Unit-4 Control of Digital Circuit Switching systems

Call-processing functions

Sequence of OperationsSequence of Operations

Signal exchangesSignal exchanges

State transition diagramsState transition diagrams

Call-processing functions1. Sequence of Operations1. Sequence of Operations A simple telephone call between two customers whose line terminates on the A simple telephone call between two customers whose line terminates on the

same exchange is described.same exchange is described.

A sequence of operations take place in which the calling and called customer’s A sequence of operations take place in which the calling and called customer’s lines and the connections to them change from one state to another.lines and the connections to them change from one state to another.

Idle StateIdle State Initially the customer’s handset is in the ‘on-hook’ condition.Initially the customer’s handset is in the ‘on-hook’ condition.

The line is idle, waiting for calls to be originated or received (state 0).The line is idle, waiting for calls to be originated or received (state 0).

Meanwhile, the exchange is monitoring the state of the line, ready to detect a Meanwhile, the exchange is monitoring the state of the line, ready to detect a calling condition.calling condition.

Call request signalCall request signal The customer sends a signal to the exchange to request a call.The customer sends a signal to the exchange to request a call.

For a telephone call this is done by lifting the handset, which causes current to For a telephone call this is done by lifting the handset, which causes current to flow in the line.flow in the line.

Calling line identificationCalling line identification The exchange detects the calling condition and this identifies the line which The exchange detects the calling condition and this identifies the line which

originated it. originated it.

In general, this signal appears on a termination associated with the customer’s In general, this signal appears on a termination associated with the customer’s equipment number(EN).equipment number(EN).

Equipment-number to directory-number(EN-to-DN) translation must therefore Equipment-number to directory-number(EN-to-DN) translation must therefore be performed in order to charge for the call.be performed in order to charge for the call.

Determination of originating class of serviceDetermination of originating class of service The originating class of service(COS) corresponds to the range of The originating class of service(COS) corresponds to the range of

services available to the calling customer.services available to the calling customer. It must therefore be determined before a connection can be setup.It must therefore be determined before a connection can be setup. Many more different class of service can be provided and these are Many more different class of service can be provided and these are

alterable electronically, some under the control of the customer(e.g. alterable electronically, some under the control of the customer(e.g. optional call barring)optional call barring)

Identification of calling partyIdentification of calling party If the originating COS indicates a multi-party line or shared service If the originating COS indicates a multi-party line or shared service

line, it is necessary to ensure that the correct party is billed for the call.line, it is necessary to ensure that the correct party is billed for the call.

Connection to the calling lineConnection to the calling line The exchange now makes the connection to the calling line.The exchange now makes the connection to the calling line.

Proceed to send signalProceed to send signal The exchange sends a signal (dial tone) back to the caller to indicate that it is The exchange sends a signal (dial tone) back to the caller to indicate that it is

ready to receive the identity of the line termination to which connection is to ready to receive the identity of the line termination to which connection is to be made.be made.

The exchange is now waiting for this information (state 1).The exchange is now waiting for this information (state 1).

Address signalAddress signal The caller now sends a signal to the exchange to instruct it to route the call to The caller now sends a signal to the exchange to instruct it to route the call to

the required destination.the required destination. In a telephone exchange - done by dialling or by sending tone pulses from a In a telephone exchange - done by dialling or by sending tone pulses from a

push-button telephone.push-button telephone.

Selection of outgoing line terminationSelection of outgoing line termination The exchange determines the required outgoing line termination from the The exchange determines the required outgoing line termination from the

address information that it has received.address information that it has received. Since the caller dialled the directory number (DN) of the called customer, in Since the caller dialled the directory number (DN) of the called customer, in

general, this involves DN-to-EN translation.general, this involves DN-to-EN translation.

Determination of terminating class of serviceDetermination of terminating class of service The exchange needs to determine the terminating COS of the called line, since The exchange needs to determine the terminating COS of the called line, since

this affects the procedure for handling the call.this affects the procedure for handling the call.

