working on an even faster standard called appropriately, V.fast. ance, those same pundits agree that far from being finished, the Whilst this quest for sheer performance continues apace, shipments modem is still firmly fighting back and looking forward to another of V.22bis modems operating at a stately 2.4kbit/s expect to top 2.5 decade of life. million in 1994, roughly two and a half times that of V.32 product. This renewed enthusiasm is based on two fundamental reasons: With faster operation, greater functionality and reducing prices, the outlook, for the consumer at least, is rosy indeed. 1. The introduction of ISDN, the Integrated Services Digital Net- work. The hype of the eighties delivered little in the way of actual service, SO that towards the end of that decade it was Principles of operation viewed with an increasing amount of cynicism and scepticism. Now, ISDN has turned the corner and is a key element in the The principles of modem operation were fashioned by the limita- strategies of networking companies, but although BT are roll- tions of the telephone network for which they were originally ing out 90,000 Basic Rate or ISDN2 circuits (2x64kbit/s bearer designed (Figure 39.1). The telephone circuits of the Public channels plus 1x16kbit/s signalling channel), it is still a drop in Switched Telephone Network (PSTN) are designed to reproduce the ocean compared to some 24 million analogue lines installed speech of a quality that is reliably understandable to the human ear. in the U.K. While reasonably priced switched digital circuits Most of the intelligible sound generated by vocal chords can be are hard to come by, there remain many applications for which contained in a frequency range of 300Hz to 3400Hz, although the modem will be eminently suited, and areas where ISDN generating sounds both higher and lower in frequency. The restric- will never be available. tion of frequencies to this range also eliminates a lot of interference 2. The modem's resilience and stability as a product. The modem and ext