Facsimile transmission over GSM is complicated by the use in the limited frequency response of the channel, and the delay spread Group 3 standard of a number of data transmission rates and modem characteristic which is affected by the audio processing in both types (V.21, V.29, V.27). In order to carry the fax signals, GSM mobile and base station. However data transmission at 1200bit/s mobiles need a special adaptor to convert the multiple standards into (using CCITT V.22) and 4800bit/s (using V32) can be achieved a synchronous bit stream for transmission between mobile and quite commonly on cellular networks, as well as fax up to 7200 or network. A similar converter in the network then converts the signal 9600bit/s. back into the Group 3 protocol to interwork with fax machines in The radio link between a cellular network's base stations and a the fixed network. mobile station is a notoriously hostile environment for data trans- mission. Disturbance and interruptions come from a variety of sources, such as variability of the radio signal strength, noise and Future developments interference, and 'intentional' breaks due to signalling interchanges between base station and mobile for handover and power control. In The technology of cellular radio systems continues to develop very order to transmit data reliably over such a path, error control of some rapidly. The early 1980s saw the introduction of the first commer- form is essential. cial analogue systems and by the end of the decade trials of second The simplest form of error control is a layer 2 protocol, and the generation digital systems were already under way. Systems such as emergence of the CCITT V.42 standard has led to error correcting GSM are now entering into service and work is already starting on modems becoming readily available. Although V.42 (which con- the specification of a third generation world wide standard system. tains two protocols, the 'open' LAP-M and 'proprietary' MNP4) These developments are not introducing technology for its own was des