B-ISDN Reference Model ATM makes B-ISDN a reality. The Integrated services Digital Network (ISDN) evolved during the 80's. It carried a basic channel that could operate at 64kbps (B-channel) and combinations of this and others (D-channels) formed the basis of communication on the network. In the new B-ISDN world, this is supposed to supply data, voice and other communication services over a common network with a wide range of data speeds. To understand a lot of the terminology in ATM-land, it is necessary to understand the B-ISDN Reference Model. Just as the ISO seven-layer model defines the layers for network software, this model defines layers for the ATM network. The header is broken up into the following fields. " Generic Flow Control (GFC) " Virtual Channel Identifier (VCI) " Virtual Path Identifier (VPI) " Payload type (PT) " Cell Loss Priority (CLP) " Header Error Control (HEC) Network - to - Network interface It is necessary for the switches to know how to send the calls along. There are several techniques that could be adopted, but the most useful one for the 1P users is called Private Network-to Network Interface (PNNI)The PNNI is an interface between switches used to distribute information about the state and structure of the network to establish circuit to ensure that reasonable bandwidth and Qos contract can be established and to provide for some network management functions." Convergence Sublayer: The function provided at this layer differ depending on the service provided. It provides bit error correction and may use explicit time stamps to transfer timing information. Segmentation and reassembly sublayer: At this layer the convergence sublayer-protocol data unit is segmented and a header added. The header contains 3 fields Sequence Number used to detect cell insertion and cell loss. Sequence Number protection used to correct and detect errors that occur in the sequence number. Convergence sublayer indication used to indicate the presence of the convergence sublayer function