Testing called line terminationTesting called line termination The called line may be unavailable, either because it is busy or it is out of The called line may be unavailable, either because it is busy or it is out of

service.service. Therefore the exchange tests the state of the line before making connection to Therefore the exchange tests the state of the line before making connection to

it. it.

Status signalStatus signal A status signal, sometimes called a A status signal, sometimes called a call-progress signalcall-progress signal, is now sent back to , is now sent back to

inform the caller of the progress of the call.inform the caller of the progress of the call.

Connection to called line terminationConnection to called line termination Having discovered that the called line is obtainable and free, the exchange Having discovered that the called line is obtainable and free, the exchange

makes a connection to it.makes a connection to it.

Alerting called customerAlerting called customer The exchange sends a signal to the called line to alert the customer to receive the call.The exchange sends a signal to the called line to alert the customer to receive the call. In telephone exchange , this is done by sending ringing current to the line. In telephone exchange , this is done by sending ringing current to the line. At the same time, ringing tone is sent back to the caller as a call-progress signal.At the same time, ringing tone is sent back to the caller as a call-progress signal. The exchange is now waiting for an answer (state 2).The exchange is now waiting for an answer (state 2).

Answer signalAnswer signal When the called customer answers by lifting the handset the line is looped and current flows. When the called customer answers by lifting the handset the line is looped and current flows.

Completion of the connectionCompletion of the connection Receipt of the answer signal from the called customer causes the exchange to complete the Receipt of the answer signal from the called customer causes the exchange to complete the

connection between the line termination of the calling and called customers.connection between the line termination of the calling and called customers.

Conversational stateConversational state The connection has now been completed between the lines of the two customers and they can The connection has now been completed between the lines of the two customers and they can

converse for as long as they wish (state 3).converse for as long as they wish (state 3). The exchange supervises the connection to detect the end of the call and to charge for it.The exchange supervises the connection to detect the end of the call and to charge for it.

Clear SignalsClear Signals When each customer replaces the handset, line current ceases and so provides When each customer replaces the handset, line current ceases and so provides

a ‘clear’ signal to the exchange.a ‘clear’ signal to the exchange.

Release of connectionRelease of connection The exchange then clears down the connection between the calling and called The exchange then clears down the connection between the calling and called

line terminations.line terminations. Thus the idle state (state 0) is resumed.Thus the idle state (state 0) is resumed. 4 ways of controlling release of connections4 ways of controlling release of connections

• Release when the calling party clearsRelease when the calling party clears• Release when the called party clearsRelease when the called party clears

Normally the connection is released when the calling party clears down.Normally the connection is released when the calling party clears down.

In case of abnormal situations, difficulties arise when one party clears but the In case of abnormal situations, difficulties arise when one party clears but the other does not.other does not.

Release of connection

Called subscriber held condition(CSH)Called subscriber held condition(CSH)• If the called customer replaces the handset but the caller does not, then the called If the called customer replaces the handset but the caller does not, then the called

line is still held and the called customer is unable to originate or receive further line is still held and the called customer is unable to originate or receive further calls.calls.

Permanent glow condition(PG)Permanent glow condition(PG)• If the caller replaces the handset but the called customer does not, then the If the caller replaces the handset but the called customer does not, then the

connection is released, but the loop still present on the line looks to the exchange connection is released, but the loop still present on the line looks to the exchange like a calling condition.like a calling condition.

• A new connection will be made to it, but there will be no response to dial tone—A new connection will be made to it, but there will be no response to dial tone—called permanent loop condition.called permanent loop condition.

• Results in lamp remaining alight on a strowger selectorResults in lamp remaining alight on a strowger selector

Parked conditionParked condition• When a line is forcibly released , it is therefore necessary to place the line When a line is forcibly released , it is therefore necessary to place the line

termination in a state which the exchange can recognize as not being a calling termination in a state which the exchange can recognize as not being a calling condition, but which the customer can remove by eventually replacing the handset.condition, but which the customer can remove by eventually replacing the handset.

