transfer them to the appropriate person. Key systems usually 34.1.2 Switching matrix have a visual appearance for each exchange line showing whether it is use or not. To make an outgoing call, a free exchange line is This is the part of the system which connects one peripheral device manually selected and the required number dialled. PABXs usually to another. In step by step (Strowger) technology, the switching pool exchange lines together and require an access code to be matrix consisted of a series of line finders, group selectors and final dialled to seize an outgoing exchange line. After dialling the access selectors which, under the control of the dialled digits, routed a call code, the receipt of dial tone indicates that an exchange line has been to its final destination. Physical movement caused one pair of wires successfully seized while busy tone indicates that there are no free to be connected to one of a number of other pairs of wires depending exchange lines at that time. on the destination of the call. The number of elements (e.g. group In recent years, the differences between key systems and PABXs selectors) in the matrix was determined by the expected number of have become blurred. Key systems now offer centralised answering simultaneous calls (traffic) through that part of the matrix. Higher positions and access codes to exchange lines while PABXs offer than expected traffic levels would cause calls to be blocked and many features traditionally only available on key systems. Many hence not reach their final destination. Since the call was routed manufacturers now refer to their products as hybrid PBX key through the matrix in a step by step manner under the control of the systems. dialled digits, there was no opportunity to try alternate routes Figure 34.1 shows the main elements of a PABX or key system. through the matrix. Outside world Wiring Line and Cross Control Switching Wiring trunk connect system matrix circuits frame Telephone Figure 34.1 Main elements of a PABX or key system 34/4 PABX an