chniques used, SO data transmission in GSM is for instance by using the mobile's current location to select the dealt with differently. When the data mode is selected, the speech appropriate information for that area. coder is replaced by a rate adaptor and channel coder which apply forward error correction to the data bits, and the resulting bit stream is then transmitted across the radio path in the same burst structure 47.6.2.3 Private interconnect as for voice transmission. At the receive end the bit stream is extracted and errors are corrected up to the limit of the forward error A large user of cellular can often gain economies by leasing a direct correction scheme. If there are any errors remaining, a higher layer connection between the cellular network and their company's pri- protocol is needed to detect and correct them. vate network, thus bypassing the PSTN. Call charges for such direct GSM has defined two families of data services, termed transpar- connections are tariffed by the cellular operator at a level substan- ent and non-transparent. The transparent service applies only for- tially less than that for PSTN calls. An extra benefit of private ward error correction as described above, and the user application interconnect are that calls can be delivered direct to extensions on a must be able to cope with the residual error rate. The characteristics company's network without having to be handled by the switch- of the transparent service are constant delay and throughput but board operator, saving time and labour. variable error rate. The transparent service is of particular use in synchronous applications (eg X.25, IBM SDLC) where the higher 47.6.3 Data services layer protocol inherent in the application will correct the errors. Asynchronous applications may also use the transparent service, In analogue cellular systems, the transparent audio path between the particularly at low bit rates where the forward error correction network and mobile can be used not only for voice communication, applied by 