ata types that needed to share a public network with less time- sensitive traffic. To reiterate, ATM is a layer in the B-ISDN model. Below ATM in the stack lies SONET. Above it lie various layers that adapt the ATM transport structure to meet the needs of different media types and applications. An ATM network switches data of all types in fixed-length cells (for that reason, its also called cell relay). The cell size is 53 bytes; of this, 48 bytes are filled with customer data and 5 bytes are used for cell header information. Cell relay works because information is "chopped" SO finely that the network can easily fit many cells across one high-speed link. Although ATM has a great deal of overhead (more than 10%) it works well in fast network (155 Mbps and up) environments. ATM user equipment connects to the ATM network via a user-network interface or UNI. Connections between ATM-provider networks are made via a network-network interface (NNI). Data contained in a cell header are used by the UNI, the NNI, and ATM switches during transport. Specifically, the first two fields in the ATM header at the NNI are the 12-bit virtual path identifier (VPI) and the 16-bit virtual circuit Circuit-switched Networks identifier. The 28-bit address formed by the VPI/VCI is used in switching a call, hop- by-hop, across the network. These 28-bits change as a call passes from ATM switch to another. The forwarding switch and the receiving switch track the VPI/VCI pairs (sender and receiver) locally, but only for a brief interlude, then the cell moves on. Cells are arranged in logical connections called channels. This confuses some people who think ATM is designed to support both connection-oriented and connectionless services. Although it does support connectionless services, ATM is, over all, a connection-oriented process. A cell may not be sent until a channel has been established to carry it. ATM packages data into cells according to its type-e.g., datagram, voice sample, video frame. It does SO in accordance with the layer above A