

magnetic field which forces the contacts together. The reed-relay switch is also

of the matrix type with horizontal and vertical wires, the interconnection again

being controlled by the marker. Figure 13 shows the basic form of a common-

control exchange. Here, the subscribers are attached directly to a subscriber’s

line unit, which recognizes a request-for-call condition. When the calling

subscriber goes off-hook, the line unit indicates.

Fig 13: Centralized control exchange



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The request to the common control that seeks a free outlet from the first

switching stage, which is the concentrator, and returns dial tone to the caller as

an awareness of the call for metering purposes and reacts to a clear down signal.

The calling subscriber dials and the digits are stored in the register in the

common-control. If translation is required, the registers pass the digits to a

translator when one becomes available, and the translation is sent to the marker.

The marker examines the switching units for a free path to the called outlet and

tests the outlet to find out if it is free. If not, busy tone is returned to the caller,

if it is free, a path through the called subscriber and ring-tone is sent back to the

caller, the common control is released and the progress of the call is noted by

the call-monitoring unit. In summary therefore, we have seen that in the

strowger system, switching is done using step- step method. With common

control method, switching is done after the system has received the entire

digits/number. This number is stored in memory. The information conveyed by

the number is then translated into instructions using logic circuitry. These

instructions control the switching mechanism tom establish appropriate

connections. Having completed connections, logic circuitry becomes available

for service of other calls.

ELECTRONIC TELEPHONE EXCHANGE

A telephone exchange, in general is required to perform the following functions

as outlined below. For instance, a local exchange must provide two wire

connections to its subscribers. It also provides two-wire to four wire conversion

for trunk routes. This becomes necessary as attenuation in trunk routes is high

due to length of trunk lines and repeater amplifiers have to be used.



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Fig 13 b: Outline of Telephone Exchange

NOTE: The development of common control based on computer operation

removed some of the constraint on inter-exchange signaling. Very rapid

reception, detection and processing were possible, and for signaling between

processors care need not to be taken to counter speech imitation as was the

case in the earlier system

PRINCIPLES OF CALL SETUP WITH WORKED EXAMPLE

When a subscriber makes a telephone call a series of events take place which

have the following:

1. calling subscriber lifts handset

2. exchange detects demand for call

3. check is made to find free equipment in the exchange



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4. When equipment becomes free dial tone is returned to the caller. If

equipment is not immediately available at the exchange there may be a

long dial tone delay

5. subscriber dials digits

6. Exchange interprets the digits and routes call to its destination.

7. Check is made to determine whether or not the called subscriber is free.

8. if called subscriber is busy , busy tone is returned to the caller

9. if called subscriber is free, ring tone is returned to caller and ring current

is sent forward to ring called subscriber’s bell.

10.if called subscriber is unavailable, caller hangs up and equipment clears

down

11.if called subscriber answers, ring tone and ring current cease

12.Speech path is set up and metering commences. \when conversation

ceases called subscriber hangs and metering ceases

13.When conversation ceases called subscriber hangs and metering ceases.

14.calling subscriber hangs up and system clears down

NOTE: if an unallowed number is dialed, or service to the number

unobtainable tone is returned to the caller. From the above description we can

see that the process of signaling has at least three functions.

1. To indicate the state of the call to each subscriber by

a. Dial tone

b. Ring tone and ring current

c. Busy tone or number- unobtainable

2. To tell system what to do next by indicating the path for the call

3. To initiate a billing procedure- usually by tripping the calling

subscriber’s meter at the correct charging rate to enable the

administration to gather the revenue needed to provide the service.



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QUESTION

Write a program in Q-Basic using the flow chart given. Run it and submit

a computer print out.

OR

Write a computer program to implement the ideas of the flow chart given

in figure 14. The program will be written either in Q-Basic or C++. Run it and

submit a computer print out.

Two stage 15x15 matrix switch using link trucking start

Fig 15:

From fig. 15, the inlets are divided into three groups of 5 at A-switches, and 3

of each group of 5 inlet can be connected to a B- switch at any one time where

they are then connected to any 3 of the 15 possible outlets. Fig15 shows the

following interconnections.



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Inlet 1 to Outlet 15


Inlet 3 to Outlet 5


Inlet 7 to Outlet 9

Inlet 8 to Outlet 4




From the illustration above, we can discover that inlets 4,5,9,10,14 and

15 cannot be connected to any outlet even though outlets 1, 2, 6,7,11 and 12 are

free, because all 9 links are already in use. Further even with only one

connection made,, say between inlet 1 and outlet 15 as shown, them inlets 1,3,4

and 5 cannot be connected to any of the outlets 11,12,13 or 14 because the one

link between the particular A and B switches is already in use. This is called

Internal Blocking or Link Congestion and must be considered when designing

multi-stage matrix switches. This blocking situation introduces limited access

into the network. For voice traffic systems having a capacity of n channels are

lousy. The phenomenon of call congestion is described by Erlang B formula

which gives the probability that an arriving call will be intercepted by finding

all channels busy, as



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is the call arriving rate, the average service time or holding time and

k is a random variable denoting the number of calls in a given interval of time,

k= 1,2,3………nThe Evlang B formula forms the basis for judging the grade of service of a

voice traffic network.

NOTE: In telephone exchanges, it is necessary to connect the two wires from

one telephone to the two wires of another telephone. It is also necessary to be

able to guard the calling line and the called line so that neither can be seized by

another subscriber. Generally, a third wire called the private wire and

designated P-wire is used for this purpose

QUESTION

Using the flow char given in and 5 explain what happens from the time the

subscriber is off-hook till the time the called subscriber hangs up.

NOTE: The explanation must correspond to the flow chart design given.

TRAFFIC AND TRUNKING

Definition:

1. Traffic may be defined as the aggregate of calls passing over a

group of circuits or trunks. The trunks means a connecting



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circuit between two switching stages e.g. between 1st selectors

and second selectors.

2. Trunking may also be defined as the provision of adequate

plant to carry traffic and inter connection of the selectors in

such a way as to route this traffic in the most efficient and

economic manner.

Measurement of traffic:

To find out how many circuits are needed on a given route, it is therefore

necessary to know how much traffic there is. To do that one must be able to

measure traffic; the unit of measurement is the “ERLANG” which is a

dimensionless quantity, quantified or example in mins/min

Suppose that four telephone circuits exist between two places and it is found

that in a particular hour, half hour period the circuits carry respectively 25, 15,5

and 24; each circuit was assumed busy for the period indicated and so the total

occupied time was 25+15+5+24=69mins. The average occupancy during the

half hour was therefore 69/30=2.3 erlangs. Therefore the measure of average

occupancy is the erlangs. Needless to say the traffic may have fluctuated during

this period. At instants when all the four circuits were busy, the carried traffic

was 4 erlangs while there may have been instants of no occupancy i.e zero

erlangs.

Telephone traffic can be said to mirror all business and social activities of the

community. It increases in prosperous times, decreases in time of depression

and shows sudden peaks in emergencies and disaster periods.

In the other hand, the main object of trunking is to provide adequate plant for

present needs as economically as possible in order to provide the required

service at reasonable cost to the subscriber.



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The Erlang and “probability”:

For a single circuit, the traffic cannot be more than one erlang because it

can carry only one call at a time. The erlang can thus be defined as the traffic

flow which would continuously, occupy one circuit. In a practical case, no

circuit can ever be continuously occupied because it must become free for a

short period after each call in order to be seized by the next call. The traffic on

one circuit is there fore always a fraction of an erlang, this fraction being the

proportion of the time for which the circuit was engaged. This may be

illustrated as follows:

If a single circuit carries C calls of average duration t, it will be engaged for a

total time of c x t (min). If these calls occur in a period T (Min), the proportion

of this time for which the circuit is engaged is;

and this is equal to traffic A erlang.

The probability of finding a given circuit engaged at any given time is

numerically equal to the proportion of time for which the circuit is engaged and

therefore to the traffic in erlangs.

In practice, it would be uneconomic to provide so many selectors that no

subscriber would ever find the one he needed already engage. It is therefore

necessary to be able to calculate the probability of circuits being engaged under

given conditions in order to assess whether adequate service will be given by a

particular arrangement or quantity of selectors. The probability of any given

event occurring is calculated as the number of occasions on which it might

occur. The probability of “certainty” is 1 and the probability of “impossibility”

is 0. All other probability is fraction, i.e they are less than 1 and greater than

zero (0).



