telecommunication system engineering lecture 2

7/29/2019 Telecommunication system engineering lecture 2

1/35

Telecommunication system

engineering

Introduction to Telephony

Lecture # 213-02-2013



2/35

Telecommunications

Telecommunication deals with the service ofproviding electrical communications at adistance.

The world wide public switched telephonenetwork (PSTN) is immense.

This same PSTN serves as a vehicle for videoconferencing and point to point connectivity.

The primary concern of this course is to describethe development of PSTN, why it is build like thisand how it is evolving.



3/35

A simple telephone connection

A common telephone is a device connected tooutside world by a pair of wires.

It consists of a hand set and a cradle with asignalling device mainly push buttons these days.

The handset is made of two electro-acoustictransducer, a mouthpiece (transmitters ormicrophone) and an earpiece (receiver orspeaker).

There is a side-tone, that is the sound ofspeakers own voice heard from the earpiece. Thelevel of the side-tone must be controlled ?



4/35


(contd)

The transmitter converts acoustic (sound) energy into theelectrical energy through carbon granule transmitter.

To transmit an electrical signal, a dc potential of 3-5 V isrequired across the its electrodes.

This voltage is supplied by central battery in the switchingcentre, that has been standardized at -48 V dc.

Off-hook

The condition where handset is separate from the cradle. It is an activestate i.e., the closed loop between the network and the telephone and a

tone can be heard. On-hook

The condition where handset is on the cradle. It is an idle state i.e., theopen loop between the network and the telephone.



5/35


(contd)

Dual-tone multi-frequency signaling

DTMF (dual tone multi frequency) is the signal that you generate when you press

an ordinary telephone's touch keys. With DTMF, each key you press on your phone

generates two tones of specific frequencies. So that a voice can't imitate the tones,

one tone is generated from a high-frequency group of tones and the other from a

low frequency group. Here are the signals you send when you press your

Touchtone phone keys:


Frequency 1209 Hz 1336 Hz 1477 Hz

697 Hz1 2 3

770 Hz 4 5 6

852 Hz 7 8 9

941 Hz * 0 #


6/35


(contd)

System limitations Length limitation for a 19 gauge loop connection for

the standard central power supply.

Limitations are attenuation and voltage drop at the

transmitters. Supported length for operation is 30 km, for an

efficient handset.

The length can be increased by adding amplifiers at

suitable distanced to maintain operation.

Introduction to TelephonyA B


7/35


(contd)

The system mentioned previously is for two subscribers only,

what if there are multiple users in the picture?

Build more personal loops among the users.

There should be a sort of switching mechanism to select the specific

required user.


A 8-point mesh connection Subscriber connected in star arrangements


8/35

Traffic, sources and sinks

Trafficis the term that quantifies the usage of

the telecommunication system.

Sources are the initializers of the call.

Sinks are the call destination points.

Nodal points or nodes are termed as switches.



9/35

Telephone network: Introductory

terminologies Telephone network

A systematic development of interconnecting transmission mediaarranged so that one telephone user can other with in the network.

Subscriber line

The telephone line connecting a subscriber to a switch are subscriber

lines. Trunks

The telephone line connecting one switch to another is called trunk.

Local Exchange

A local exchange has a serving area, which is geographical area in whichthe exchange is located: all subscribers in that area are served by that

exchange. Toll area

A toll area includes many exchanges, toll calls are same as long distancecalls



10/35

Essentials of traffic

Most important part of telecommunications engineeringpractice is to determine, the number of trunks requiredbetween the exchanges, it is termed as dimensioning ofroute.

Key parameters in dimensioning

Traffic path: it is a line, path, time-slot, frequency-band, trunk, orswitch.

Calling rate: number of times a traffic path is used for a period oftime.

Holding time: the duration for which the traffic path was occupied.

Carried traffic: is the volume of the traffic actually carried by aswitch.

Offered traffic: maximum volume of the traffic that the switch canallow .



11/35

Typical traffic intensity



12/35

Busy Hour definitions

Busy Hour

The busy hour to one hour period for which thenumber of calls attempted are the greatest.

Peak Busy HourThe peak hour every day.

Time consistent busy hour

The one hour period for which the average call-attempt count is greatest over the days underconsideration.



13/35

Measurements of telephone traffic

Traffic measurements are used for long term

planning of the network.

Lets define the telephone traffic as the product

of number of calls originated during a period of

one hour (C) and average holding time (T).

A = C x T

It is dimensional less quantity, but it is also usedas call-seconds, call-minutes or call-hours.



14/35

Measurements of telephone traffic

(contd)

Traffic density

number of simultaneous calls at a givenmoment.

Traffic intensity

The average traffic density during 1-hourperiod.

The preferred unit of traffic intensity is Erlang, it is also adimensionless quantity.

1 Erlang means that one circuit is occupied for 1 hour.



15/35

Blockage, Lost calls, and grade of

service

In order to understand these terms lets first

consider

Peg count: calls offered

Usage: Traffic carried

Overflow: call encountering trunk busy

Grade of service:Grade of service = number of lost calls / Peg count



16/35

Handling of lost calls

Lost calls are handled in numerous ways some

of them are.

