 activation = Auxiliary framing bit S = Reserved for future standardization Multiframing bit Figure 44.7 Basic access interface. Layer 1 frame structure User/network interfaces 44/9 250us resulting in a bit rate of 192kbit/s with each bit approximately made of the address by the network for routeing purposes and no 5.2us long. Figure 44.8 shows that there is a 2 bit offset between information about its value will be held outside the Layer 2 entity. transmit and receive frames. This is the delay between a frame start The Service Access Point Identifier (SAPI) is used to identify the at the receiver of a terminal and the frame start of the transmitted service that the signalling frame is intended for. Consider the case signal. It also shows a 10 bit offset between the D channel leaving of 1.420 telephones sharing a passive bus with packet terminals. The a terminal, travelling to the NT and being echoed back in the E two terminal types will be accessing different services and possibly channel. This 10 bit delay is made up of bus and transmission delays different networks. It is possible to identify the service being in- in the NT. A frame contains several L bits, which are balanced bits voked by using a different SAPI for each service. This gives the to prevent a build up of DC on the line. network the option of handling the signalling associated with differ- ent services in separate modules. In a multi-network ISDN it allows Layer 2 routeing to the appropriate network. The value of the SAPI is therefore fixed for a given service. VARIABLE FRAME ADDRESS CONTROL LENGTH INFORMATION CHECK The Terminal Endpoint Identifier (TEI) takes a range of values 01111110 FIELD SEQUENCE 01111110 that are associated with terminals on the customer's line. In the MULTIPLE OF simplest case each terminal will have a single unique TEI value. It is important that no two TEIs are the same and therefore the network has a special TEI management entity which allocates TEIs on Figure 44.8 Layer 2 structure request and ensures their correct use