GSM services and features

In addition to speech, GSM offers a wide range of data bearer services up to 9.6kbit/s suitable for connection to circuit switched or packet switched data networks. GSM also supports Group 111 facsimile as a data service by use ofan appropriate converter.

A comprehensive range of supplementary services are offered by GSM, including call forwarding, call barring, multi-party service, advice ofcharge and others. A full description is provided in the GSM Recommendations, and further detail of cellular services is provided later in this chapter.

An important feature ofGSM is the short message service (SMS). This allows transmission of alphanumeric messages of up to 160 characters to or from a mobile via a service centre. If the message cannot be delivered due to mobile being switched off, or outside of the coverage area, the message is stored at a service centre and re-transmitted when the mobile registers again. Received messages can be displayed on the mobile and stored in the SIM forfuture reference. A related service is cell broadcast which allows mess­ages ofup to 93 characters to be sent to all mobiles within a specific geographical area, for example to deliver traffic or weather reports.

Roaming

Naturally, with a pan-European system, roaming of subscribers between networks is specified by GSM. When a mobile first swit­ches on in a foreign PLMN (Public Land Mobile Network), the local MSC/VLR will determine the identity ofthe home PLMN from the mobile network code which is part of the IMSI. The home HLR will be interrogated to establish whether roaming is permitted and for authentication. The home HLR then passes the subscriber data to the local (foreign) VLR and registers the foreign location of the mobile. Calls to and from the roamed mobile can then take place.

Services

The primary purpose ofall cellular radio networks is to offer speech telephony service to its customers. In addition most networks offer a range of supplementary and value added services to enhance the basic product.

In analogue systems, basic telephony is provided directly by the audio path between mobile and network. Other than some linear speech processing to increase the channel's signal to noise performance, the audio path is transparent across the speech band, allow­ing other sounds (tones, non-voice signals etc) to pass through undistorted. By contrast GSM (and other digital systems) use a speech coder tailored to voice characteristics. They therefore pro­vide a fully acceptable telephony service, but non-voice signals can suffer distortion across the non-audio transparent path.

Figure 47.14 Location updating

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