In a SPC system, there are four statesIn a SPC system, there are four states

Line stateLine state RelayRelay StatesStates

LL K K

Line idleLine idle 0 0 0 0

Line calling (but not yet connected) Line calling (but not yet connected) 1 1 0 0

Line connectedLine connected 0 0 1 1

Fourth state(1 1) -Fourth state(1 1) -indicates line is parked.indicates line is parked.

Signal Exchanges

Forward signalsForward signals Signals sent in the direction away from the caller ( and towards the Signals sent in the direction away from the caller ( and towards the

called line).called line).

Pass from the caller to the exchange and from the exchange to the Pass from the caller to the exchange and from the exchange to the called customer.called customer.

Backward signalsBackward signals Signals sent towards the caller ( and away from the called line)Signals sent towards the caller ( and away from the called line)

Pass from the called customer to the exchange and from the exchange Pass from the called customer to the exchange and from the exchange to the caller.to the caller.

Timing of signals exchanged for a local call

Following are the Handshake signals…………..

The call request signal is answered by the proceed-to-send signal.The call request signal is answered by the proceed-to-send signal.

The address signal is answered by a call-status signal.The address signal is answered by a call-status signal.

The answer signal is a response to the alerting signal.The answer signal is a response to the alerting signal.

The caller responds to the answer signal by commencing the The caller responds to the answer signal by commencing the conversation.conversation.

The backward clear signal is a response to the forward clear signal (or The backward clear signal is a response to the forward clear signal (or vice versa)vice versa)

Signal Exchange diagram for a Local call

State Transition diagrams

A call progresses from one state to another in response to events.A call progresses from one state to another in response to events.

An EventAn Event Arrival of a signal Arrival of a signal

Results in the performance of an action Results in the performance of an action TaskTask

Present state + new event = Task (Performing this result in the next Present state + new event = Task (Performing this result in the next state)state)

STD(State Transition Diagram)STD(State Transition Diagram) clearly describes the sequence of clearly describes the sequence of operations.operations.

SDL(Specification and Description Language)SDL(Specification and Description Language) An international An international standard produced by CCITT (Consultative Committee for standard produced by CCITT (Consultative Committee for International Telegraph and Telephony)International Telegraph and Telephony)

Basic symbols used State Boxes->State Boxes-> These are labelled with a number and a title. Additional These are labelled with a number and a title. Additional

information may also be included in the box if requiredinformation may also be included in the box if required..

Event Boxes->Event Boxes-> These have an indented arrow, indicating whether the event These have an indented arrow, indicating whether the event corresponds to the receipt of a forward or backward signal.corresponds to the receipt of a forward or backward signal.

Action Boxes->Action Boxes-> These are rectangular boxes, except when the action is the These are rectangular boxes, except when the action is the sending of a signal. The box then has a protruding arrow, indicating whether the sending of a signal. The box then has a protruding arrow, indicating whether the signal is sent forward or backward.signal is sent forward or backward.

Decision Boxes->Decision Boxes-> The basic symbol is the diamond-shaped box used in The basic symbol is the diamond-shaped box used in computing flow charts. However, if more than two decisions are possible, the computing flow charts. However, if more than two decisions are possible, the modified symbol shown in Fig is used.modified symbol shown in Fig is used.

State 0

Operate Switch

Accept

Test

State 1 State 2

0 Idle

Connector (from other diagram)

State box

Event box

Action boxes

Normal action

Send signal

Decision box

Connectors (to other diagrams)

Yes

No

State no State description

Event name

Symbols used in state transition diagrams- Basic Symbols

Seize

Symbols used in state transition diagrams- Multiple decision box

Line state ?

busy free invalid

State Transition diagrams

Can be drawn for a complete system, for a subsystem or for Can be drawn for a complete system, for a subsystem or for functional units within it.functional units within it.

The amount of detail shown by the STD may be expected to The amount of detail shown by the STD may be expected to increase at the lower levels of system implementation.increase at the lower levels of system implementation.

