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Telecommunication system
engineering
Introduction to Telephony
Lecture # 213-02-2013
Introduction to Telephony
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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.
Introduction to Telephony
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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 ?
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A simple telephone connection
(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.
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A simple telephone connection
(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:
Introduction to Telephony
Frequency 1209 Hz 1336 Hz 1477 Hz
697 Hz1 2 3
770 Hz 4 5 6
852 Hz 7 8 9
941 Hz * 0 #
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A simple telephone connection
(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
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A simple telephone connection
(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.
Introduction to Telephony
A 8-point mesh connection Subscriber connected in star arrangements
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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.
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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
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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 .
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Typical traffic intensity
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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.
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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.
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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.
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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
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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)
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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.
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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
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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
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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
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Erlang B table
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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
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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
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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.
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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
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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.
30 trunks will carry 20 Erlang of traffic, trunk to
Erlang ratio is 3:2.
120 trunks will carry 100 Erlang of traffic, trunk to
Erlang ratio is 6:5.
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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
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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
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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.
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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.
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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.
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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.
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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.
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Introduction to Telephony
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Sample problem 2
Introduction to Telephony