ce call is in progress. The short message service is being enhanced by the introduction of the Enhanced Message Service (EMS). This enables formatted text, pictures and sounds (melodies) to be sent. It uses the same basic format as SMS but larger data elements are sent and the header format is slightly changed. Further improvements in the service include a multimedia message service (MMS) which allows full colour pictures, animations and the like to be sent. GPRS and EDGE GPRS or General Packet Radio Service is a packet-based service that enables the GSM system to carry data at rates up to 114 kbps although normal data rates of around 30 to 50 kbps are likely to be experienced. In addition to this it will give a continuous connection to the Internet. It uses the same modulation (GMSK) as GSM and complements the existing services such as SMS. To upgrade the system to carry GPRS it is necessary to convert the system from what is termed a circuit switched network where the circuit is switched for the period when the user requires a particular circuit, to a packet switched network. The handsets, however, cannot be converted and new handsets are required when upgrading from GSM to GPRS. Packet switching is the key to GPRS and it enables the system to be used only when data is being sent or received. Instead of devoting a complete channel to a mobile user for the duration of the call, as is normally the case for GSM, the channel is only used when data is actually being sent and as a result the available radio resource is shared between several users. Overall this means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users that can be supported at any time naturally depends upon how much data is being transferred. A further update that can be used with GSM is known as EDGE. This stands for Enhanced Data rates for Global Evolution, and it is a standard Cellular telecommunications 269 originally called GSM 384. Often systems using EDGE are kn