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Form the explanation above, it follows that the probability P, of a

particular selector being found to be engaged at any instant during a specified

period is given by

Effect Of Traffic On Plant Provision:

Grade of Service: This is the proportion of calls which are allowed to fail in

the busy hour owing to the limitation, for economic reason, of the amount of

switching plant. Calls which fail in this way are referred to as lost calls. The

grade of service, B, for a group of circuits can therefore be calculated as

Examples: In the busy traffic offered to a rank of selectors is 200 erlangs, the

average call duration is 3mins, and 20 calls were lost in the course of the busy

hour. Calculate the grade of service, what is the traffic carried?

SOLUTION

Step 1. Express the call duration t as a fraction of an hour

= t = 3/60 = 1/20 or 0.05hr

Step 2. Using the relation C = A/T

Where C = calls offered

A = traffic offered

C = 200/0.05 = 4000 calls offered

Step 3. Grade of service B

B = calls lost / calls offered



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= 20/4000

= 0.005 (or 1 lost call in 200 )

Step 4. Traffic carried = No of calls carried x time

= calls offered – calls lost/ period

= 4000 – 20/20 =199 erlangs

Which is the traffic carried or

A = (C – L)/ T

The expression carried traffic was carefully used above. It is not the same

as offered traffic. For example, 20 erlangs may be offered to circuits in which

case a lot of the offered traffic would fail to secure a circuit, hence congestion

would result.

QUESTION

Define traffic congestion, looking at the erlang tables of factorizes and make

some predictions on the congestion stake of some circuits. Hence, calculate the

grade of service.

Note: It is possible to calculate statistically the degree of congestion or grade

of service as it is known give the amount of traffic in Erlangs and the number of

circuits and their arrangement. However it is a lot easier to look up the

information in Erlangs tables of factorials. Such tables are used to calculate the

grade of service.

Erlang statistical distribution:

This expresses the relationship between grade of service, average traffic,

number of trunks and availability in the following equations



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Where a = traffic lost, in traffic’s units

A = total traffic offered, in traffic units

B = grade of service

N = number of trunks (ccts) available hence, the lost traffic “a” is given by

Note: average traffic carried by the network in any period is given by

Where T is the period and hi is holding time

i = 1, 2, 3, ------- m

QUESTION

1. A total of 2 traffic unit is offered to a full availability group of 6 trunks.

Calculate the grade of service provided and find also the traffic carried by

the first and last trunks. Assume that calls to the group arrive in pure

statistical chance order.

2. (a) Derive the erlang mathematical model for simulation of traffic flow in

communication networks.



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(b) Establish the statistical relation for a suitable of poisson arrival time.

3. It is found that during the busy hours that the average number of calls in

progress simultaneously in a certain full availability group of selectors

was 15. all the selectors are in use simultaneously for a period of

30seconds. Calculate the traffic in traffic unit offered to the group in the

busy hour.

SOLUTION TO NO1.

Recall: grade of service B, is given by

Where A = 2, N = 6 given

B = 26/6!

1 + 2 + 22/2! + 23/3! + 24/4! + 25/5! + 26/6!

64 / 720

1 + 2 + 2 + 8/6 + 16/24 + 32/120 + 64/720

= 0.012 T. U.

Note: the traffic offered to the second trunk is that lost by the first and is given

by

A2 = 4 = 1.333 T. U.

1 + A 3



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Hence, traffic carried by the first trunk

= 0.66 T.U. (2 – 1.333 = 0.667)

Traffic offered to the 6th trunk

26/5! = 0.073 T.U

1 + 2 + 22/2! + ………..+ 26/5!

The traffic offered to the hypothetical 7th trunk

= 27/6! = 2N-1 = 0.024 T. U.

1 + 2 + 22/2! + …………+ 26/6! N-1!

Hence traffic carried by the 6th trunk = 0.073 – 0.024

= 0.040 T. U.

SOLUTION TO NO. 3

Let, C = Number of calls per hour

T = Average duration of a call in hours

A = the traffic flow in traffic units then

A = C x T

The traffic lost will be A x B traffic unit

If B = proportion of call lost

The traffic carried = traffic offered - traffic lost

Therefore 15 = A – AB

Or A = 15

1 - B

Hence B = 30/ (60x60)

= A = 15/ [(1-30) /60x60] = 15.13 T.U



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Hence the traffic offered to the group of trunks during the busy hours is 15.13

T.U.

QUESTIONS

1. Mention two ways in which a call may be blocked.

ANSWERS

a) There may not be free route Circuit.

b) There may be no path available between the inlet carrying. the

incoming call and the free rout circuits.

SWITCHING AND SIGNALING

Signal systems link the variety of switching systems, transmission systems and

subscriber equipment in a telecommunication network to enable the network to

function as a whole.

Types of Signaling System

We have four mail categories:

i) loop disconnection – dc signaling;

ii) multi-frequency – ac signaling

iii) voice- frequency – ac signaling

iv) common channel signaling

LOOP DISCONNECT SIGNALING

Many years ago, this was the universal means at the subscriber’s disposal

for indicating the number he wishes to call- the telephone dial. Although it is

still present with many handsets today. The major disadvantage is that it

operates slowly when compared with the standard of modern electronics. The

slow motion places a definite limit on the speed at which signals can be sent to



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the exchange. When any digit is dialed, a governor inside the dial causes it to

rotate back automatically at a fixed speed causing a series of pulses to be sent

down the subscriber’s line. The time between the last of the pulses for one digit,

and the first pulses of the next is called the inter digit pause. It is this pause that

allows the exchange to recognize the end of a digit. A typical sequence, with

average operating times (finally the pulses are sent at a rate of 10 per second) is

show in fig 16.

Fig 16: Ideal output from telephone dial

MULTI- FREQUENCY (MF) SIGNALING

Modern n handsets are fitted with key pads instead of dials to facilitate a

much more rapid transfer of signals between handset and exchange. Generally,

the keypads send out frequencies instead of pulses to represent a digit. Most

systems use two frequencies to represent a particular digit. Modern exchanges

respond directly to the mf signals and the call be set up very quickly.

VOICE FREQUENCY (VF) SIGNALING.

The normal telephone channel occupies a bandwidth of 300 – 3400 Hz

which it is allocated. If ac signaling is to be used it must operate at frequencies

within this range and it is therefore known as voice frequency signaling

Note: the frequencies used can be either inside the normal speech band (in-band

signaling) or outside that band and within the 0-4000hz range (out –of-band

signaling)



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Since signaling is done by tones within the base band of the telephone channel

it is possible to use VF signaling when the channels are multiplexed on to a

common carrier either line or radio.

Since the signaling frequency is within the 300-3400hz speech band width

there are obvious problems associated with this signaling method. It cannot be

operated during speech and the equipment must be able to distinguish between

a speech pattern and a signal. There are two parameters available for variation.

Signal frequency and the signal recognition time. Other consideration that will

assist in distinguishing between speech and signal are

i) speech at the signal frequency is accompanied by other frequencies;

ii) more than one signal frequency could be used;

iii) the signals could be coded burst of the signal frequency

CHOICE OF FREQUENCY

This must have relationship with the frequency characteristics of speech.

As shown in fig 17 the energy level in English is predominant at lower

frequencies (maximum at 500Hz or there about) and it falls gradually over the

rest of the band.

Fig 17: variation of energy with frequency in English speech



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This then suggest that high signal frequency should be used to reduce the

possibility of imitation by speech frequencies. However, there are other

considerations that suggest frequencies at the upper end of the band should not

be used.

Reason: at the higher frequencies there is an increase in crosstalk, hence

low level transmission is necessary. If the amplitude is low the receiver must be

very sensitive thereby raising the prospect of imitation signaling by low level

speech. In addition the variation of amplitude with frequency is quite marked at

the high frequencies so any change in signal frequency will result in a

significant change in signal amplitude. Between these considerations some

compromise is necessary and in practice the frequency chosen should lie

between 2040-3000

SIGNAL DURATION

When a signal is detected there is need to delay before action is taken by

delaying the recognition until it has persisted for some time the chance of signal

imitation is reduced significantly. Using a guard circuit and a 40ms recognition

delay, the probability of the signal receiver responding to a speech frequency is

reduced and to a low level. The effect of signal recognition delay frequency is

greatly enhanced if a guard circuit is used in the receiver to increase the

rejection of imitation signal



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Fig 18: showing block diagram of system using guard circuit

From the above fig 18 the energy coupled from the speech channel is

amplified and then passes along two paths.

i) Through a band pass filter tuned to the signal frequency, forming part

of the signal circuit.

ii) Through the guard circuit which includes a band stop filter allowing

all but the signal frequency to pass.

