oted earlier, the ATM cell is multiplexed in virtual channels and virtual paths. A sequence of ATM cells belonging to a particular type of service or destination is a virtual channel. A virtual path is a number of virtual channels sharing one single link, which bundles different VCs into one VP. Also, to simplify routing and switching of cells belonging to a particular destination and particular type of service, VCs are combined into a single For the cell to be able to identify its virtual channel and virtual path, identifiers are needed. Cell routing is based on the values of VPI and VCI. The VPI has 8 bits for UNI and 12 bits for NNI. Payload Type (PT) The payload type basically indicates the type of information a cell is carrying (user in- formation or management information). User information may be about congestion in the network, while management information may be to distinguish between different types of cells, such as whether it is an associated cell or a resource management cell. Cell Loss Priority (CLP) CLP is a 1-bit information field that gives the priority for cell loss, basically used for traffic management. There are two priorities for ATM cell loss: 0 and 1. Cells with CLP = 1 are discarded before cells having CLP = 0, in periods such as congestion. Generic Flow Control (GFC) This is a feature of a UNI cell header which basically helps the user to control the traffic flow for obtaining a desired quality of service. It is a 4-bit space in the UNI cell. 8.3.2 ATM Protocol Layers ATM protocol layers (see Figure 8.7) are based on the B-ISDN protocol reference model consisting of three planes: user, control and management planes. The user plane handles user-related information such as flow control, while the control plane handles control-related information such as call control. ASYNCHRONOUS TRANSFER MODE (ATM) Management User Plane Control Plane Upper layer ATM Adaptation Layer ATM layer Physical layer Figure 8.7 ATM protocol layer Physical Layer Generally it is said that the physical layer is the ph