are called network node interfaces (NNIs). 6.8.2 Cell Structure of ATM The ATM cell is 53 bytes long with 48 bytes reserved for carrying the pay- load and 5 bytes for the header (see Figure 6.60). This size was a compromise between the 64 bytes favored by the data community and the 32 bytes pre- ferred by the voice community. The data fields of a cell are shown in Figure 6.60 are explained in the following sections. 6.8.2.1 Generic Flow Control (GFC) The GFC field is used at a user interface only to control the data flow between the first ATM switch and the user node. Inside the network (i.e., in the NNI), this field is used for virtual path identification together with the other VPI fields. This is the only difference in the cell structure between UNI and NNI. 6.8.2.2 VPI and VCI The majority of the header is taken up by VPI and VCI. Together they iden- tify an individual circuit. They have only local significance and they change Data Communications ATM has constant cell size of 53 bytes, five of them used for a header GFC = Generic flow control (inside the network this field is used for VPI) VPI = Virtual path identifier VCI = Virtual channel identifier PT = Payload type CLP = Cell loss priority HEC = Header error control bytes Figure 6.60 Structure of an ATM cell (UNI). on the way through the network as explained in Section 6.2.4. They are used in the same way as the logical channel number in X.25 or the data link con- nection identifier of frame-relay technology. 6.8.2.3 Payload Type Payload type specifies whether the cell contains user information or informa- tion to be used by the network itself, for example, for O&M. The network can use these maintenance cells between nodes to perform operations in, for example, a congestion situation. 6.8.2.4 Cell Loss Priority The cell loss priority bit carries information between an ATM user system and the network. For example, in a congestion situation the network may use this field to define the priority of cells in the queues or to decide which cells are discarded fi