Can specify precisely and unambiguously the functions to be Can specify precisely and unambiguously the functions to be performed by a system and its subsystems.performed by a system and its subsystems.

Important tool and is useful at every stage in the lifecycle of a Important tool and is useful at every stage in the lifecycle of a switching systemswitching system

Life cycle of a switching system

Initial specificationInitial specification

Design (of both hardware and software)Design (of both hardware and software)

Laboratory testingLaboratory testing

Manufacturing testingManufacturing testing

Installation and commissioningInstallation and commissioning

Acceptance of serviceAcceptance of service

Operation and maintenanceOperation and maintenance

Training of staffTraining of staff

Common control

Performs a specific call-processing functionPerforms a specific call-processing function

Thus , the control of the switching system employs functional Thus , the control of the switching system employs functional division.division.

A common control is brought into a connection only when A common control is brought into a connection only when required and released when it has performed its task.required and released when it has performed its task.

Switching networks are lost-call systems. However when Switching networks are lost-call systems. However when common controls are busy, calls offered to them are not common controls are busy, calls offered to them are not normally lost; they are delayed.normally lost; they are delayed.

Common control unit has been designed using relays, electronic Common control unit has been designed using relays, electronic digital circuits and stored-program control(SPC)digital circuits and stored-program control(SPC)

Common control

A single SPC control can perform all the call processing needed A single SPC control can perform all the call processing needed for an exchange, leading to centralized control.for an exchange, leading to centralized control.

The principal methods involved in the interconnection of The principal methods involved in the interconnection of common controls and the trunks of switching networks in order common controls and the trunks of switching networks in order to exchange signals areto exchange signals are Auxiliary switching networksAuxiliary switching networks Tree networksTree networks Use of the main switching networkUse of the main switching network BusesBuses ScanningScanning

Auxiliary switching networks

A separate small network using switches similar to those in the main A separate small network using switches similar to those in the main switching network can be used to connect common controls to trunks switching network can be used to connect common controls to trunks as required. as required.

Since a number of trunks may request the use of a common control at Since a number of trunks may request the use of a common control at the same time, contention can arise.the same time, contention can arise.

A circuit in a common control used to resolve this contention is called a A circuit in a common control used to resolve this contention is called a one-only selector, an allotter, or an arbiter.one-only selector, an allotter, or an arbiter.

If a function is provided by several common controls, then further a If a function is provided by several common controls, then further a one-only selector is needed to select a particular free control to respond one-only selector is needed to select a particular free control to respond to an incoming request.to an incoming request.

Its operation is as follows…………

If the arbiter is free, then an incoming seize signal is accepted by it for the If the arbiter is free, then an incoming seize signal is accepted by it for the common control.common control.

If it is busy, incoming seize signals are rejected.If it is busy, incoming seize signals are rejected.

If it is free, but two or more seize signals are received simultaneously, only If it is free, but two or more seize signals are received simultaneously, only one is accepted and the others are rejected.one is accepted and the others are rejected.

The other alternative is , incoming requests are queued.The other alternative is , incoming requests are queued.

Seize signals are stored and , when the common control gives a clear Seize signals are stored and , when the common control gives a clear signal, a previously received seize signal is accepted.signal, a previously received seize signal is accepted.

Tree Networks

If there is a single common control, or a small number, it may be If there is a single common control, or a small number, it may be more economic to provide each with a selecting tree than to use more economic to provide each with a selecting tree than to use a more complex switching network.a more complex switching network.

A tree consisting of n relays or electronic elements can connect A tree consisting of n relays or electronic elements can connect a common control to 2a common control to 2n n trunks.trunks.

Use of Main switching network

Instead of using an auxiliary switching network, the main Instead of using an auxiliary switching network, the main switching network may be employed for connecting common switching network may be employed for connecting common controls using the controls using the crank-backcrank-back principle. principle.

This principle is extensively used in the TXE4 system, where it This principle is extensively used in the TXE4 system, where it is called serial trunking.is called serial trunking.

