E5122COMMUNICATION SYSTEM

EXCHANGE Telephone exchange is a place/centre

where telephone calls are connected to enable voice conversation.

3 types of exchange: Local Tandem International

Local exchange-connect to the customer (subscriber)

Tandem exchange- A telephone central office switch that links telco end offices together and does not connect to the customer directly

International exchange-connects exchange to entire country and oversea.

• Switched Services:• Switched calls are dialable  (users dial a telephone

number to make a connection.• People can reach anyone on public network by

dialing a telephone numbers.• Charges are based  on  amount of time calls

connected.• Can be used for voice/data/video/ image traffic.• Can be used with analog and digital telephone lines.• When callers hang up, network line and equipment

are free to be used by another person or data device.

As the number of telephones increases, so does the number of transmission lines used for calls between them, at a surprisingly large expense.

Switching equipment is used to share these expensive transmission lines.

A network consisting of at least one switching system (exchange) and accommodated transmission lines (optical fiber microwave) is referred to as a telephone network.

1 phone-3 phone

1 phone-7 phone

1 phone-Many phone

National Exchange Hierarchy

MSC

LE

DSC

ZSC

S

Main switching

center

Local Exchange

District switching

center

Zone switching

center

Subscriber

Function Of Exchange Record customer’s call meter Monitor switching process between

exchange and customer Record customer’s request (meter

termination) Provide supervision tone such as dial and

busy tone. Control conversation quality and

exchange service.

Manual Switching

SWITCHING NETWORK Concentrator (Penumpu) Distributor (Pengagih) Expander (Pengembang) Multiplexer

CONCENTRATOR

CONCENTRATORDISTRIBUT

OR

CONCENTRATOR A device that can funnel many line into

few lines. Output line will be fed into distributor. Example: 1000 line entered concentrator

and only 100 line comes out from concentrator.

Concentration -- many lines funneled into a few trunks

Distribution or Routing -- interconnecting trunk lines

Expansion -- a few trunks expanded to many lines

DISTRIBUTOR

CONCENTRATOR

EXPA

NDER

Line 1Line 2

Line 1000

Line 1Line 2

Line 1000

Distributor A device between

concentrator and expander.

Receive output line from concentrator and fed it into expander.

Expander Receive output

line from distributor.

Expand the line back into source line.

Example: receive 100 line from distributor and expand it back to 1000 line.

MULTIPLEXER A process where multiple analog

message signals or digital data streams are combined into one signal.

The aim is to share an expensive resource.

Method of multiplexing: TDM (Time Division Multiplexing) FDM (Frequency Division Multiplexing)

TDM FDM

MUX

Line 1Line 2

Line 1000

DEM

UX

Line 1Line 2

Line 1000

Stored Program Control (SPC) Block Diagram

DriverScanner

Ringer

Memory

CPU

Control Director

File

Switching# 2

# n

# 1# 1

# 2

# n DTMF

1 2 n

Block Diagram Function Memory-save user’s status activity in software Control director-software that control all

system File-1)save payment data 2) prepare special

schedule for other services DTMF (Dual Tone Multi Frequency)-decodes

signal produced during dialing process Scanner-detect phone lift status and signaling

from cellular line Driver-1) connect/disconnect switch 2)

provide path for tone and dial signal CPU-control and execute stored software Ringer-generate signal to show incoming call

SPC Advantages Maintenance and monitoring process are

easier because exchange operation can be changed any time using programming

Signaling are easier and more efficient. Additional channel doesn’t need

additional signaling equipment. Process is faster than manual method.

SPC Disadvantages Minor damage will stop exchange

operation. Need extra building and equipment (ex:

air conditioning room and programming) Need to train all workers to operate the

process. Maintenance are dependence on supplier.

SIGNALLING

The information/instruction exchange between different section in a telecommunication system.

Involved section: Caller with exchange Exchange with exchange Exchange with receiver

Signal type MeaningSeizure Signal when the handle is

lift (off-hook)Clear-forward Signal when the handle is

put back (on-hook)Address Dial tone to exchange

a) Caller to exchange

Signal type MeaningRinging tone Signal to indicate

incoming call

b) Exchange with receiver

Signal type MeaningAnswering Signal when receiver lift the handle (off-hook)Clear-back Signal when receiver put back the handle (on-

hook)

c) Receiver with exchange

Tone type MeaningDial Tone (DT) Tone when receiver lift the handle.Busy Tone (BT) Tone when the dialed number is in used.Number Unobtainable Tone (NUT)

Voice message when the dialed number is not in service.

Ring Tone (RT) Tone when the call is connected.

d) Tone from exchange to caller/receiver

CHANNEL ASSOCIATED SIGNALING (CAS)

Voice and control signal are send in the same channel/circuit.

Can be controlled by digital or non-digital exchange.

Signaling speed is slow.

(CAS)Exchange A Exchange B

Traffic channel

Signaling transmitter/receiver

COMMON CHANNEL SIGNALING (CCS)

Voice signal and control signal are send on different channel.

Control signal will be detected and execute control duty and line switching.

