ace', and covering timing, ence points interface structure and OAM implementation), header error control, idle (empty) cells and cell delineation and scrambling. From the The characteristics of the interfaces at the TB and SB reference ATM viewpoint, the mappings into SDH and the HEC field and cell points are described. There are two physical layer options at 155.52 delineation and scrambling algorithms are perhaps the most import- Mbit/s: cell-based and SDH based. The options at 620 Mbit/s are for further study. Transmission frame adaptation functions: SDH based interface. The interface bit-stream is based on SDH as described in 41.4.11.4 B-ISDN model applied to functional groups CCITT Rec. G.709. The ATM cell stream is mapped into the C-4 container and then packed into the VC-4 virtual container A list of functions which might be included in each of the B-UNI along with the VC-4 path overhead. The ATM cell boundaries functional groups is given, although these lists are not exhaustive: are aligned with the STM-1 octet boundaries. The C-4 is not an integer multiple of the cell length so cells will cross C-4 1. B-NT1 functions: line transmission termination; transmission boundaries. The H-4 will be set at the sending side to indicate interface handling; OAM functions. the next occurrence of a cell boundary. This may be used 2. B-NT2 functions: adaptation functions for different media and optionally to supplement the mandatory HEC cell delineation topologies; functions of a distributed NT-2; cell delineation; mechanism. (Figure 41.4.) concentration; buffering; multiplexing/demultiplexing; re- HEC field and cell delineation. The HEC field of the cell header source allocation; usage parameter control; adaptation layer is calculated across the entire cell header and the code used is functions for signalling (for internal traffic); interface handling capable of single bit error correction and multiple bit error (TB and SB); OAM functions; signalling protocol handling; detection. The 8-bit HEC field is the remainder of 