ATM connections and ATM switching
ATM is connection-oriented, and ATM connections are logical connections, or virtual circuits. In an ATM network, you can create logical connections called virtual paths (VPs) and virtual circuits (VCs) on physical links. As shown in Figure 63, you can create multiple VPs on a physical link, and each VP can be demultiplexed into multiple VCs. Cells from different users are transmitted over different VPs and VCs, which are identified by virtual path identifier (VPI) and virtual channel identifier (VCI).
Figure 63: Physical link, VP, and VC
ATM uses VPI/VCI pairs to identify a logical connection. When a connection is released, all the involved VPI/VCI pairs are reclaimed for new connections.
As shown in Figure 64, an ATM switch forwards ATM cells by looking up the switching entries and changing the VPIs/VCIs. The ATM switching by changing VPIs only is VP switching, and the connection established through VP switching is Virtual Path Connection (VPC). The ATM switching by changing VCIs only or changing both VCIs and VPIs is VC switching, and the connection established through VC switching is Virtual Circuit Connection (VCC).
Figure 64: ATM switching
ATM interfaces support only manually created permanent virtual circuits (PVCs) and permanent virtual paths (PVPs), not switched virtual circuits (SVCs) created through the exchange of signals.