The outputs from (i) and (ii) are compared. When a signal arrives, the signal

circuit responds strongly and the guard circuit weakly, leading to a strong

positive signal being detected by the signal detector circuit. When speech is

present; any imitation signal passing through the signal circuits are attenuated

by the strong signal passing through the guard circuit, thus reducing the chance

of spurious imitation.

Types of Voice Frequency Signaling:

(i) Pulse signaling

(ii) Continuous signaling



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Pulse signaling:

We recognize a signal by its length and its sequence. The following

points should be noted about pulse signaling.

1) It has a higher signal repertoire than continuous signaling.

2) It can be transmitted at a higher voltage level and therefore provides a

better SNR.

3) It is less influenced by interference.

4) It complicates the dc/ac and ac/dc conversions because the pulses have to

be carefully timed.

5) It requires a memory facility at the receivers for pulse recognition.

THE TELEPHONE NETWORK AND STRUCTURE

The telephone network has developed dramatically in recent years so that

it is now possible to make calls automatically between subscribers separated by

thousands of miles away. It should be recalled that long – distance calls pass

through several stages of switching and several possible transmission links

before reaching their destination, and to make such calls possible many facet of

telecommunications must be integrated and reasonable compromises made by

systems designers.

Although the structure of telephone networks has developed piecemeal as

demand has increased, it still has some identifiable forms. The passage of a call

through a national network can be represented by the multi level diagram show

in figure 19.

Public switched telephone network (PSTN) or the plain old telephone system

(POTS) is perhaps the most stupendous telecommunication network that has

been in existence over 100 years. A unique feature of the telephone network is

that every piece of equipment, technique, or procedure owned by different

corporation is capable of working with each other. You may compare this fact

that it is almost impossible (P.T.O) to interface the first IBM computer with its



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latest system. The enormous complexity of the telephone network is managed

by using a hierarchical structure, world-wide standardization, and

decentralization of administration, operation and maintenance. Any

telecommunication network may be viewed as consisting of the following

major system.

1. Subscriber and instrument or equipment.

2. Subscriber loop systems

3. Switching systems

4. Transmission systems

5. Signalling systems.

Fig. 19: Telephone Network Switching Hierarchy

Table 1: Terminology

C C I T T U S A UK

Primary Centre Toll Centre Group Switching Centre



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Secondary Centre Primary Centre District Switching Centre

Tertiary Centre Sectional Centre Main Switching Centre

Trunk exchange Toll Office Trunk exchange

Trunk network Toll network Trunk network

Trunk circuit Trunk Trunk (circuit)

Local exchange End (central) Local exchange

Office

Junction Circuit Inter-Office Junction

Trunk

From fig 19, we can see several levels of switching that combine to form

the complete network. It is usual to think of the system in two parts. The first is

the junction network, serving the subscriber and consisting of the link from the

subscriber to the local exchange, from local exchange to primary switching

center, and back to the called subscriber via another local exchange. The trunk

network, which is concerned only with calls passing at primary centre level and

above. Thus the primary centre is associated with both parts of the network.

The number of exchange at the various levels depends on several factors:

the physical extent of the network, the number of subscribers, the amount of

traffic, the forecast growth and the transmission methods used. Beyond the level

of the national system is a layer that access to the international network. This

layer may consist of one or more international (usually called gateway)

exchanges.

Numbering schemes:

In modern systems, the numbering scheme used by a telephone

administration to allocate subscribers numbers has an underlying plan, and there

are a few constraints on the development of the plan that must be taken into

account:

(i) It must provide each subscriber with a unique number within the national



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network

(ii) The allocation to areas must be able to meet forecast growth, for several

decades.

(iii) The number of digits should not exceed that recommended by CCITT.

In principle, (i) is easy to satisfy if (ii) have been met. The length of the

number recommended by CCITT is 11-n where n is the country code. If, for

example, n is 2, then national number should not exceed nine digits in length.

These digits are used to denote the subscriber’s number on the local exchange,

the exchange within a given area and the area within the national numbering

scheme. In many local exchanges there is a maximum capacity of 10,000 lines,

thus the last four digits of the national number are allocated to the subscriber’s

number in the exchange of the remaining five digits in our example, the first

two would denote the area code and the remaining three the exchange within

the area. Thus the number has the form show in fig. 20.

Toll call Area Exchange Subscriber’s

Prefix Code Code number

Fig. 20: National Telephone Number

For automatic long dialing a prefix is necessary to indicate to the

exchange equipment that a trunk call is being made.

Note: In calculating the length of the national number, the prefix is not

included.

With the anticipation of a general use of mobile systems, plans were

developed to relate numbers to people rather than premises. Hence a personal

numbering scheme was introduced so that access can be made via any handset.

This makes use of the 11-digit number where n=0



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DIGITAL SYSTEMS

Here, we will concentrate on the switching and transmission used in the

public switched telephone networks (PSTN), including the integrated services

digital network for voice and data, known as ISDN.

In digital exchange, most of the controls are handled by microprocessor

devices, single or in clusters, which are driven by software. The software must

be efficient, reliable, secure, understandable and well documented.

In analogue exchange the usual figure of merit used is the grade of

service, or probability of blocking, but in digital switches, the blocking is

virtually zero. In these systems the major problems concern delay in the

processing of calls caused by the processor units becoming overloaded.

QUESTION

Write brief note on the international gateways.

DIGITAL SWITCHING

The digital switch can have many structural forms depending on the

application, the number of connections required and the technology used. The

system show schematically in fig. 18 is a local exchange and it shows that there

are two types of switched involved. A subscriber switch to act as a concentrator

and a control switch has a distribution function.

Fig. 20: Digital Telephone Exchange



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Note: SI—signaling interface

CCS----common channel signaling

What is the function of a concentrator?

The architecture of the subscriber switch will depends to a large extent on

the number of subs to the exchange, if a small rural exchange is being

connected, the switch may act as no more than a multiplex or being the

mechanism whereby up to 30 speech channels are time division multiplexed on

to a single PCM carrier. In such a case, since a group of 30 channels is the

smallest PCM link available, it may be sensible to allow all subscriber lines full

availability access. However, if the number of subscriber is somewhat larger, a

concentrator will be necessary. As in analogue systems. A concentrator

provides significantly fewer lines on the outlet than are attached at the input.

The fact that the inlets are subscribers indicates that generally the mean traffic

per inlet line will be less than 0.1 erlang thus a very low loss probability can be

achieved even if the number of outlets is no more than a fraction of the number

of inlets.

Notice that if a digital switch (exchange) has to carry a traffic originating

on an analogue line then interworking is difficult hence a special interface units

required are regained to ensure smooth operation; as you can see in Fig, 20.

TIME SWITCHING AND SPACE SWITCHING:

Recall that one of the main functions of digital switching is to inter-

connect a calling subscriber on an incoming line with a called subscriber on an

outgoing line. In PCM system, the time slot arrangement is used. In fig 21, for

example, five subscribers on the inlet side of a multiplex are able to call

subscribers on the output side, if for instance, subscriber C on the inlet timeslot

3 wants to be connected to subscriber V on outlet timeslot 4, the inlet sample

must be delayed in the switch for one timeslot before being sent out on



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timeslot4 for the connection. The type of switch designed to provide the

appropriate delay is called a time switch. The sample from the inlet channel is

stored in a buffer and then read out when the appropriate output timeslot

arrives.

Fig. 21: Time Switching

QUESTION

Draw the block diagram representation of telephone switching hierarchy.

Explain what you understand by junction network and trunk network. Hence

give reason for the insertion of prefix “0” in automatic long distance dialing.

Fig 22, illustrates the way in which this is done. Synchronization is

obtained from the frame and mutliframe alignment signals in the PCM format.

Each input time slot word is stored in a buffer. A control store holds

information on the time at which each sample has to be read out. At the

appropriate time, this control store connects the data buffer to the output lines.

The control store instructions are derived from a central processing unit which

responds to the call request. What are the limitations associated with time

switching? Note the limitations of time switching led to the adoption of space

switching.



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Fig, 20. Buffer delay for time switching.

When there is more than one incoming and outgoing PCM line, time

switching will not be able to meet the requirement. But effective

communication system must be able to provide access for any inlet line channel

on any outgoing line channel and this is what space does. It provides linking

between the particular inlet and outlet lines. But for complete flexibility in the

operation of the digital switch, some combination of time and space switching

is required. It is possible to achieve full switching with just one time switch (T)

in conjunction with one space switch (S), but this will create internal blocking.

Most often a three-stage arrangement is used, either STS or TST. A more

general TST switch is given in fig. 23. The time switch is split into several

units, each having M PCM links of L Channels Consequently, if the time switch

is non-blocking it will have an outlet highway of N = ML time slots.



