e carefully considered. If the 30kbit/s limit on New standards are being developed by the CCITT for transferring modems is achieved and can successfully be combined with com- digital data over Group 111 analogue connections and improve- pression techniques operating a 4:1 reduction in line transmission ments in both speed and error correcting capability have been requirements, potential throughputs of 120kbit/s will provide recommended. CCITT study groups are currently looking at con- modems with ISDN basic rate performance. The question is, will nectivity between digital Group 1V fax and Group 111 fax, a sure they need to? sign of a healthy market. Already products are having to be developed to help computer Apart from the fax boom, other applications, which improvements serial communications ports keep up with current modem perfor- in dial up technology have made possible, include local area net- mance as users fall into the speed trap of assuming their desk top work interconnection. Formerly the sole province of analogue or equipment can run fast enough to make full use of it. Significantly digital leased lines, dial up technology now provides for Internet those vendors who have a foot in both the modem and ISDN camps Protocol (IP) addressing over switched circuits. The continuing still believe there is a lot of mileage left in the modem. Their future viability of this trend from leased circuit to dial up circuit, typically is not in trying to match ISDN in terms of performance, but provid- for back up applications, is the speed with which ISDN can erode ing innovative, added functionality, for increasingly cost effective this technological advance. communications. Increasingly there will be a choice to be made between high speed modems over switched analogue lines and ISDN over switched 39.7 References digital lines, as PTTs fulfil their promises on basic rate ISDN availability. For the moment, costing up analogue switched, digital Braue, J. (1991) The state of the modem. Data Communications leased and ISDN a