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Telephone exchange is a place/centre where
telephone calls are connected to enable voice
conversation.
3 types of exchange:
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Local
Tandem
International
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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.
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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.
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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-Many phone
1 phone-7 phone
Main
switching
center
MSC
ZSC
District
switching
center
DSC
LE
Subscriber
Zone
switching
center
S
Local
Exchange
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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.
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Concentrator (Penumpu)
Distributor (Pengagih)
Expander (Pengembang)
Multiplexer
CONCENTRATOR
DISTRIBUT
OR
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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.
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Concentration -- many lines funneled into a
few trunks
Distribution or Routing -- interconnecting
trunk lines
Expansion -- a few trunks expanded to many
lines
Line 1000
DISTRIBUTO
R
EXPANDER
CONCENTRAT
OR
Line 1
Line 2
Line 1
Line 2
Line 1000
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Distributor
A device between
concentrator and
expander.
Receive output line
from concentrator
and fed it into
expander.
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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.
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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:
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TDM (Time Division Multiplexing)
FDM (Frequency Division Multiplexing)
TDM
FDM
MUX
Line 1000
DEMUX
Line 1
Line 2
Line 1
Line 2
Line 1000
#1
#1
#2
#2
Switching
#n
DTM
F
#n
1 2
Scanner
n
Ringer
Driver
CPU
Memory
Control
Director
File
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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
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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.
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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.
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The information/instruction exchange between
different section in a telecommunication
system.
Involved section:
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Caller with exchange
Exchange with exchange
Exchange with receiver
a) Caller to exchange
Signal type
Meaning
Seizure
Signal when the handle is lift
(off-hook)
Clear-forward
Signal when the handle is put
back (on-hook)
Address
Dial tone to exchange
b) Exchange with receiver
Signal type
Meaning
Ringing tone
Signal to indicate incoming
call
c) Receiver with exchange
Signal type
Meaning
Answering
Signal when receiver lift the handle (off-hook)
Clear-back
Signal when receiver put back the handle (on-hook)
d) Tone from exchange to caller/receiver
Tone type
Meaning
Dial 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.
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Voice and control signal are send in the same
channel/circuit.
Can be controlled by digital or non-digital
exchange.
Signaling speed is slow.
Exchange A
Exchange B
Traffic channel
Signaling transmitter/receiver
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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.
Exchange A
Exchange B
Traffic channel
Signaling line
Signaling transmitter/receiver
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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
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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.
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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
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The traffic intensity offered by each user is:
A = CH / T Erlangs
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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 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 call
H=50 minutes
Traffic 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.
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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.
Solution
C
= 240 calls
H
= 5 minutes
A
= CH/T
= (240 calls x 5 min) / 60 min
= 20 E
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Find traffic intensity
for a system if 60
calls are received in one hour, each call lasting
5 minutes
Solution
C=60 calls
H=5 minutes
A
= CH/T
A
= 60 x 5 / 60
=5E
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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
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.
Solution
Incoming traffic, Ain= 200 calls x 3 min / 60 min = 10
E
Outgoing traffic, Aout= 120 calls x 2 min / 60 min = 4
E
Total traffic = Aout + Ain = 14 E
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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)
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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.
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Grade Of Service, B is used to observe and
measure how many calls are offered, carried
and lost in the system.
B=
B=
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Number of calls lost
Number of calls offered
Traffic lost
Traffic offered
The lower this number, the higher the GOS.
 For example, if GOS = 0.05, one call in 20
will be blocked during the busiest hour
because of insufficient capacity
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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.
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1200 calls are offered to a channel and 6 calls
are lost. Duration of a call is 3 minutes. Find:
Offered traffic, A
b) Carried traffic
c) Lost traffic
d) GOS, B
e) Congestion time
a)
a)
b)
c)
d)
A = CH/T = (1200 x 3) / 60 = 60 E
Carried traffic = [(1200-6) x 3] / 60 = 59.7 E
Lost traffic = (6 x 3) / 60 = 0.3 E
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
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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 intensity
s= Average holding time for a call
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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