Csn Call Processing Functions

Unit-4 Control of Digital Circuit Switching systems

Call-processing functions 

Sequence of Operations

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Signal exchanges

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State transition diagrams

Call-processing functions 1. Sequence of Operations 

A simple telephone call between two customers whose line terminates on the same exchange is described.

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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.

Idle State 

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

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

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

Call request signal 

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

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

Calling line identification 

The exchange detects the calling condition and this identifies the line which originated it.

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

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

Determination of originating class of service 

The originating class of service(COS) corresponds to the range of services available to the calling customer.

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It must therefore be determined before a connection can be setup.

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Many more different class of service can be provided and these are alterable electronically, some under the control of the customer(e.g. optional call barring)

Identification of calling party 

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.

Connection to the calling line 

The exchange now makes the connection to the calling line.

Proceed to send signal 

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

Address signal  

The caller now sends a signal to the exchange to instruct it to route the call to the required destination. In a telephone exchange - done by dialling or by sending tone pulses from a push-button telephone.

Selection of outgoing line termination  

The exchange determines the required outgoing line termination from the address information that it has received. Since the caller dialled the directory number (DN) of the called customer, in general, this involves DN-to-EN translation.

Determination of terminating class of service 

The exchange needs to determine the terminating COS of the called line, since this affects the procedure for handling the call.

Testing called line termination  

The called line may be unavailable, either because it is busy or it is out of service. Therefore the exchange tests the state of the line before making connection to it.

Status signal 

A status signal, sometimes called a call-progress signal, is now sent back to inform the caller of the progress of the call.

Connection to called line termination 

Having discovered that the called line is obtainable and free, the exchange makes a connection to it.

Alerting called customer    

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. 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).

Answer signal 

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

Completion of the connection 

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.

Conversational state  

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). The exchange supervises the connection to detect the end of the call and to charge for it.

Clear Signals 

When each customer replaces the handset, line current ceases and so provides a ‘clear’ signal to the exchange.

Release of connection   

The exchange then clears down the connection between the calling and called line terminations. Thus the idle state (state 0) is resumed. 4 ways of controlling release of connections • •

Release when the calling party clears Release when the called party clears

Normally the connection is released when the calling party clears down. In case of abnormal situations, difficulties arise when one party clears but the other does not. 

Release of connection 

Called subscriber held condition(CSH) •

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Permanent glow condition(PG) •

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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 calls.

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 like a calling condition. A new connection will be made to it, but there will be no response to dial tone— called permanent loop condition. Results in lamp remaining alight on a strowger selector

Parked condition •

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 condition, but which the customer can remove by eventually replacing the handset.

In a SPC system, there are four states Line state Line idle Line calling (but not yet connected) Line connected

Relay

States

L 0 1 0

K 0 0 1

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

Signal Exchanges Forward signals 

Signals sent in the direction away from the caller ( and towards the called line).

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

Backward signals 

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

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

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

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The answer signal is a response to the alerting signal.

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

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

Signal Exchange diagram for a Local call

State Transition diagrams ■

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

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An Event Arrival of a signal

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Results in the performance of an action Task

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

STD(State Transition Diagram) clearly describes the sequence of operations. SDL(Specification and Description Language) An international standard produced by CCITT (Consultative Committee for International Telegraph and Telephony)

Basic symbols used ■

State Boxes-> These are labelled with a number and a title. Additional information may also be included in the box if required.

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

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

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

Connector (from other diagram)

State 0 State description

State no 0

State box

Idle

Event name Event box

Seize

Normal action

Operate Switch

Action boxes Send signal

Decision box

Accept

No Test Yes

Connectors (to other diagrams)

State 1

State 2

Symbols used in state transition diagrams- Basic Symbols

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 functional units within it.

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

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

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

Life cycle of a switching system ■

Initial specification

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Design (of both hardware and software)

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Laboratory testing

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Manufacturing testing

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Installation and commissioning

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Acceptance of service

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Operation and maintenance

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Training of staff

Common control ■

Performs a specific call-processing function

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

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

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

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

Common control ■

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

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The principal methods involved in the interconnection of common controls and the trunks of switching networks in order to exchange signals are ◆ ◆ ◆ ◆ ◆

Auxiliary switching networks Tree networks Use of the main switching network Buses Scanning

Auxiliary switching networks ■

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

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

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

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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 to an incoming request.

Its operation is as follows…………

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

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If it is busy, incoming seize signals are rejected.

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

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The other alternative is , incoming requests are queued.

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

Tree Networks ■

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 a more complex switching network.

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

Use of Main switching network ■

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

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This principle is extensively used in the TXE4 system, where it 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 shown in Fig.

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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 transmission bridge.)

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

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

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When the second connection has been tested for continuity, the 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.

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

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

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

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

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This uses a split link, which does not complete the connection until the 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 in Fig

Use of the Main switching network ■

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

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

Buses ■

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 connected by a common bus.

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

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

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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.

Functional units interconnected by a bus

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CPU = Central processor unit.

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I/O = Input/output ports

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MMI = Man-machine interface

Scanning ■

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

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

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However scanning presents the following requirements, which may conflict: ◆

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In every cycle the scanner must connect the common control to each trunk for a sufficiently long period to exchange the required signals. 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 trunks being scanned.