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Fig. 23: Network Representation of a digital switch

The purpose of the TST unit is to allow a particular call, which occupies

a specific channel into one of the time switches, to be connected to a particular

outlet channel. Basically, the switching is between highways on either side of

the space switch. Each highway has N timeslots and in order for a particular call

to be connected say H1 to H3, it must find a timeslot which is free in both

highways,. This slot may not be the same as the required incoming and

outgoing slots for the call, and so some time delay, provided by the time

switches, is necessary.

To understand the behavior of the TST switch in terms of the link

systems considered earlier it is important to appreciate the fact that for each

timeslot there will be a different set of calls in progress and the connections

between the highways will last for only one time slot period than the new

connections will be established. This can be represented by having space

switches one for each timeslot. See fig. 24



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Fig. 24: Analogue Equivalent of TST Switch

Whether or not blocking occurs in the TST unit depends entirely on the

dimension of the space switch and since modern systems switches are

comparatively inexpensive, they are usually large enough to make blocking

negligible. For total non-blocking there must be least as many outlets as inlets

on the switches, and the space switch highways must have 2N-1 time slots,

where N is the number of time-slots in a link to a time switch.

Digital switches are uni-directional and that implies that two paths are

required to connect two channels X and Y, and for conversation from X to Y

and the other for conversation from Y to X. to reduce the control process, the X

to Y slot is chosen according to whatever rules are used by the designer, and Y

and X interconnection is allocated a fixed number of time slots from it e.g one,

or half a frame. By this arrangement, if the X to Y connection is available, then

Y to X must be free.

QUESTION

Differentiate between analogue and digital switches.

Explain briefly the precautionally measures normally taken during design to

ensure non-blocking operations using digital switch.



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MOBILE COMMUNICATION SYSTEM

Mobile communication system is broadly divided into two viz:-

1. Cellular mobile system

2. Cordless mobile system.

Both have something in common; they rely on radio transmissions for the

final link with the subscriber: it is envisaged that in the nearest future, the

public switched telephone network with fixed telephone handset will eventually

disappear completely and be replaced by mobile units allowing individual

subscribers the facility of global travel with continuous personal

communication.

In this topic we will concentrate attentions on European systems. Recall

that the first generation is the analogue systems while the second generation is

the digital systems. The essential feature of all cellular networks is that the final

link between the subscriber and fixed network is by radio. This has a number of

implications viz:-

1. Radio spectrum is a finite resource and the amount of spectrum

available for mobile communications is strictly limited;

2. The radio environment is subject to multipart propagation, fading and

interference and is not therefore an ideal transmission medium;

3. The subscriber is able to move and this movement must be

accommodated by the communications system.

Due to the limited amount of the spectrum (radio) allocated to cellular

radio the number of carrier frequencies available is limited. Hence, it becomes

necessary to re-use the available frequencies many times in order to provide

sufficient channels for required demands. This introduces the concept of

frequency re-use and with it the possibility of interference between cells using

the same carrier frequencies. The question now is: How can we contain with a

fixed network with ever increasing or expanding subscriber or customer

capacity; with a view of providing efficient and reliable service.



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Hence, it becomes imperative that with a fixed number of carrier frequencies

available the capacity of the system can be increased only by re-using the

carrier frequencies more often. We do this by making the cell sizes smaller. But

this action has prizes to pay.

1. It increases the likelihood of interference (known as co-channel interface)

between cells using the same frequency.

2. If a mobile is moving it will cross cell boundaries more frequently when

the cells are small.

Whenever a mobile crosses a cell boundary, it must change from the

carrier of the cell which it is leaving to the carrier of the cell which it is

entering. This process is known as “handover” this action cannot be performed

instantaneously hence there will be loss of communication while the handover

is being processed. If the cell sizes are smaller (say micro cells) then handover

may occur at a very rapid rate.

QUESTION

Explain briefly the principle of handover in mobile communication?

PROPERTIES OF THE RADIO CHANNEL

The radio channel in cellular system has major influence on the overall

system design. This has already been evident in the way in which frequency re-

use is implemented based on radio attenuation proportional to D4. Cellular radio

systems are categorized by the fact that the heights of antennas at both base

station and mobile are usually low compared to the distance of separation. The

model is given in fig. 23



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Fig. 25: Plant Earth Propagation Model

For an isotropic antenna, it is assumed that it radiate energy equally in all

directions.

ANALOGUE CELLULAR RADIO

With the development of the integrated circuits technology, radio

equipment can now be miniaturized, and relatively sophisticated operations can

be implemented at low cost. This development allows the sharing of a number

of radio channels on demand using frequency division multiple across (FDMA)

where a particular channel is assigned to a specific user only when a telephone

call is IN PROGRESS. An example is trunked mobile radio system in which a

number of radio channels is shared with different groups of users.

Mobile telephone service is designed to connect mobile units with dial

telephone exchanges. The more advanced system provides fully automotive

operation, giving the mobile user the identical telephone capabilities of a

regular fixed subscriber. Earlier systems required the mobile user to manually

place e cell through an operator. Each mobile unit is assigned a conventional

telephone number in the central office and is given the same treatment as a land

telephone.

A basic telecommunication network can be described as the

interconnection of subscriber’s instruments to ensure that one subscriber can be

linked to another within and outside the same locality, using cables. Switches



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and transmission equipment. The different elements are further classified as

independent network, like the local Area Network (LAN) comprising the

exchange and the local cabling to the subscriber’s premises.

It is also common to hear about the switching network (Interconnection

of Switches) comprising the local exchange (LE) the primary switching centre

(PC), the secondary switching centre (SC). Transit Exchange (TE), among other

up to international switch.

From the foregoing analysis, the cellular Mobile Network can also be

defined as the interconnection of Mobile Subscribers or Mobile stations (MS)

and the public Switched Telephone Network (PSTN)

Land Mobile Radio Services has been in use for over a century now. The

world Administrative Radio Conference (WARC) an organ of international

Telecommunication Union (ITU) which is responsible for the administration of

the Radio frequency Spectrum, had allocated certain bands for use in the land

Mobile Radio services. These comprises of the very high frequency (VHF) and

the Ultra high frequency (UHF) with channel spacing of 12.5 KHz and 25 KHz,

respectively.

Different national Telecommunication Administrators have used these

frequencies within their geographical boundaries, for the provision of fixed and

Mobile services to their Nationals. In Nigeria, these bands of frequencies have

been assigned to provide private and pubic organizations whose access to the

pubic switched Telephone Network (PSTN) has been made difficult as a result

of the area where such organizations are located. Primarily, those affected in

this regard include oil companies that require the need of communication

between the oil rigs scattered in the reverine areas (for off-shore and on–shore

exploration activities); the construction companies that need to be in constant

touch with progress made in their works including communication with the

ground and the top of high rise buildings, forest guards for constant touch with



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their centre and some governmental institutions, where co-ordination of

government projects in the rural areas are necessary.

It is therefore a statement of fact that the introduction of cellular Mobile

Radio System in Nigeria is a welcome development in the sense that business

activities in the country have been given a new impetus. Although , the cellular

Mobile Telephone Service is new in the country have been introduced in 2001,

but based on the information received on the subscription to the network so far,

it is encouraging to note that demand for the services would continue to grow

with a high level of enthusiasm.

THE CONCEPT OF THE SYSTEM

The evolution and development of Mobile cellular Technology is partly

as a result of the phenomenon of cable cost escalation and partly a contribution

towards the March to the Personal communication state. The conventional

telephone subscriber cable distribution where cables are used for primary and

secondary cable layout is not an attractive option for some rural environment.

This is due to the inhibiting factors of geography and cost. The rural terrain

makes access a problem, and the use of long span cables makes the cost

prohibitive. These two factors would exert a discouraging influence on any

administrator seeking to invests in rural telephone service. Not only that the

immobility inherent in the pubic switched Telephone Network ( PSTN) which

makes it absolutely impossible to answer a call while on the move was another

disturbing factor. Hence, these posed a challenge to Telecommunications

Engineers.

However, the development of cellular Technology and its capabilities

served to fulfill the desired technical phone service distribution. The concept

allows the network to be populated with intelligence equipment ranging from

the Mobile Switching service Centre (MSC) to the subscriber unit. Attached to

this is the increasing independence and functions ascribed to these units. The



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control and functions hitherto carried on Cables are now carried in radio waves.

The stationary or fixed location characteristic associated with subscriber units is

now replaced with flexibility and mobility. This is because the radio

transmission phenomenon now introduced for communication between the

switch and the subscriber unit can be located tens of kilometers away from the

nearest cell site. This is in contrast to the cable distribution system where

subscriber unit can only be .located a mere few kilometers.

