he can only occur due to design faults (bugs) which have not been acyclic read out from all these stores. detected during the design and development phase. The number of Telephone customers' lines are generally on individual pairs em- combinations of external inputs and system states in programs of ploying analogue transmission. The interface to the analogue cus- this size makes it impossible to guarantee the complete absence of tomer's line includes certain functions which require voltage and such bugs. power levels incompatible with the digital concentrator switch and To provide some degree of fault tolerance, it is first necessary to which, therefore, have to be provided at the individual line circuit design the software in discrete modules with defined interfaces (line card). The line interface functions are generally referred to as between them. This enables more comprehensive testing of possible the Borscht functions: combinations within each module than would be possible on the 31/16 Principles of switching systems complete system. It also allows checks to be applied to input and 31.6.3 Network aspects output conditions at the interfaces. These checks can either be used to trigger the hardware changeover mechanism or to reinitialise the The introduction of spc has enabled electronic exchanges to provide software module to an acceptable state. customer and administration features not readily available with The modular design of software also facilitates the evolution of earlier control systems. Examples include the ability to transfer calls features during the life of the system and the implementation of at will and allow calls to wait on a busy line. Where new features distributed control. are self contained within an exchange they can be introduced by The primary function of the software is to set up and clear down installing a new build of software. If the operation of a feature calls on demand. Calls arise concurrently in real time and in a requires identical operation at all exchanges, the practical pr