ards. It could be a network employing passive optical devices with time-diversion multiple 44.8.5.2 ATM layer access (TDMA) equipment, or in the future a cellular radio network with a restricted throughput. This layer specifically addresses the ATM cell structure and coding. The B-NTI will provide the interworking between this range of As mentioned in Section 44.8.2 and shown in Figure 44.14 the cell media, rates, transmission standards supported by the network oper- is composed of a 5 octet header and a 48 octet information field. The ators and a common interface for all terminal equipment and cus- primary role of the header is to identify cells belonging to the same tomer premise networks. This will ensure portability of CPE across virtual channel in the ATM stream. To do this the header contains a the whole range of satellite, cellular and terrestrial networks. The routeing field comprising a Virtual Channel Identifier (VCI) and a functions that therefore could be performed by the B-NT1 in addi- virtual path identifier (VPI). A virtual channel is fully defined at the tion to the standard transmission conversion may also include rate interface by the combination of VCI and VPI, since two different conversion from the standard interface rate to a wide range of rates virtual channels in different virtual paths may have the same VCI. 44/16 The Integrated Services Digital Network 9 octets 261 octets STM-1 AU-4 PTR ATM cell 53 octets VC-4 path overhead Figure 44.14 Mapping of ATM cells into the STM-1 frame The header also contains a Generic Flow Control (GFC) field, optimised. However it is hoped that a minimum of AAL proto- which is used to control traffic flow during short term overload cols need be derived with only minimal degradation to some of the conditions, a Payload Type (PT) field, used to indicate whether the services. Currently four classes of service have been defined based information field contains user or network information, a Cell Loss on the timing relation between source and destination, the bit 