Therefore, the advent of this type of technology has made its application

to rural development attractive because it possesses attributes conducive to rural

communication setting,

The attraction for rural application include

• Its cellular characteristic whereby no subscriber cable link is required.

• Its long range capability from the nearest cell site.

• It suitability for pay phone in isolated locations

• Relative ease of installation.

• Comparatively law maintenance factor

• Solar power source capability

The cellular concept has the following advantages:

(a) Larger subscriber capacity

(b) Efficient use of the radio spectrum

(c) Nationwide compatibility

(d) Service to hand-held portables as well as vehicles

(e) High-quality telephone and data service to the Mobile user at relatively

low cost

THE THEORY OF CELLULAR MOBILE NETWORK

A basic cellular mobile radio network is shown in fig. 26. The network

components consist of the following:



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• Cells (cell sites) and Radio Base Stations (RBS) Linked by radio

medium to mobile subscribers.

• Mobile Services Switching centre (MSCs) Linked by digital or analogue

Links to the Radio Base stations, within the service area,

• A Link between the MSCs and the existing public switched Telephone

Network (PSTN).

In addition to these compliments, certain theoretical requirements must be

present if the full benefits of the cellular mobile Radio system are to be fully

realized.

BASIC THEORITICAL REQUIREMENTS

The cells within each service area must overlap to ensure that Mobile

Subscriber within the service area roam from one cell to another cell via a

phenomenon of handoff and handover without break in conversation.

The cells in the boundary between adjacent service areas must be

contiguous to ensure uninterrupted conversation when a Mobile Subscriber

moves from one service area to another.

The mobile service Switching centers must be linked via an international

consultative committee for Telegraph and telephone (CCITT) No. 7 signaling

protocol in order to ensure that data interchange about a subscriber crossing

from one service area to another in a mult- service area configuration are not

lost. The mobile subscriber in such a situation would be encouraged in a

phenomenon of automatic registration (a concept that arises from the mobile

subscriber telling the new mobile service switching centre (MSC) service area

of her presence or identification)

The mobile service Switching centre most be linked either by digital or

analog means to existing PSTN to ensure free flow of traffic from the mobile to

PSTN subscriber and vice versa.



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There must also exist a shareable medium the limited frequency (channels

available) band which the individual mobile subscribers must compete for.

The mobile service Switching centers in a multi-service area network must

be compatible with one another, that is to say that all must have the same

cellular standard.

The cellular network must not interfere with existing mobile and fixed

private radio system. i.e. it must conform with international consultative

committee for Radio regulation with respect to frequency assignment.

Note: If all the foregoing requirements are attained, then a truly cellular

mobile RADIO Network must in theory work efficiently. But it is difficult to

realize this in practice due to certain factors, mainly economics considerations

and geographical size of the area where the cellular mobile system operates.

SYSTEM DESIGN CONSIDERATION

Bearing in mind the existence of telephone exchanges in the country, NITEL

in collaboration with digital Tele-communications of U.S.A, otherwise called

Mobile Telecommunication Services (MTS), decided to incorporate the mobile

Service Switching centre (MSC) in the already existing exchanges housing the

Public Switched Telephone Network (PSTN).this is before the advent of GSM

in Nigeria.

The traditional problem faced by Mobile radio system designers has been

how to balance the apparently conflicting requirements of area coverage and

user capacity. These requirements conflict because, if a base station is to

provide service to mobiles over a wide area, it must have high power and be

situated on the highest point available in the required coverage area. But

following this strategy means that the channels allocated to the transmitting site

cannot be re-used for another service for a very considerable distance.



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The Figure,26(a) shown below is the Basic Telephone Network showing the

position of the mobile services switching centre (MSC); while Figure 26(b)

shows the cellular mobile Radio System Network..

Where TC = Tertiary Centre, SC = Secondary Centre, PC = Primary Centre

LE = Local Exchange, SUB = Subscriber

Fig. 26(a): Basic telephone network with MSC

BS: BASE STATION, LE: LOCAL EXCHANGE, MS: MOBILE STATON

MSC: MOBILE SERVICES SWITCHING CENTRE



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T: TERMINAL EQUIPMENT, TE: TRANSIT EXCHANGE

Fig. 26 (b) Cellular mobile radio system network

Note: The cell transmitting antenna is a single, driven, vertical, unidirectional

element mounted on top of the transmitting tower, see figure 1 (c) below. The

cell receiving antenna system is shown one-third of the way down the

transmitting tower in the figure and consists of half-wave vertical dipole

antennas, each with 90 corner reflector.

Vertical half-wavedipole receivingantenna

Vertical Polarizedtransmitting antenna



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Fig. 26 (c) Showing a cell receiving and transmitting antenna tower.

RADIO BASE STATION EQUIPMENT

A radio base station (MBS) is the radio interface between the mobile

subscribers and the mobile services switching centre (MSC). It acts as a radio

relay point for both speech and data (control signals). In this regard, it is

connected to the MSC on a point-to-point circuit basis, using twisted wire pair

coaxial cable, optic fiber or radio medium

The type of the radio base station equipment in use in the country is the RBS

883. A typical base station is made up of the following equipment as shown in

Figure 27(a)

(i) Radio channel group (RCG).

This is the heart of the RBS equipment

A typical configuration of RCG is made up of one control channel, a number of

guide channels and one signal strength meter suitable to serve a cell.

(ii) Exchange Radio Interface is one of the functional units of the base

station and it is equipped with devices dedicated to data communication for

signaling between the MSC and the base stations and vice versa. In the cabinet

that houses the ERI are equipment such as 32- channel per multiplexers for

PCM link between the base station and the MSC.

(iii) Power supply- This is what energizes the system before it commences

work. It is operating at about 26.4 V dc level.

MOBILE STATION (MS)

The Mobile Subscriber is called the mobile station. It is the replica of the

telephone set used in the public switched Telephone Network (PSTN)

Different manufacturers ranging from:

• Mobile station installed in cars

• Transportable ones that can be carried around.



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This type is a dual purpose telephone that can be used in a mobile phone

installed in a car but it can also be easily removed to be placed in a boat, a

cottage or just to be carried around as the need arises:

• Hand-held, which is a small portable unit with low output power.

• There are also mobile phones that are permanently installed in fixed

locations beyond the coverage areas of PSTN, and used as coin boxes.

These types can also be used as public telephones on trains and shops

where charging information are sent on the radio voice channel.

Note: Irrespective of the types, the basic configuration of the mobile station is

the same.

Fig. 27 (a): Functional units of a base station



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Fig. 27 (b): Functional units of a mobile station

BASIC STRUCTURE OF A MOBILE STATION

The functional block diagram of a mobile station is given in figure 27 (b).

The functional units include:

• The operating part which contains a push button keypad and a display of

dialed digit system. This operating part is micro-processor controlled;

• The control part is also micro-processor controlled, and handles specific

tasks such as data signaling on the radio part such as the selection of

channel, activation of transmitter, opening of channel path, etc. as well

as communication with the operating parts. For example during

reception of the dialed B-number to be sent on the radio path.

• The radio part; the radio part consists of a transmitter, receiver, power

amplifier (with power output ranging from 1w maximum for hand-held

and the car installed types);

• The duplex filter: This is used for simultaneous transmission and

reception via the same antenna on the radio path.



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MOBILE STATION POWER SYSTEM

The power system for the MS usually comes with the station in pair (for

purpose of working state and standby system) of rechargeable scaled batteries.

In addition, a charger also comes with the mobile set, suitable for charging from

main (220-240V)

THE ANTENNA SYSTEMS

Two types of standard antenna are available for use in the base stations:

The omni-directional for circular calls or directional for sector calls.

Fig. 28(a): Base station antenna system

For the case of mobile station, most common antenna available are the ¼

wave length and collinear antennas. Glass mounted antenna are also available

for mounting on the glass or cars. The ¼ wave and the collinear can also be

found on the roof top of cars.

Fig. 28(b): Mobile station antenna system



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CELLULAR MOBILE RADIO OPERATION AND TECHNIQUES

The cellular mobile Telephone uses FM Systems. The major elements of the

improved mobile telephone service include the following:

Terminal Unit: This unit performs the necessary control, signaling, and

switching functions to interface the local telephone exchange with the radio

base station equipment. In addition to its connection to the dial office, the

control terminal may also have a trunk connection to a switchboard for

operator’s assistance on certain types of calls.

