e frame. The address has only local significance and is must specify the particular bearer service required for the call (i.e. known only to the two end points using the LAP. No use can be speech, 64kbit/s unrestricted or 3.1kHz audio) and any terminal 44/10 The Integrated Services Digital Network Octet Protocol Discriminator Length of call reference value (in octets) Call Reference Value Message Type Other information elements as required Figure 44.9 Signalling message structure compatibility information which must be checked at the destination. terminal is allocated a feature number which is invoked by the This information element may also be used to specify low layer terminal signalling that number to the network. This mechanism is terminal characteristics such as data rate. Where applicable the very similar to that currently offered on PABXs where the PABX non-voice application to be used on the call may be specified via the terminals have feature keys which can be programmed to corre- High Layer Compatibility information element (i.e. Group 4 FAX, spond to a particular supplementary service. teletex, videotex or slow scan video). In functional signalling, the terminal must have knowledge of the As well as bearer services and teleservices the ISDN is able to supplementary service being invoked and have the associated sig- support a range of supplementary services similar to those that are nalling protocol implemented. Both the terminal and the network available on PABXs and private networks today. Two generic layer then hold records of the supplementary service call state. The 3 signalling protocols have been specified for the control of sup- functional protocol is based on using the facility information ele- plementary services, namely stimulus and functional signalling ment conveyed in the basic call control messages if the supplemen- procedures. tary service is invoked during call establishment or call clearing and In stimulus procedures, the terminal is not required to have knowl- in the FACILITY message 