Fully controlled by computer.

CCSExchange A Exchange B

Traffic channel

Signaling line

Signaling transmitter/receiver

Traffic Engineering – Traffic Intensity

Holding Time - the length of time that a resource is being held (the duration of a phone call)

Traffic volume - for an interval is the sum of all the traffic holding times for that interval

Traffic intensity = traffic volume / time interval which is a measure of demand

Erlangs - describe traffic intensity in terms of the number of hours of resource time required per hour of elapsed time

TRAFFIC UNITS The international dimensionless unit of

telephone traffic is called the Erlang after A. K. Erlang (1878 –1929) a Danish scientist.

Defined as one circuit occupied for one hour. 1 Erlang= 1 Call–hour / hour 1 Erlang= one circuit busy 100% of the time 1 Erlang= two circuits busy 50% of the time Traffic of one Erlang refers to a single resource

being in continuous use, or two channels being at fifty percent use, and so on.

1 hour of continuous use of one channel = 1 Erlang

1 Erlang = 1 hour (60 minutes) of traffic 1 Erlang= 1 call last for 1 hour or 2 calls with

an average call duration of 30 minutes 2 E =2 telephone operators who are both

busy all the time 0.5 E =a radio channel that is occupied for

30 minutes during an hour 10 erlangs =300 two-minute calls in an hour

The traffic intensity offered by each user is:

A= Traffic intensity H=The average holding time of a call C=The average number of call

requested/hour T=1 hour / 60 minutes / 3,600 seconds

A = CH / T Erlangs

Example 1 A call established at 1am between a mobile and

MSC. Assuming a continuous connection and data transfer rate at 30 kbit/s, determine the traffic intensity if the call is terminated at 1.50am.

Solution:C=1 callH=50 minutesTraffic intensity (A) = CH/T= (1 call)*(50 mins)/(60 min) = 0.833 E

Note, traffic intensity has nothing to do with the data rate, only the holding time is taken into account.

Example 2 Consider a PSTN which receives 240

calls/hr. Each call lasts an average of 5 minutes. What is the outgoing traffic intensity to the public network.

SolutionC = 240 callsH = 5 minutesA = CH/T= (240 calls x 5 min) / 60 min = 20 E

Example 3 Find traffic intensity for a system if 60

calls are received in one hour, each call lasting 5 minutes

SolutionC=60 callsH=5 minutesA = CH/TA = 60 x 5 / 60= 5 E

Example 4 If a group of user made 30 calls in one

hour, and each call had an average call duration of 5 minutes, find traffic intensity.A = CH/T

= (30 x 5) / 60= 2.5 E

Example 5 A system received 120 outgoing calls with

duration of 2 minutes and 200 incoming calls with duration of 3 minutes. Find outgoing traffic, incoming traffic and total traffic.

SolutionIncoming traffic, Ain= 200 calls x 3 min / 60 min =

10 EOutgoing traffic, Aout= 120 calls x 2 min / 60 min

= 4 ETotal traffic = Aout + Ain = 14 E

Grade of Service (GOS), B How to compare the quality of services

provides by different service providers? What is the probability of not being able to

make a call? What is the probability of waiting before a call

is connected? All these can be explained by the Grade of

Service (GOS)

When a user attempts to make a telephone call, the routing equipment handling the call has to determine whether to accept the call, reroute the call to alternative equipment, or reject the call entirely.

Rejected calls occur as a result of heavy traffic loads (congestion) on the system and can result in the call either being delayed or lost.

If a call is delayed, the user simply has to wait for the traffic to decrease, however if a call is lost then it is removed from the system.

The Grade of Service is one aspect of the quality a customer can expect to experience when making a telephone call.

Grade Of Service, B is used to observe and measure how many calls are offered, carried and lost in the system.

The lower this number, the higher the GOS.

B = Number of calls lost Number of calls offered

B = Traffic lost Traffic offered

For example, if GOS = 0.05, one call in 20 will be blocked during the busiest hour because of insufficient capacity

For GOS = 0.02. This means that two users of the circuit group out of a hundred will encounter a call refusal during the busy hour.

Example 6 1200 calls are offered to a channel and 6

calls are lost. Duration of a call is 3 minutes. Find:

a) Offered traffic, Ab) Carried trafficc) Lost trafficd) GOS, Be) Congestion time

a) A = CH/T = (1200 x 3) / 60 = 60 Eb) Carried traffic = [(1200-6) x 3] / 60 =

59.7 Ec) Lost traffic = (6 x 3) / 60 = 0.3 Ed) B = Lost traffic / Offered traffic

= 6/1200 = 0.005

e)Congestion time = B x 1 hours (second)= 0.005 x 60 x 60= 18 seconds

Call Intensity Call intensity, λ which is the mean

number of calls offered per time unit, and mean service time s.

The offered traffic is equal to: λ = A / s

A=Traffic intensitys= Average holding time for a call

Example 7 Given traffic intensity of 90 E and holding

time for a call is 3 minutes. Find call intensity.

λ = A / s = (90 x 60) / 3 = 1800 call