Radio Base Station: This installation includes the transmitter, receiver,

duplexer, and antenna units and any necessary control equipment for connection

with the terminal unit over wire line or carrier facilities. A duplexer is a system

whereby transmission and reception may occur concurrently by using two

different frequency carriers for each signal. This enables conversation without

the push to talk requirement. Depending on the terrain, antenna height, and

required operating range, the required output of the transmitter unit may vary

anywhere from 20 to 250watts.

Mobile Radio-Telephone :These units each include a small integrated control/

logic unit, which can be located within convenient reach of the vehicle driver,

and a compact transmitter, receiver package (including duplexer) which can be

located at any out-of-way place (e.g. under a seat or in the vehicle luggage

compartment) The mobile Installation also includes an antenna. The transmitter

output and antenna ensure proper area coverage.

Principles of Operation: The most basic feature of the mobile telephone

system that gives it the flexibility to offer all the rest of its advanced features is

the automatic selection and marking of a radio channel for each cell. Whenever

there are channels idle and available for traffic, the terminal unit selects one and

activates the associated base station transmitter modulated by an idle marking

tone. All idle units scan over their available channels until the idle tone is

detected and lock onto the marked channel.



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The next call in either direction is then established on this channel, with

the participating mobile unit remaining locked to it. In the process of

completing the call, the terminal unit moves the idle marker to another free

channel; again looking idle mobile units to a common channel eliminates the

necessity for the mobile users to monitor the channels to find a free one or to

maintain watch on a separate calling channel.

If all channels are busy, the mobile unit will illuminate its busy lamp

when the telephone handset is removed from the cradle. It also offers the unique

feature of providing the conventional engaged signal in the handset ear piece

when “all-channels-busy” situation exist. A fixed subscriber placing a call to a

mobile unit when all channels are busy may receive the “all-trunks-busy” tone

indication or an optional voice announcement.

To maintain system order each mobile unit is assigned a unique seven-

digit identification and selective calling number. Mobile units will respond only

upon receipt of their assigned number. On mobile-originated calls, the unit

transmits this same number back to the terminal unit as identification so that the

call will be routed to the proper line circuit for completion. When the full

capacity of the seven identification digits is not required, the number of digits

can be reduced accordingly. This can result in a saving of time required for

selective calling and identification.

In the cellular phone, each user communicates via radio from a cellular

telephone set to the cell-site base station. This base station is connected via

telephone lines to the mobile telephone switching office (MTSO), otherwise

called MSC.

The MTSO connects the user to the called party. If the called party is

mobile, the connection is made to the cell site that covers that area in which the

called party is located, using an available radio channel in the cell associated

with the called party.



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The critical consideration in this system is to design the cells for

acceptable levels of channel interference/ as the mobile user travels from one

call to another, the MTSO automatically switches the user to an available

channel on the new cell and the telephone conversation continues uninterrupted.

Consistent with most mobile communications systems, is the polarization

of antenna. The antennas are vertically polarized to guarantee uniform reception

and transmission in all directions, regardless of the direction in which the

mobile vehicle is traveling. The mobile antenna is a half-wave vertical whip,

usually mounted at the top centre of the rear window of the mobile vehicle, and

is used for receiving and transmitting.

Each cellular telephone contains a programmable read-only Memory

(Prom) or Erasable programmable read-only Memory (EPROM) - called a

numeric assignment module (NAM) - that is programmed to contain the

following:

a) The telephone number-also called the electronic service number

(ESN)- of the phone:

b) The serial number of the phone as assigned by the manufacturer.

c) Personal codes that can be used to prevent unauthorized use of the phone.

When the phone is “on-the-air” it automatically transmits its serial

number to the MTSO. The serial; number is used by the MTSO to lock out

phone service to any phone that has been stolen. This feature, of course,

discourages theft of the units. The MTSO used the telephone number of the unit

to provide billing information.

When the phone is used in a remote city, it can be place in the roam

mode so that calls can be initiated or received and yet the service will be billed

via the caller’s ‘’home town’’ company.

To place a call the following sequence of events occurs.

1) The Cellular Subscriber initiates a call by keying in the telephone

number of the called party and then presses the send key.



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2) The MTSO verifies that the telephone number is valid and that the

user is authorized to place a call.

3) The MTSO issues instructions to the user’s Cellular phone indicating

which radio channel to use.

4) The MTSO sends out a signal to the called party to ring his or her

phone. All of these operations occur within 10 secs of initiating the call

5) When the called party answers, the MTSO connects the trunk lines

for the two parties and initiates billing information.

6) When one party hands up, the MTSO frees the radio channel and

completes the billing information.

Note: While call is in progress, the cellular Subscriber may be moving from one

cell area to another, so the MTSO does the followings:

a) Monitors the signal strength from the cellular telephone as received at the

cell base station. If the signal drops below some designated level, the

MTSO initiates a ‘’hand-off’’ sequence.

b) For ‘’hand-off’’, MTSO inquires about the signal strength as received at

adjacent cell sites.

c) When the signal level becomes sufficiently large at an adjacent cell site,

the MTSO instructs the cellular radio to switch over to an appropriate

channel for communication with that new cell site.

Note that MTSO means same thing as MSC.

SPECIAL FEATURES OF CELLULAR MOBILE

Some of the features that make cellular mobile Radio system attractive are

cell planning, frequency re—use and roaming capabilities.

Cell planning: The sizes of cells in CMRS are dependent upon a number of

factors, namely:



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- Radio Transmitter output

- Frequency band used

- Height and location of the Antenna Tower

- The type of Antenna

- Topography of the Area

- Radio Receiver sensitivity.

Although, the above factors determine the size of a cell; the traffic

capacity to a great extent conflicts with the transmitter power requirements.

This is so because when a cellular network is being planned; planning data are

usually based on estimation of subscribers within a cell area, as a result cells

within urban area where traffic is high require smaller size, and thus lower

transmitter power output while in the rural areas with lower traffic capacity,

would require a large cell and proportional higher transmitter power output.

However, with increase in traffic demands, cells are preplanned using cell

splitting and overlaid cells. Cell splitting involves putting more base stations in

area where traffic demand has risen especially in urban areas.

Cell planning arrangement is shown in fig.29, below:

Fig. 29: showing cell planning arrangement



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FREQUENCY REUSE

The principle of frequency re-use arises from the fact that each cell is

assigned a group of channels. Frequency re-use is a fundamental concept in

cellular radio systems, but such systems need careful planning to avoid

degradation by co-channel interference, i.e. interference with calls in one cell

caused by a transmitter in another cell that uses the same set of frequencies.

Frequency re-use is therefore the fundamental target of cellular radio systems,

and implementing this to maximum effect has been the subject of much study

since the inception of the cellular principle.

CHANNEL ASSIGNMENT

In assigning frequencies to cells and thus re-use of frequencies two

requirements must be met; if the problem of co-channel interference is to be

eliminated. The first is that the frequency spacing between any two channels

(within the same cell) must be as large as possible (be equal to the number of

cells in the cluster). A major step forward was the introduction of dynamic

channel allocation (or trunking), which is an essential feature of cellular radio

and has substantially increased spectrum utilization. Although the cellular

concept which relies on frequency re-use is still in its infancy, it has the

potential to produce a radio telephone service to individuals on the move, either

through vehicular installations or by means of hand-held portable equipment.

The setting up of the UK national network and its associated facilities has

produced a demand which has exceeded all expectations. It is the availability of

low-cost microprocessor and other large-scale integrated circuits which has

brought about the most recent advances in mobile radio.

Roaming: Another main attraction of the cellular Mobile Radio System is the

capability of roaming whereby a mobile station already in conversation moves

from one cell area to another without loss of call. In such a situation the mobile

service switching centre (MSc) automatically does the switching from one base



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station to another via a phenomenon of handoff and handover; used to

disconnect the mobile from one Base Station (BS) and to connect it to another

where the signal strength is stronger.

Roaming can take place within an MSc service area or between MSCs

(where the cells between the MSCs Overlap). Manual roaming also exist;

whereby the cells between the MSCs do not overlap and there is no data link

between them. In this case mobile subscriber moving from one MSC to another

would be required to make manual registration to indicate her presence. To

achieve manual roaming, subscribers will request call forwarding on no reply

service from their home MSC (MSC– H) to the other MSCs.

Fig. 30: Network arrangement for TACS

In TACS the BSs are connected by permanent links to mobile switching

centers (MSCs) which are Computer Controlled telephone exchanges

specifically designed for handling cellular services. The MSCs in turn are

connected to the PSTN and to other MSCs. The arrangement is shown in fig 30.



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It enables MSs to communicate with other MSs and to non-mobile users. It also

allows cells to be connected to MSs who are not in their home area.

