telecommunication switching systems part 02 udhay prakash jntuh

7/30/2019 Telecommunication Switching Systems Part 02 Udhay Prakash Jntuh

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Telecommunication Switching SystemsAutomatic Switching system

Udhay Prakash

Lecturer, Dept. Of ECE, JNTUH [email protected]


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Content under consideration Drawbacks of Manual switching system

Why an Automatic switching system?

Automatic Switching system Strowger switching system

Crossbar switching System


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Manual Switching system Manually operated switch boards

Cord pair Ajack for each line and two plugs on a long

flexible wire for making the connection.

Cord pairs appeared in rows on a shelf in

front of the operator, and the jacks (calledline appearances) were mounted on avertical panel.

Connection establishment operator picks up a cord & plugs into jack

corresponding to line requesting service,obtained from calling party Then plugs other end of cord pair into the

correct outgoing line jack.


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Functions of amanual switching system

Functions Interconnection Control

Alerting Attending Information receiving Information

transmitting Busy testing supervisory


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Limitations ofManual Switching system

1) Common language factor Operator should know the languages spoken by

subscriber, for communicating with them2) Less degree of privacy3) Wastage of time in connection establishment/release If the operator lately notices the end/initialization of

conversation.

4) At switching system, subscribers are recognized bytheir name, profession or organization


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Automatic switching system Automation achieved by utilization ofsignalling tonesfor

Establishing,Maintainance,Releasing a telephone conversation

Signalling is carried out with specific signals or messages thatindicate to the other end what is requested of it by this connection.

Signalling examples on subscriber lines : Off-hook condition:Exchangenotices that subscriber hasraised telephone hook (dc

loop is connected) and gives a dial toneto the subscriber. Dial: Subscriber dials digits and they are received by theexchange.

On-hook condition:Exhange notices that subscriber has finished call (subscriberloop is disconnected), clears the connection,and stops billing.


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Signalling Tones Functions of Signalling tones

Dial tone Respond to calling subscriber to obtain the identification of called

subscriber.

Call in Progress Tone Inform the calling subscriber that the call is being established. Ring Tone

Ring the called party

Busy Tone

Inform the calling subscriber if the called party is busy Number Unobtainable Tone

Inform the calling subscriber if the called party is unobtainable for somereason


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Signalling Tones Dial Tone (33Hz/50Hz/400Hz)

Call in Progress Tone (400Hz/800Hz)


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Signalling Tones Ringing Tones (400Hz/133Hz)

Busy Tone (400Hz)


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Signalling Tones Number Unobtainable Tone (400Hz)


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Strowger switching system Why automatic switching system?

To improve the speedandcarry more load

First electromechanical switching system was

developed byconnolly and Mcig in 1879.

But, Strowgers step-by-step switching,

developed in 1888, was most popular. In october 1891, strowger Automatic

telephone exchange was formed.


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Strowger switching system A moving wiper moved upto

and around a bank of many

other contacts, making a

connection with any one ofthem.


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Strowger switching system Reasons of its success and survivability:

High system availability

Comprehensibility

Cheapness and simplicity

Language independent (standard)

High degree of privacy (security)

Fast establishment/release (efficient)


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Rotary Dial Telephone Subscriber identification

Howto identifythe subscribers? In Manual Switching system

Name/department/profession

In Automatic Switching system

Numbering Plan How to transfer identity?

Pulse-dialling Multi-frequency dialling


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Rotary Dial Telephone Pulse dialling

A train of pulses is used to represent a digit in thesubscriber number

Successive digits are distinguished by a pulse(inter-digit gap)


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Rotary Dial Telephone Rotary or Pulse dialling


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Rotary Dial Telephone Subscriber signalling


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Strowger switching components Basic elements in performing switching function:

Uniselector - single rotary switch with a bank of contacts

Two motion selector -


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Strowger switching components


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Strowger switching components Typical Uniselector representation


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Strowger switching components Typical representation of a uniselector


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Strowger switching components Example of a uniselector scheme


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Strowger switching components Two-motion selector


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Step-by-step switching What does step-by-step mean?

The wiper steps forward by one contactat a time and moves as many contacts as

the number of dial pulses received. Construction

Using uniselector

Using two-motion selector Combination of both


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Step-by-step switching


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Step by step switching Configuration of a step-by-step switching

system


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Step-by-step switching Line equipment part (pre-selector stage)

Function Provide access to common switching resources

Category Selector hunters

Line Finder


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Step-by-step switching Selector hunter

There is one dedicated selector hunter for eachsubscriber to search and seize a free selector from

the switching matrix part. Usually 24-outlet uniselectors are used as selector

hunters.


