lternatives is easy but potentially misleading. International. Connecting and installing ISDN is expensive, whereas the call Clark, A. (1991) Error control and modem communications, Com- charges are identical to analogue dial up lines, being based on munication and Computer News. May. distance and time but independent of the volume. By comparison Dennis, T. (1990) Dial up modems. Datacom. June. with analogue voice lines, they offer considerably more bandwidth, Doll, D. (1978) Data Communications; Facilities, Networks and the quality of end to end digital connections and practically instan- System Design. John Wiley & Sons. taneous call set up. So in reality it is not a like for like comparison. Enslow, P. (1988) The OSI Reference Model and Network Architec- Based purely on today's tariffs, and compared with a V.32 alter- tures. Frost & Sullivan, London. native, ISDN can only be justified by substantial use, where its Minoli, D. (1991) Telecommunications Technology Handbook. Ar- greater bandwidth can reduce call charges to recoup the much tech House, Boston. higher initial outlay. By comparison, used on a single location for Scott P.R.D. (1980) Modems in Data Communication. NCC Publi- more than three hours a day, it could well be cheaper to use digital cations, Manchester. leased lines like KiloStream. For a genuine comparison, the nature This page intentionally left blank Multiplexers J Hoolan Dowty Communications Ltd Contents Introduction 40/3 Time Division Multiplexers 40/3 Statistical time division multiplexing 40/6 High order multiplexing 40/8 Multiplexing and packet switching 40/9 X.25 and OSI 40/10 Physical layer standards 40/13 Multiplexers in communications networks 40/14 40.9 The future of multiplexing 40/16 This page intentionally left blank Introduction 40/3 40.1 Introduction problem created by the growth in remote computer processing during the 1960s. Computer terminals (or consoles) were locally The multiplexer is one of the most important components in com- attached in early computers but as compute