The cellular network must keep track of all the MSs that are subscribers

to the network. It does this by forming traffic areas which consists group of

cells. In all directions hence in such a situation it is possible to calculate the

power density at a distance or from the antenna. If the antenna radiates a total

power pt, the power at any distance r from the antenna is the power passing

through the surface of a sphere of radius r. The surface area of the sphere is 4 r2

and the power received per unit area is given by

Pa = pt watts/m2 (1)

4 r2

At sufficiently large value of r the wave becomes a plane wane. The power

received by an antenna placed in this field is

Pr = Pa Ae (2)

Ae is known as the ‘effective aperture’ of the antenna and is the equivalent

power absorbing area of the antenna. The effective aperture of an isotropic

antenna when used as a receiver can be shown to be Ae – 2/4

Hence the power received by such an antenna is

Pr = Pa X 2/4

But Pa = Pt / 4 r2

i.e.

Pr = Pt 2 (3)

(4 r) 2

Note: The isotropic antenna has unity gain in both the transmit and receive

modes. A non-isotropic transmit antenna will have gain of Gt and the product

Pt Gt is known as the effective radiated power (ERP). In mobile radio ERP is

used as the standard method of quoting transmitted power. In effect, if the ERP

is quoted as 100W (50dBm) and the antenna gain is 10W ( 40dBm ).



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A non–isotropic receive antenna will have gain Gr and, in such cases, the

received power would be given by

Pr = Gt Gr Pt 2 4

(4 r) 2

This expression indicates that the attenuation is proportional to distance squared

(d)2.

SPACE WAVE PROPAGATION

In mobile radio, we consider the height of both transmit and receive

antennas above the earth’s, surface. If the height of the base station antenna is

hl and the height of the mobile antenna is h2, the system may be represented as

shown also in Fig. 25, earlier where the separation between transmitter and

receiver is d. In the Figure 26, it is assumed that‘d’ is small enough to neglect

the earth’s curvature. From there we see that there will be both a direct and

ground reflected wave. The direct path length is dd and the reflected path length

is dr. It can be seen from the geometry of the system that

dd = d2 + (h1 – h2)2 (5)

Using the binomial expansion and noting that d hl or h2, the length of the direct

path approximates to

dd d 1+0.5 h1 - h22

d

Similarly

dr d 1+0.5 h1- h22

d

The path difference is thus



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d = dr - dd, i.e.

d = 2h1h2 (6)

d

The corresponding phase difference between direct and reflected path is

Ø = 2 X 2h1h2 = 4 h1h2

d d ( 7)

The total received power is thus

Pr = Pt ( / d) 2 X 1 + e Ø 2 (8)

is the reflection coefficient and for low angles of incidence the earth

approximates to an ideal reflector with

= -1, i.e.

Pr = Pt ( / d)2 X 1 – e Ø 2 (9)

But 1 – e Ø = 1 – Cos Ø – j Sin Ø,

Hence (1 – e ø) 2 = (1 – Cos ø) 2 + Sin 2 ø.

If ø << 1 then, Cos ø = 1 and Sin ø = ø

Pr = Pt ( / d)2 4 h1h22

d

Hence

Pr = Pt h1 h22

d 2 (10)

This is the 4th power law used in the frequency re-use calculation and is

known as the plane earth propagation equation. The loss is given by:

loss (dB) = 40 log10 d – 20 log10 h1 – 20 log10 h2.

This means that the loss increases by 12 dB each time the distance is

doubled. It should be noted that this equation is not dependent on frequency,



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which is a surprising result. But this is based in the assumption that hl and h2

are much smaller than d and the earth is flat and perfectly reflecting.

PRINCIPLES OF FADING

It should be noted that as the mobile moves, there are substantial

amplitude fluctuations in the received signal known as fast fading. A typical

variation of signal strength with distance is shown in Fig,34.

Fast fading (due to local multipart) is also accompanied by a slower variation in

mean signal strength known as slow fading

Fig. 31: Fading due to multipart propagation

Fast fading is observed over distance of about half a Wavelength and can

produce signal strength variations, in excess of 30 dB. Slow fading is produced

by movement over much longer distanced, sufficient to produce gross variations

in the overall path between base station and mobile.

SHORT-TERM FADING (FAST FADING)

When a mobile unit is stationary the received signal strength will be

formed by the vector sum of the various signals reaching the antenna and will

give constant amplitude. But when the mobile is moving it is assumed that the

signal received will be the vector sum of N reflected signal of equal amplitude

which arrive at the receiving antenna at a random phase angle. This is accepted



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as a reasonable model for the cellular environment where there is not usually a

direct line of sight path between transmitter and receiver. (If there is direct line

of sight component this will alter the nature of the fading envelope and its

statistics).

Applying the central limit theorem it can be shown that the received

electric and magnetic field components have independent Gaussian

distributions. This may lead us to conclude that the envelope of the resultant

received carrier has amplitude which has a Raleigh distribution given by

Pa (a) = a exp ( -a2 ) (11)

2 2

In the above expression 2 is the mean square value (i.e. mean power) of

the carrier envelope and ‘a ‘is the instantaneous amplitude of the envelope.

The distribution function is shown in Fig. 32.

Fig. 32, Resultant carrier envelope distribution function

Note that the probability density function has a peak value of

0.6 at a = where is the

rms value of the received signal. The corresponding cumulative distribution

function (CDF) is Prob(a<A)= Pa(A)=1–exp(A) (12)

2 2



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When the CDF is known it is possible to determine the average number

of times per second that the signal envelope crossed a particular level in the

positive direction. This is known as the level crossing rate. The level crossing

rate is related to the velocity of the mobile V and the wavelength of the received

carrier and, for a vertical monopole antenna, we have that

N (Ao) = V P exp (- 2). (13)

Where P = Ao and Ao is the specified level.

The situation is shown in Fig. 33.

Note: N (Ao) is a maximum when Ao is 3 dB below the rms carrier level. This

can be explained by the observation that if Ao is low, the envelope is this level

for a large proportion of the time and hence the number of crossings per second

decreases.

A similar situation

Fig. 33,

Fading experienced by a mobile is observed when Ao is set at a high

level, the envelope being below this level for a large percentage of the time

which again reduces the average number of crossing per second.



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The duration of a fade is the interval of time that the envelope remains

below the level Ao and this is also shown in fig. 33. Hence the average duration

of fades below the level Ao is

(Ao) = Prob [a < Ao]

N (Ao)

However, prob [a < Ao] = 1 –exp (- 2), hence the average fade duration for a

vertical monopole antenna is

(Ao) = [ exp ( 2) - 1] (14)

PV 2

QUESTIONS

1. Mention one special feature of cellular networks. Hence, state three

implications associated with such characteristic.

2. Briefly discuss the problems that may be encountered in attempting to

transmit high speed base band digital data over the public switched

telephone network (PSTN).

DIGITAL CELLULAR RADIO

There are a number of significant advantages to be achieved with digital

systems which makes their adoption as second generation systems attractive.

The global system for mobile communication (GSM) was chosen after

satisfying the following criteria based on the available digital techniques.

They include:

i. High subjective voice quality

ii. Low infrastructure cost

iii. Low mobile equipment cost

iv. High radio spectrum efficiency

v. Capability of supporting hand held portables

vi. Ability to support new services



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vii. The ability to coexist with existing systems.

THE GSM TECHNOLOGY

In GSM the voice wave form is digitally encoded before transmission.

The system is based in TDMA (Time Division Multiple Access) hence

individual users are given access to the radio channel for a limited period and

transmit a burst of binary information. Before we go into the details of the

system, let us have a look at the principles of GSM operation and compare it

with the analogue system. RBS BSC MSC Destination.

GSM has two main objectives: pan- European roaming, which offers

compatibility throughout the European continent, and interaction with the

interacted services digital network (ISDN), which offers the capability to extend

the single-subscriber line system to a multi-service system with various services

currently offered only through diverse telecommunication networks system

capability was not an issue in the initial development of GSM, but due to the

expected rapid growth of cellular service, 35 revisions have been made to GSM

since the first issued specification. The first commercial GSM system, called

D2, was implemented in Germany in 1992.The Figure below shows the external

environment of the base station sub-system.

Fig 34: showing the external environment of the BSS.