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Step-by-step switching Line finder

Associated with the first set of selectors inswitching matric part, there is one line finder for

each selector. Usually built using uniselectors or two-motion

selectors.


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Selector hunter based access Suitable for large switches with fairly heavy traffic.


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Selector hunter based access


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Line finder based access Suitable for small switches with low traffic


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Line finder based access


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Step-by-step switching Switching network part

Group selector + final stage


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Switching network & connection


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Step-by-step switching Control function

Performed by circuits associated with the selectors.

Contact banks

Control and supervisory signals (P[private]-wire) Voice signals(positive & negative wires)

Control and supervisory signals are carried fromstage to stage by means of contacts in one of thebanks.

A selector X is said to have seized another selector Yin the next stage when positive, negative and privatewires have been connected to those of Y.

m a ons


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m a onsof strowger switching system

1) Inability to use tone dialling (DTMF) directly

Possible only ifthe central office is equipped with a conversion device.

2) The switch requires the successful sequential (step-by-step, time related)operation

of several relays, and a sizable voltage and current is switchedeach time a switch is

stepped.

Mechanical reliabilityof the switches is low Requiresregularmaintenance byskilled people Also,generates large amounts of electrical andmechanical noise.

3) No flexibility to address new subscribers, as it is a hard-wired switching

arrangement.

Li it ti


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Limitations

of strowger switching system4) As all switches (except the first one) are shared among many incoming

lines in tandem operation, a call may be blocked partway throughdialling sequence, even thoughthe called line is free.

5) Dependence on moving parts and contacts that are subject to wear andtear.

6) Difficulties in maintenance and adjustments.

Solution: Switching systems requiring less maintenance andadjustment after installation are expected


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CROSSBAR SWITCHINGHow to address the limitations of strowger switching system?


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Cross bar switching system Development of crossbar switching

First patent, USA, 1915

Second patent, Sweden, 1919

First crossbar switching system, AT&T, 1938.

Common Control subsystems were firstintroduced in crossbar exchanges


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Typical crossbar switch


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Principles of common control Common controlprovides more sophistication and flexibility

in the way calls are routed. Uniform numbering

The same number is dialled, no matter from whichexchange the call originates

It is convenient for customer to make phone call to aspecific party any where.

It is difficult to implement uniform numbering indirect control switching system unless as Director isavailable.


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A Multi-exchange network


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A Multi-exchange Network Partly connected network (directcontrol)

A level is reserved in each strowger exchange foroutgoing calls.

The corresponding outlets are connected to a specific

neighbour exchange. Call from A to F

Called subscriber:1457 Route 1: A-B-C-J-F

Dialling sequence: 01-04-03-01-1457

Route 2: A-I-H-G-F Dialling sequence: 02-05-01-02-1457


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A Multi-exchange Network Difficulties faced by direct control Identification number of a subscriber is route

dependent

A user must have knowledge of the topologyof the network and the outlet assignments ineach exchange.

Depending on from which exchange the calloriginates, the number and its size vary forthe same called subscriber.


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A Multi-exchange Network How to overcome these difficulties in a

strowger exchange? The routing is done by the exchange instead of

dialling A uniform numbering scheme is used

Uniform Numbering Scheme

Exchange identifier + subscriber line identifier


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Common Control Fundamental features

A user is assigned a logical number which isindependent of the physical line number used to

establish a connection to him. The logical address is translated to actual physical

address for connection establishment by an addresstranslation mechanism.

Call processing takes place independent of theswitching network.


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Common control Diagram of common control subsystem

Call processing subsystem Digit receiver and storage register

Register sender Initial translator

Final translator

Charging circuits

Operation control Maintenance circuits

Event monitor


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Common control


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Common control Control functions

Event monitoring Events at line units

Events at trunk junctors Events at interexchange signalling receiver/sender

units


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Call processing Digit receiver and storage register: receive and store the

dialling number from calling party.

Initial translator (office code translator): determine the routefor the call through the network, charging method/rate.

Final translator (subscriber code translator): determine theline unit to which a call must be connected and category of

the called line. Register sender: transfer the route digit and dialled digit

using proper signalling.


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Charging Depends on the type of subscriber

Depends on the service of the subscriber Operation and maintenance

Controlling of the switching network Map-in-memory & Map-in-Network

Administration & Maintenance Exchange equipment, subscriber lines, trunks


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Touch tone dial telephone Disadvantages of rotary dial telephone

Low dialling rate

Limited usage

Limited signalling capacity

Development of touch tone dial phone Development in 1950s

Applied in 1964 Touch dial arrangement

DTMF: Dual Tone Multi-Frequency


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Touch tone (push button) dialarrangement


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Touch tone dial telephone


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Typical IC based DTMF circuit


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Tone dialling advantages It is quicker and dialling of all digits takes the same time.

