s of ISDN code is used to identify a particular country. The principles relating capabilities: low layer which relate to bearer services and high layer to an ISDN number in relation to the user/network reference con- which, together with low layer capabilities, relate to teleservices. By figuration are that an ISDN number shall be able unambiguously to the network capabilities it means all the technology and techniques identify a particular: which are available in the ISDN to support the telecommunication services. This involves the transmission, switching, signalling and 1. Physical or virtual interface at reference point T, including control procedures which will exist in various parts of the network, multiple interfaces. or interconnected networks, involved in supporting a call end-to- 2. Physical or virtual interfaces at reference point S, including end. These network capabilities are described by various "connec- multiple interfaces, for point-to-point configuration. tion types", which use the attribute method for this description and Interfaces at reference point S for multipoint configurations are described in Recommendation 1.130. (for example, passive bus). An ISDN connection is a connection established between refer- ence points; thus, it is the physical or logical realisation of an ISDN The ISDN numbering plan, Recommendation 1.331, indicates that connection type. Each ISDN connection can be categorised as the international ISDN numbering shall consist of three parts the belonging to a connection type depending on its attributes of infor- country code, the national destination number and the subscriber's mation transfer rate, signalling access protocol and performance number. The country code is used to select the destination country which are all examples of ISDN connection type attributes. An (or geographical area) and varies in length as outlined in Recom- ISDN connection is composed of connection elements as shown in mendation €.163. The national (significant) number is used to select Figure 44.2, 