6.1.2 Control Elements (CE) changes which deal specifically with freephone service, signal transfer points for CCITT No.7 Common Channel Signalling, wide- All CEs have similar hardware comprising a microprocessor, with band System 12 exchanges handling up to 30 X 64kbit/s connections associated memory and a Terminal Interface (TI). and cellular radio switching applications (also called mobilophone Most important hardware modules are described in the following system). sections. Development of ISDN feature capability for System 12 was started some years ago and already System 12 ISDN field trials were held in a number of countries. 32.6.1.3 Analogue Subscriber Module (ASM) A feature which has recently been gaining importance in Europe is Centrex (see Chapter 35). A combined ISDN/Centrex package is The ASM, which can be equipped as a paired module, provides the seen as particularly attractive for business customers. ISDN Centrex interface for up to 128 subscriber lines which are terminated by line features for System 12 are presently under development and the circuits. initial Centrex installations are already serving analogue subscriber- The module is controlled of two TCEs which are connected in a dual controlled pair configuration (also called the crossover mode). Along with the System 12 ISDN product version, a combined ISDN/ Centrex package will be available. 32.6.1.4 Digital Trunk Module (DTM) 32.6 System 12 architecture The Digital Trunk Module (DTM), performs the preprocessing and control for a single 30 or 31 channel PCM trunk. The System 12 flexible architecture, which allows extensions and The DTM converts the various PCM formats which can be used new services to be introduced as required, can be used for the entire on a digital trunk to the standard format used in System 12 ex- range of exchange applications: changes. System 12 architecture 32/9 Auxiliary Control Elements Control Analog Operator Subscriber Interface Module Module Control Control Trunk Digital Trunk Module Module Control Control Digital 