Recall that an ATM cell contains a 5-byte header and a 48-byte payload. A 5-byte space is not sufficient to contain a large address space for the source or destination endpoints. In packet- or frame-based networks, placing the source and destination addresses in each frame is necessary so that routers and switches can forward frames correctly. When ATM transports a cell from source to destination, however, only the specific VC needs to be known.
ATM uses the VPI/VCI identifier combination to deliver cells through a network. An ATM edge device places the VPI/VCI values in a cell before presenting the cell to an ATM switch. The switch then references a forwarding table, relating the VPI and VCI values to outbound switch ports.
For comparison purposes, a UNI cell contains an 8-bit VPI and a 16-bit VCI address. An NNI cell expands the VPI to 12 bits and has a 16-bit VCI value. These values are only locally significant to an ATM switch and do not have to be globally unique.
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