Lost call held (LCH)

Lost call cleared (LCC)

Lost call delayed (LCD)



17/35

Handling of Lost calls

Lost Calls Held (LCH)

This concept assumes that the telephone user willimmediately reattempt the call on receiving the congestionmessage and will continue to redial. The user hopes toseize the next available switching equipment is free.

Lost Call Cleared (LCC)

On receiving the congestion signal, the user will hang upand wait for a while for attempting the call. Such callsusually disappear from the system.

Lost Call DelayedThis concept assumes that the user request is automaticallyput in a queue, where it waits for its turn to be routed.


18/35

Probability distributions curves for

traffic theory

The origination calls at an exchange closely fit a family of probability

distribution curve following the a Poisson distribution.

Most distribution curves are two parameters curves, mean and variance.

mean is average

variance is parameter for dispersion


19/35

Smooth, Rough and Random traffic

VMR (variance to mean ratio)

It is the coefficient of over-dispersion, and characterizes the

traffic patterns.

Smooth traffic: VMR < 1

Random traffic: VMR = 1

Rough traffic: VMR > 1


20/35

Erlang traffic formula

Erlang traffic formula is commonly used for dimensioning the route (finding the

number of trunks required to accommodate required traffic).

The parameters that are basically dealt with are for computing the traffic are

Call arrival and holding time distribution

Number of traffic sources

Availability of the traffic sources

Handling of lost calls

nis number of serving channels

A is mean traffic offered

EB

is the grade of service


21/35

Erlang B table


22/35

Waiting systems (Queuing)

Queuing system is required when the scenario

of lost call delay (LCD) is encountered.

There are four ways (queuing discipline) to

select the waiting call from the queue.

First come first serve

Random selection

Last come first serve

Priority selection


23/35

Waiting systems (Queuing) contd

Grade of service for the queuing system is

defined by the probability of delay.

Other factors include

Average delay on all calls

Length of the queue


24/35

Dimensioning and efficiency

Efficiency is rate of utilization of resources.

100 % efficiency will be achieved when all the

trunks are busy with calls for all the times.

Practically achieving 100%, will mean that

there will be a lot of lost calls, hence

decreasing the grade of service.

Optimize dimensioning will mean a trade off

between grade of service and efficiency.


25/35

Alternate routing

One way to achieve higher efficiency is to usealternate routing.

Lost calls are routed through routes other thenthe direct route to the destination.

The basic problem is to increase circuit groupefficiency.

X Y

Z

Direct route


26/35

Efficiency versus circuit group size

Increase in the circuit group (using the higher

number of trunks), increases the efficiency.

For a grade of service p = 0.01

11 trunks will carry 5 Erlang of traffic, trunk to

Erlang ratio is more than 2:1.


Erlang ratio is 3:2.


Erlang ratio is 6:5.


27/35

Basis of network configuration

Tandem Exchange

Mesh Configuration

Usually applied for

higher traffic among

exchanges

Star Configuration

Usually applied for

lower traffic

among exchanges Double star configuration

Star connections utilizing an

intervening(dominant) exchange

for the connection among them

selves

Note: In practice most networks

are compromise between mesh

and star configurations


28/35

Hierarchical networks

Hierarchal networks are systematic networks

developed to reduce trunk groups.

This network gives orders of importance to

the exchanges making up the network.

1A 1B

2A1 2A2 2B1 2B2

3A1 3A2 3A3 3A4 3B1 3B2 3B3 3B4

The rank of the exchanges is based on the sizes of the boxes


29/35

Connection through hierarchical

networks1A 1B

2A1 2A2 2B1 2B2

3A1 3A2 3A3 3A4 3B1 3B2 3B3 3B4

1A 1B

2A1 2A2 2B1 2B2

3A1 3A2 3A3 3A4 3B1 3B2 3B3 3B4

The case with no

congestion

between 2B1 & 2B2

The case with

congestion between

2B1 & 2B2. The

dotted line is

showing final route.

The route for the overflow traffic through the highest exchange level is the Final route.


30/35

The trend away from a hierarchical

structures

There are trends that are pushing away thehierarchal networks, but they will remain inthe system for foreseeable future.

The reasons are Satellite communications allowed direct routes

between continents.

Optic fibres offered higher performance and

higher bandwidths over long distances.

Optimum routing through signalling system 7.


31/35

Routing method

There are generally three routing methods between intermediate nodes(switches).

Right through routing

The originating exchange determines the route of the route from source to

destination.

Addition of a new exchange in the network will require the updated at allexchanges.

Own exchange routing

This routing allows changes in routing as call proceeds to its destination.

Minimal switch modification is required for the addition of new exchange.

It is suited when alternate switching is mostly encountered.

Computer controlled routing

The switching is computer based.

The route is mapped in memory based on network details.


32/35

Quality of service

The most common measure of quality of serviceis customer satisfaction.

It is usually measured by how well the customer

can hear the calling party Other factors include

Delay before receiving dialling tone

Post dial delay

Correctness of billing

Reasonable cost of the customer services.

Responsiveness to servicing requests.


33/35

Sample problems 1

The two neighboring exchanges that were

build a decade ago, had 5 trunks between

them, with the increase of users now, the

grade of service has declined to 0.4. Howmany more truncks will be required to

improve the scenario. Further more the user

requirements is expected to rise by 10% within an year. How many more truncks will be

needed to meet these future requirements.



34/35



35/35

Sample problem 2


telecommunication system engineering lecture 2

Documents