ling; Cell Loss Priority (CLP) HEC header generation and verification; cell delineation; transmission frame adaptation; transmission frame gener- Header Error Control (HEC) ation and recovery. 2. ATM Layer: generic flow control; cell header generation and The coding of the fields is as follows: extraction; VPI/VCI translation; cell multiplexing and demulti- plexing. 1. GFC field: 4 bits, UNI only. 0000 when not used. Coding for 3. ATM Adaption Layer: further study. AAL Segmentation and Re-assembly sublayer (SAR); CS 2. Routing field: At the UNI = 8 bits VPI + 16 bits VCI. Specific PDU Segmentation; CS PDU Re-assembly. values are assigned for meta-signalling virtual channel identi- AAL Convergence sublayer: convergence (service depend- fication and general broadcast signalling virtual channel ident- ent). ification. At the NNI = 12 bits VPI + 16 bits VCI. Specific Management plane Control plane User plane Higher layers Higher layers ATM adaptation layer ATM Layer Physical layer Figure 41.15 B-ISDN protocol reference model (Fig. 1/1.321) Standards for ATM 41/15 Table 41.2 Cell header fields at the NNI Table 41.4 Examples of services Header field Number of bits Class of service Example of service Virtual Path Indicator (VPI) Class A Circuit emulation Virtual Channel Indicator (VCI) Class B Variable bit rate video and audio Payload Type (PT) Class C Connection oriented data transfer Reserved (Res) Class D Connectionless data transfer Cell Loss Priority (CLP) Header Error Control (HEC) values are assigned for idle cell identification, physical layer to peer operations. It describes in detail the four types of AAL OAM cell identification, code for the use of the physical layer, relating to the four classes of service defined in 1.362, in each case unassigned cell identification. addressing the functions which may be performed in the AAL to Payload type field: 2 bits for payload type identification. User enhance the service provided by the ATM layer. In particular the information = 00. Other values for further study. SA