Download - E5122 COMMUNICATION SYSTEM
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