ormation flows, including user information and control infor- description technique has now been widened. The following ca- mation flows to and through an ISDN, and to take into account the tegories of supplementary service have currently been standardised: separate channel signalling nature of the ISDN. In order to construct the ISDN protocol reference model, a fundamental generic protocol 1. Number identification supplementary services; (Recommenda- block has been identified and is shown in Figure 44.3. tion 1.251). In particular, the model has been designed SO that protocols in an 2. Call offering supplementary services; (Recommendation ISDN can be studied in a structured and uniform way with account 1.252). being taken of the wide range of communication modes and capa- 3. Call completion supplementary services; (Recommendation bilities that can be achieved in the ISDN. For example: 1.253). Multiparty supplementary services; (Recommendation 1.254). 1. Circuit-switched connection under the control of common- 'Community of interest' supplementary services; (Recommen- channel signalling. dation 1.255). 2. Packet-switched communication via the circuit-switched or 6. Charging supplementary services; (Recommendation 1.256). packet channel. 7. Additional information transfer supplementary services; (Rec- Signalling between users and network-based facilities. ommendation 1.257). 4. End-to-end signalling 5. Combinations of above in multimedia communications. 44.4.2 Network aspects ISDN numbering ISDN numbering and addressing principles are detailed in Recom- 44.4.1 Network capabilities mendation 1.330, and Recommendation 1.331 (E.164) describes the numbering plan for the ISDN era. It has been agreed that the ISDN The ISDN functional description defines a set of network capa- numbering plan should be based on and evolve from the existing bilities which enable bearer services and teleservices to be offered telephony numbering plan, and therefore the telephony country to customers. The services require two different level