Fewer dialling errors result.

End-to-end signalling is possible.

Additional push buttons are available (*, #, A, B, C, D) foractivationof supplementaryservices.

Supplementary(call transfer, ...) and value added services(telebanking,..)


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Design considerations Choice of code

Imitation of code signal by speech and musicshould be difficult.

Band separation Easy to recognize a specific frequency

Easy to regulate

Reduce the probability of false response


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Design considerations Choice of frequencies

Attenuation and delay distortion characteristics of thetelephone network circuits

Choice of power level High enough for reliability Power level should be planned according to

attenuation characteristics of the channel

Signalling duration Longer duration is helpful to combat talk-off, but

inefficient. Dialling habit


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Design considerations


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Crossbar switching Also called coordinate switching system

Principle: to provide a matrix of m*n sets of contacts with only n+m

activators or less.

Intersecting horizontal and vertical wires form a set of contact points

A contact point pair acts as a cross-point switch and remains separated or

open when not in use.

Contact points are mechanically mounted (and electrically insulated) on aset of horizontal and vertical bars.


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Typical Crossbar switch


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Crossbar Switching To activate a crosspoint, the

corresponding vertical and

horizontal electromagnets must be

energised.

Encircled crosspoint shows the

unenergised crosspoint, that was

activated.


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Cross point switching Establishment of a connection

Scheme1 Step 1: Energize horizontal bar

Step 2: Energize vertical bar Step 3: De-Energize horizontal bar

Scheme 2 Step 1: Energize vertical bar

Step 2: Energize horizontal bar

Step 3: De-Energize vertical bar


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Crossbar switch configurations Non-blocking crossbar configuration

N2switching elements for N subscribers

N/2 simultaneous conversations

Cross-point used depends on the calling


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66 crossbar matrix


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Crossbar switch configurations Modified non-blocking scheme

Diagonal cross-point matrix

N(N-1)/2 elements

The number of elements is the same as that offully connected network

Connection establishment

Energize the corresponding horizontal bar Energize the corresponding vertical bar


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Diagonal (lower triangle) cross point matrix


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Crossbar switch configurations Comments on non-blocking schemes

Large number of switching elements

Difficult to implement in practice

Not cost-effective

Solution Blocking crossbar switching


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Crossbar switch configurations Blocking crossbar switches

Aiming to reduce the number of cross-pointswitches

Category: Single stage/Multistage Methodology:

Two subscribers share one vertical bar Number of bars reduced

Number of cross-point switches remains the same

All subscribers share a number of vertical bars Numbers of bars and cross-point switches reduced


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Blocking crossbar switches Scheme 1

2NK switches N denotes the number of subscribers

K is the number of simultaneous connections

Operate four bars to establish a connection (eg: A-B) Energize horizontal bar A

Energize free vertical bar P

De-Energize horizontal bar A

Energize horizontal bar B Energize free vertical bar P

De-Energize horizontal bar B


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Transfer line support Locally non-blocking and externally blocking


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Transfer Line support Blocking both locally and externally


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Crosspoint Technology The cost of crossbar system increases in

proportion to the number of crosspoints Challenges Reduction of the size of a crosspoint Reduction of the cost of a crosspoint Improvement of switching time


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Crosspoint Technology Category of crosspoint technology


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Reed Relay crosspoint


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Crossbar exchange organization Basic building blocks of crossbar exchange

Link frames Primary stage crossbar witches

Secondary stage crossbar switches Links

Control markers Control the connections between inlets and outlets

Registers


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Link frames


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Crossbar Exchange organization Organization of a crossbar exchange Line unit (two-way units) Line link frames

Markers and registers Originating or terminating calls Group unit (unidirectional)

Trunk link frame and associated hardware

Handling local, outgoing, incoming, terminatingand transit calls


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Crossbar Exchange organization


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Crossbar Exchange organization Call processing (three stages)

Stage 1 Preselection (performed by originating marker)

Stage 2 Group selection (performed by registers, translators

and senders) Stage 3

Line selection (performed by terminating marker)


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References Thyagarajan Viswanathan,

Telecommunication switching systems, PHI.

Roger L. Freeman, Fundamentals of

Telecommunications, 2e, Wiley-interscience,ISBN 0-471-71045-8.

Stephen J Bigelow, joseph J Carr, steve winder,

Understanding Telephone Electronics, 4e,Newness, 2001.

telecommunication switching systems part 02 udhay prakash jntuh

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