. The values that TEIs can take fall into the ranges: 1. () - 63 for automatic assignment TEIs which are selected by the 44.5.4 Layer 1 - Primary rate access user. 64 - 126 for automatic assignment TEIs selected by the network The primary rate access is contained in Recommendation 1.431 on request. and is based on existing primary rate PCM multiplex structures, and 127 for a global TEI which is used to broadcast information to therefore reflects the two alternate standards of 1544kbit/s and all terminals within a given SAPI. 2048kbit/s. In Europe the 2048kbit/s standard contains 30 B chan- nels, one D channel and a 64kbit/s synchronisation channel. The The combination of TEI and SAPI identify the LAP and provide primary rate access is permanently activated and operates in a point a unique Layer 2 address. A terminal will use its Layer 2 address in to point manner and therefore does not require any activation/deac- all transmitted frames and only frames received carrying the correct tivation procedures. An HDB3 line code is used over coaxial or address will be processed. screened balanced pairs and it has a reach capability restricted to In practice a frame originating from telephony call control has a 6dB loss. SAPI that identifies the frame as "telephony" and all telephone equipment will examine this frame. Only the terminal whose TEI 44.5.5 Layer 2 agrees with that carried by the frame will pass it to the Layer 2 and Layer 3 entities for processing. The layer 2 structure and features are based upon the High Level Data Link Control (HDLC) procedures shown in Figure 44.8 and 44.5.6 Layer 3 has the following fields. The general structure of the layer 3 signalling messages is shown in 1. The flag acts as a unique delimiter and additional circuitry Figure 44.9. The first octet contains a protocol discriminator which ensures that the flag pattern never occurs in the bits in the rest gives the D channel the capability of simultaneously supporting of the frame. additional communications protocols in the future. 2. The 