2.1 The service switching function

As Chapter 1 intimated, the term triggering in intelligent networks refers to the process of breaking out from normal call-control procedures in digital telephone exchanges to remote service logic. Triggering is fundamental because the original purpose of the IN was to enable call control software to recognise pre-assigned events and conditions and suspend call processing whilst it obtained further call processing instructions from an external service control point.

However, when IN was introduced, the software which handles this breaking out process could not simply be added alongside existing call processing functions. It had to be built in as an integral part of the basic switch call control. Because different telephone exchange manufacturers used different design philosophies for their call control, achieving a standard for the break-out procedures was difficult. In order to achieve progress in building the standard recommendations for enhancing telephone networks with IN features, it was necessary to introduce a hypothetical modelling device known as the service switching function (SSF) to the telephone exchanges.

The illusion was created of an apparent separation between the new IN access function (the SSF) and the existing call control function (CCF). In fact, because the introduction of IN triggers could only be achieved by fundamental modifications to existing call processing software, including the embedding ...