Serial trunking in the TXE4 system- Local call

Serial trunking in the TXE4 system- Local call

The connections used in making an own-exchange call are The connections used in making an own-exchange call are shown in Fig.shown in Fig.

When a customer calls, an initial connection (path1) is made to When a customer calls, an initial connection (path1) is made to a register via a through link (i.e. a link which does not contain a a register via a through link (i.e. a link which does not contain a transmission bridge.)transmission bridge.)

When the register has received the address information, a When the register has received the address information, a connection is then made between the calling and called connection is then made between the calling and called lines(path2).lines(path2).

This uses a link that contains a transmission bridge and This uses a link that contains a transmission bridge and provides supervisory facilities.provides supervisory facilities.

When the second connection has been tested for continuity, the When the second connection has been tested for continuity, the first connection is released.first connection is released.

Serial trunking in the TXE4 system-Outgoing junction call

Serial trunking in the TXE4 system-Outgoing junction call

The connections made for a outgoing junction call are shown in fig.The connections made for a outgoing junction call are shown in fig.

An initial connection is made from a calling line to a register , via a An initial connection is made from a calling line to a register , via a through link , as before.through link , as before.

When the address information has been obtained, a connection (path2) When the address information has been obtained, a connection (path2) is made from the sender side of the register to an outgoing junction via is made from the sender side of the register to an outgoing junction via another through link.another through link.

The register then sends out the required routing digits over the junction.The register then sends out the required routing digits over the junction.

A final connection(path3) is then made between the calling line and the A final connection(path3) is then made between the calling line and the junction.junction.

This uses a split link, which does not complete the connection until the This uses a split link, which does not complete the connection until the register has finished sending the routing digits.register has finished sending the routing digits.

Use of the Main switching network

Another example of the use of the main switching network is the Alcatel system 12 shown Another example of the use of the main switching network is the Alcatel system 12 shown in Figin Fig

Use of the Main switching network

Control of the exchange is distributed among a number of Control of the exchange is distributed among a number of processors, each performing a specific function.processors, each performing a specific function.

These communicate with each other and with customers These communicate with each other and with customers connections by means of temporary connections made through connections by means of temporary connections made through the main switching network as required.the main switching network as required.

Buses

When it is necessary to exchange signals between a number of functional units, When it is necessary to exchange signals between a number of functional units, but the signal exchanges need only take place one at a time, the units can be but the signal exchanges need only take place one at a time, the units can be connected by a common bus.connected by a common bus.

If information is transmitted in serial mode, the bus can be a single wire.If information is transmitted in serial mode, the bus can be a single wire.

If parallel transmission is used, a wire is needed for each digit (e.g. eight wires If parallel transmission is used, a wire is needed for each digit (e.g. eight wires to transmit 8-bit bytes)to transmit 8-bit bytes)

An example is the use of a bus to connect a processor unit, or units , with stores An example is the use of a bus to connect a processor unit, or units , with stores and input and output ports as shown in fig.and input and output ports as shown in fig.

Functional units interconnected by a bus

CPU = Central processor unit.CPU = Central processor unit.

I/O = Input/output portsI/O = Input/output ports

MMI = Man-machine interfaceMMI = Man-machine interface

Scanning

In scanning, electronic gates, forming the equivalent of a rotating In scanning, electronic gates, forming the equivalent of a rotating switch, connect a common control to each trunk in turn.switch, connect a common control to each trunk in turn.

Since at any time, only one trunk can communicate with the common Since at any time, only one trunk can communicate with the common control, there can be no contention.control, there can be no contention.

However scanning presents the following requirements, which may However scanning presents the following requirements, which may conflict:conflict: In every cycle the scanner must connect the common control to each trunk In every cycle the scanner must connect the common control to each trunk

for a sufficiently long period to exchange the required signals.for a sufficiently long period to exchange the required signals. The period of a complete scanning cycle must be sufficiently short for the The period of a complete scanning cycle must be sufficiently short for the

common control to detect every change that occurs in the states of the common control to detect every change that occurs in the states of the trunks being scanned.trunks being scanned.