etworks, IEEE Trans. on Commun., COM-30, July, pp. 1493-1497. Bell (1971) Transmission systems for communications, Bell Tele- phone Laboratories. Rossiter, T.J.M. et. al. (1982) A modular transmultiplexer system Bennett, W.R. (1940) Cross modulation requirements on multichan- using custom LSI devices, IEEE Trans. on Commun., COM-30, nel amplifiers below overload, Bell Systems Technical Journal, (7), July. (19) October. Tucker, D.C. (1948) Journal Inst. Electrical Engineers, Part III, CCITT (1985) Volume III, CCITT, Geneva. Connor, F.R. (1973) Introductory topics in electronics and telecom- munications - Noise, Edward Arnold, London. Young, P. (1983) The power of speech, George Allen, London. Time division multiplexing Michael J Simmonds Telindus Ltd Luc Ceuppens Ind Ing Telindus Ltd Contents General definitions 21/3 Digital time division multiplex structure 21/4 The digital hierarchy levels 21/5 21.4 The T carrier framing and coding formats 21/6 The CEPT PCM-30 framing format 21/9 21.6 T1 and PCM-30 alarms and error conditions 21/10 Coding schemes 21/12 Examples of digital network services 21/14 21.9 References 21/16 This page intentionally left blank General definition 21/3 General definition Reference Point In telephone systems a time division multiplexer, better known as a TDM, is generally defined as a device that distributes a number of channels periodically in time through the intermediary of pulse modulation. Each pulse corresponds to a channel and is interleaved between those of other channels. Hence, a time division multiplexed signal is always composed by means of synchronous sampling of the channels, with pulses shifted with respect to each other. This is illustrated in Figure 21.1. The interleaved channels form one frame of a duration corresponding to the sampling period Ts. The pulse modulation used can be analog (e.g. PAM, PPM) or digital (PCM). With time division multiplexing time is a relative value, i.e. TDM requires a reference point in the frame cycle to which the receiver must be synchronised 