functions necessary for providing switched services. The C-plane structure shares the physical and ATM layers with the U-plane as shown in Figure 20.3. It also includes ATM adaptation procedures and higher-layer signaling protocols. Management plane (in other literature called the M-plane). The management plane provides management functions and the capability to exchange information between the U-plane and the C-plane. The M-plane contains two sections: layer management and plant management. The layer management performs layer-specific management functions, while the plane management performs management and coordination functions related to the complete system. We will return to Figure 20.3 and B-ISDN/ATM layering and layer descriptions in Section 20.6. 20.4 THE ATM CELL: KEY TO OPERATION 20.4.1 ATM Cell Structure As we mentioned earlier, the ATM cell consists of 53 octets, 5 of which make up the header and 48 make up the payload or "info" portion of the cell. 5 Figure 20.4 shows an ATM cell stream, delineating the 5-octet header and 48-octet information field of each cell. Figure 20.5 shows the detailed structure of cell headers at the user-network interface (UNI) (Figure 20.5a) and at the network-node interface (NNI)6 (Figure 20.5b). We digress a moment to discuss why a cell was standardized at 53 octets. The cell header contains only 5 octets. It was shortened as much as possible, designed to contain the minimum address and control functions for a working system. It is obvious that the overhead is non-revenue-bearing. It is the information field that contains the revenue- bearing payload. For efficiency, we'd like the payload to be as long as possible. Yet the 5 Under certain situations, there are 6 octets in the header and 47 octets in the payload. 'NNI is variously called network-node interface or network-network interface. It is the interface between two ATM network nodes or switches. 20.4 THE ATM CELL: KEY TO OPERATION Header (includes Information field - routing information) 48 octets 5 octets Cell stream