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Where MS means mobile station

BSS - base station sub-system

NSS - network and switching subsystem

OSS - operation subsystem

MULTIPLE ACCESS SCHEME

o General description: GSM is a combination of FDMA and TDMA. The

total number of channels in FDMA is 124, and each channel is

200khz.both the 935-960Mhz downlink have been allocated 25Mhz for a

total of 50Mhz.if TDMA is used within a 200Khz channel, 8 time slots

are required to form a frame, frame duration is 4.615ms, and the time slot

duration burst period is 0.577ms.there is a DCS1800 system, which has

the same architecture as the GSM, but it is up converted to 1800Mhz.the

downlink is 1805-1880Mhz (base TX) and the uplink is 1700-1785Mhz

(Mobile Tx).

o Constant Time Delay Between Uplink And Downlink: the numbering

of the uplink slots is derived from the down link slots by a delay of 3

times slots. This allows the slots of one channel to bear the same slot

number in both directions.

In this case, the mobile station will not transmit and receive

simultaneously because the two time slots are physically separated. Propagation

delay when the mobile station is far from the BTS (Base Transreceiver Station)

is a major consideration. For example, the round trip propagation delay between

an MS and BTS which are 35km apart is 233µs. as a result, the assigned time

slot number of the uplink and down link channels may not be the same (less

than 3 time slots apart).the time compensation for the propagation delay

(sending to the mobile station via Slot associated control channel (SACCH) is 3

time slots minus the time advance. Recall that the solution to the propagation

delay earlier mentioned is to let the BTS compute a time advance value. And



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the key purpose to allow significant guard time by taking into account that

BCCH (Broadcast control channel) is using only even time slots.

o Frequency hoping: GSM has a slow frequency hoping radio interface.

The slow hoping is defined in bits per hop. Its regular rate is 217 hops/s,

therefore with a transmission rate of 270 kbps, the result is approximately

1200 bits/hop.

o Different types of time slots: each cell provides a reference clock from

witch the time slots are defined. Each time slot is given a number (TN)

which is known by the base station and the mobile station. The time slot

numbering is cyclic.

o Burst and training sequences: In TDMA, the signal transmits in bursts.

The time interval of the burst brings the amplitude of a transmitted signal

up from a starting value of 0 to its normal value. Then a packet of bits is

transmitted by a modulated signal. Afterward, the amplitude decrease to

zero. These bursts occur only at the mobile station transmission or at the

base station if the adjacent burst is not transmitted.

BASIC OPERATIONS OF THE DIGITAL CELLULAR RADIO (GSM)

The word cellular loosely translated refers to a honey comb like structure of

a network such as the one being constructed by Celtel Wireless. The area

covered by a cellular network is divided into cells and each cell has a base

station which receives and transmits calls made to and from cellular phones.

Each cell is in turn connected to a mobile switching centre (MSC), and can

handle many calls simultaneously. The MSC directs your calls to the

destination cell in the network of telephone exchange area, and because some

calls will be destined for the fixed line telephone network, in such situations,

the MSC will route the calls to the Telephone Network Exchange popularly

called public switched telephone network (PSTN) which is currently managed



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by NITEL. From there the call finds its route to the specific fixes line telephone

being called. If the call is originating from NITEL line, MSC will transmit the

call to as Celtel base station from where it is routed to the receiving subscriber.

All these protocols happen in milliseconds.

Note: Inserting a SIM card and entering your PIN (personal identification

number) which ‘Logs’ you onto the network activates your cell phone. Once the

PIN has been accepted, you are ready to make and receive calls. SIM card

(subscriber identity module) is the key to making and receiving calls, and also

ensures accurate billing each moment. If you remove the SIM and fit it to

another compatible phone, the calls you make will be charged to your account.

SIM cards come in two sizes, large and small you should be supplied with the

right size to fit your phone.

Smart phone - a combination of mobile phone and pocket computer.

Roaming – some digital mobile phones can be used abroad. This is called

roaming. All charges are higher than normal; however, you can pay for

incoming calls. To use abroad, arrangements have to be made with your

network before you travel and they may require some deposit in order to grant

you access.

THE GSM RADIO INTERFACE

The radio subsystem constitutes the physical layer of the link between mobile

and base stations.

The main attributes of GSM interface are:

1. Time division multiple access (TDMA ) with 8 channels/ carrier

2. 124 radio carriers in a paired band (935 to 915 MHz) mobile to base

station, (935 to 960 MHz) base to mobile, inter-carrier spacing 200 KHz.

3. 270. 833.kb/ S per carrier

4. Gaussian minimum shift keying with a time band-width product BT= 0.3



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5. Slow frequency hopping ( 217 hops/ second )

6. Synchronization compensation for up to 233 absolute delay.

7. Equalization of up to 16 time dispersion

8. Down link power control

9. Discontinuous transmission and reception

10.Block and convolution channel coding coupled with inter leaving to

combat channel perturbations.

It should be recalled that the first pair of carriers in the GSM system are

890.2 MHz and 930 MHz .This gives a spacing of 45 MHz. GSM recommends

that the carriers 1 and 124 are not used due to energy of the modulated carrier

lying outside the nominal 200 KHz bandwidth. Multiplexing techniques is

employed in GSM system. Each cell can have from 1 to 15 pairs of carriers and

each carrier is time multiplexed into 8 slots. The carriers and their associated

time multiplexed slots from the physical channels of the GSM system. The

operation of the radio subsystem is divided into a number of logical channels

each of which has a specific function in terms of handling the transmission of

information over the radio subsystem.

There are two major categories of this channel viz:-

1. Traffic channels (TCH)

2. Signaling channels

Under signaling channels, we have

a) Common control channel (CCCH)

b) Broadcast control channel (BCCH) and

c) Dedicated control channel (DCCH)

The radio subsystem requires two channels for its own purposed. They are:

a) The synchronization channel (SCH)

b) The frequency correction channel (FCCH) some tasks performed

by the physical layer include:



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1. Create physical channels by building data bursts and

transmitting them over the radio path.

2. Map the logical channels onto the created physical channels,

taking into account the through put needs of particular logical

channels.

3. Apply error protection to each logical channel according to its

particular needs.

4. Monitor and control the radio environment to assign dedicated

resources, and combat changes in propagation characteristics by

functions such as handover and power control.

QUESTION

What is the significant of slow frequency hopping in GSM system?

Hint: Since fading is frequency dependent, slow frequency. hopping may be

used to over come fading.

GSM MODULATION, CODING AND ERROR PROTECTION

There are several stages of coding and decoding in the GSM traffic

channels. These are shown in Fig.35.

Fig. 35, Coding and Decoding in GSM



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The channel coding is most efficient when bit errors are uniformly

distributed within the transmitted bit stream. However errors due to fading

cause errors to occur in burst. The problem is reduced by a technique known as

bit inter-leaving. Clearly it is necessary to have two speech blocks available

before a normal burst can be formed. This requires an interval of 40ms and

hence the coding delay for GSM is of the order of 40ms. The radio path is

subject to multipath propagation which can produce a delay spread of several

microseconds. This becomes apparent at the receiver as inter-symbol

interference.

The form of modulation used in GSM is the Gaussian minimum shift

keying (GMSK).Two problems need to be addressed here.

1. Minimum bandwidth

2. Minimum error probability.

It should be recalled that the standard frequency shift keying uses two

separate carriers f0 and f1 to transmit binary o and binary 1. In order to produce

the smallest error probability the carriers’ f0 and f1 must be orthogonal, i.e. they

must have a correlation coefficient which is Zero. Also to minimize the

bandwidth of the transmitted signal it is necessary to determine the minimum

difference between f0 and f1 which will produce orthogonal signals and this is

called minimum shift keying. If the number of cycles of f0 in the interval T (

where T is the duration of bit period ) is no, then the number of cycles of M in

the same interval to achieve orthogonality must be n1= no + 0.5 or ( f1- f0 ) =

1/2T. Hence MSK is effectively FSK with the minimum frequency difference

between f1 and fo.

If MSK is considered in terms of the modulation of a single carrier

frequency fo, then the instantaneous frequency is given by

fI = fc + afd. (15)

Where a = + 1 and fd is the carrier deviation.

For MSK the carrier deviation fd = ¼ T.



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Hence, considering the MSK signal in terms of frequency modulation gives an

expression for the modulation carrier of

1

Vc (t) = cos (2 fct + a fd dt) (16)0

QUESTIONS

1. Mention three ( 3 ) attributes of GSM interface and explain briefly what

each represents.

2. What do you understand by through- put in relation with data

transmission?

3. Differentiate between fast fading and slow fading.

4. Why is frequency planning a major issue in the design of a cellular

system?

5. Fading is a frequency selective phenomenon discuss.

6. Given that a carrier frequency of 900 MHz and a mobile speed of 48Km

/h the level crossing rate at Ao= 3 dB. Calculate N (A0)

7. What constitutes the physical layer of in the GSM technology?



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Ece 542

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