rformed by the above two functions. A further function 'operational' is necess- Figure 43.1 Generalised principle of common control ary to enable the best use to be made of the network and to cater for switching system and operating organisation facilities. The type of switching equipment, direct control or common con- trol, and the type of transmission equipment have significant in- In common control switching (Figure 43.1), the common control fluence on the type of signalling system. equipment (register, translator, marker) deals with the call connec- tion set-up through the switchblock, being released from a call on connection establishment to deal with other call connections. The 43.1.1 Type of switching system marker controls the switching. The incoming line is the incoming mark at the switchblock. Address information inputs the register Direct control switching systems, e.g. step-by-step, do not allow for which refers to the translator. The resulting translation establishes separation of the supervisory and address signalling functions. The the outgoing mark identifying the outgoing line. The marker swit- dialled address positions the switches directly, the numbering ches the two marks together through the switchblock. The transla- scheme controlling the routing of call connections. The supervisory tion facility permits automatic alternative routing. and address signalling functions are combined in the network line Common control switching permits separation of the supervisory signalling systems. The signalling limit tends to be limited by the (line) and address (selection) signalling functions over the network permissible decadic address pulse distortion. (Figure 43.2). Line signalling system - d.c., v.f., outband or p.c.m., v.f. shown typically v.f. line recr. Outgoing Incoming Speech circuit v.f. line recr. Line signalling (per line) Inter register m.f. signalling (common equipment) Register Register Figure 43.2 Line and interregister signalling 43/4 Signalling systems 43.1.2 Influence of type of transmi