IAS TIPS

CALL PROCESSING FUNCTIONS

Sequence of operation

Similar basic process must be performed by switching systems in any network, whether circuit switched. They were summarized for telephony in section 3.5. They will now be described in more detail for a simple telephone call between two customers whose lines terminate on the same exchange. A sequence of operations takes place in which the calling and called customers’ lines and the connections to them change from one to another as described below.

Idle state

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). 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 call. For a telephone Call, this is done by lifting the handset, which causes current to flow in the line. The calling signal is also sometimes known as a seize signal, since it obtains a resource from the exchange.

Calling line identification

The exchange detects the calling condition and this identifies the line which originated it. In general, this signal appears on a termination associated with the customer’s equipment number (EN).Equipment-number to directory-number (EN-to-DN) translation must 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. It must therefore be determined before a connection can be set up. In electromechanical exchanges it was necessary for lines with different classes of service to be segregated into different groups in order to distinguish between them. For example, ordinary lines and payphone lines in a Strowger exchange must be connected to different first selectors. In an SPC exchange, a customer’s COS forms part of the data stored for that customer in the line store; it is sometimes called a class mark. Many more different classes 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, it is necessary to ensure that the correct party is billed for the call. In a manual exchange, party identification was carried out by the operator asking the caller. In an automatic exchange, customers on two-party lines can be distinguished by the calling condition placing an earth on one line wire or the other instead of looping the line. [3]

Connection to the calling line

The exchange now makes a connection to the calling line.

Proceed to send signal

The exchange sends a signal (dial tone) back to the caller to indicate that it is ready to receive the identify 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 this is done by dialing 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 dialed 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. For example, if the call is to a customer having a PBX, the customer will have a group of lines ad any of these may be used for the connection. (This also applies to junction or trunk calls; any free circuit on the required outgoing route may be used.) If the called customer shares a party line, it is also necessary to determine which party is to be alerted.

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. In the case of a call to a PBX (or to an outgoing junction), the exchange tests each termination until either it finds a free one or all are found busy.