. The series mitted between the telephone and telephone system in an analogue of samples are then filtered to convert back to an analogue wave- format, and hence these telephones were still classified as analogue form. The most common digital encoding scheme is called Pulse telephones despite the fact that the signalling was now in a digital Code Modulation (PCM). In PCM, the analogue waveform is sam- format. The next stage in telephone set evolution was to move the 8000 times a second, and each sample is converted into an 8 bit conversion from analogue to digital voice (codec) from the tele- digital code. The bandwidth of PCM is therefore 64000 bits per phone system to the telephone itself. Telephones with a codec built second (64kbit/s). into them are classified as digital telephones. Digital systems use Time Division Multiplex (TDM) to interleave digitally encoded telephone calls onto a common highway. The Using the techniques of Time Compression Multiplex (TCM), a management of this highway (or highways) in effect creates a digital telephone is able to multiplex the digitalised voice and digital switching matrix. signals onto a single pair of wires. 34.1.3 Cross connect frame 34.2 PABX architecture The control system and switching matrix are all normally located in one part of the customer's building. The telephone instruments on Figure 34.3 shows a more detailed block diagram of a PABX. The the other hand are distributed throughout the customer's building. trunk circuits provide the basic access to and from the public The cross connect frame is therefore used to connect pairs of wires network and can be analogue or digital connections. A variety of from individual telephones to individual circuits in the system signalling systems are in operation ranging from basic loop discon- peripheral equipment. Similarly, the cross connect frame is used to nect signalling to common channel signalling on a 64kbit/s channel connect circuits from the outside world to the peripheral equipment. of a 2Mbit/s 30 channel trunk 