 for connection to circuit switched authentication. The home HLR then passes the subscriber data to the or packet switched data networks. GSM also supports Group III local (foreign) VLR and registers the foreign location of the mobile. facsimile as a data service by use of an appropriate convertor. Calls to and from the roamed mobile can then take place. A comprehensive range of supplementary services are offered by GSM, including call forwarding, call barring, multi-party service, advice of charge and others. A full description is provided in the 47.6 Services GSM Recommendations, and further detail of cellular services is provided later in this chapter. The primary purpose of all cellular radio networks is to offer speech An important feature of GSM is the short message service (SMS). telephony service to its customers. In addition most networks offer This allows transmission of alphanumeric messages of up to 160 a range of supplementary and value added services to enhance the characters to or from a mobile via a service centre. If the message basic product. cannot be delivered due to mobile being switched off, or outside of In analogue systems, basic telephony is provided directly by the the coverage area, the message is stored at a service centre and audio path between mobile and network. Other than some linear re-transmitted when the mobile registers again. Received messages speech processing to increase the channel's signal to noise perfor- Services 47/15 Random access Channel procedure Location request Update location area Old VLR Send IMSI Response Update location Subscriber data insert Subscriber data ack Cancel location Cancel location ack Location area Location confirm update accepted Update location accepted Figure 47.14 Location updating mance, the audio path is transparent across the speech band, allow- service is often combined with the "divert on no answer" ing other sounds (tones, non-voice signals etc) to pass through service. undistorted. By contrast GSM (and other digital systems) use a Divert