commendation 1.411. 44.5.1 Reference model National ISDN number The ISDN user/network interface Recommendations apply to International ISDN number physical interfaces at reference points S and T. At reference point R, physical interfaces in accordance with existing CCITT Recommen- dations (for example, X-series and V-series) or physical interfaces ISDN address not included in CCITT Recommendations may be used. There is no reference point assigned to the transmission line to the local ex- Figure 44.4 Structure of ISDN address change since an ISDN user/network interface was not envisaged at this location. User/network interfaces 44/7 Transmission Reference point Functional grouping Figure 44.5 Reference configuration for the ISDN interfaces: (a) S and T; (b) R and S In the reference model the NT1 functional grouping includes 3. H channel. This is currently defined at 384kbit/s (the Ho chan- functions equivalent to layer 1 of the Open Systems Interconnection nel), 1536kbit/s (the H11 channel) and 1920kbit/s (the H12 (OSI) Reference Model and are associated with the proper physical channel). The H channel is intended to carry user information and electromagnetic termination of the network. for services requiring high information throughput (e.g. fast The NT2 functional group includes functions equivalent to layer facsimile, videoconferencing). 1 and higher layers of the OSI Reference Model. PABXs, local area networks and terminal controllers are examples of equipment or Two main interface structures are currently defined at the T & S combinations of equipment that provide NT2 functions. reference points, a basic rate interface and a primary rate interface. The terminal equipment (TE) functional grouping includes func- The basic interface structure is composed of two B channels and one tions equivalent to layer 1 and higher layers of the OSI Reference D channel, 2B+D. The B channels can be used independently, that Model. Digital telephones, data terminal equipment and integrated is, in different connections at the sam