ATM connections and ATM switching
ATM is connection-oriented, and ATM connections are logical (virtual) connections. In an ATM network, you can create logical connections called virtual paths (VPs) and virtual circuits (VCs) on physical links.
As shown in Figure 40, 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). ATM uses VPI/VCI pairs to identify virtual connections.
Figure 40: Physical link, VP, and VC
ATM interfaces support only manually created permanent virtual circuits (PVCs), not switched virtual circuits (SVCs) created through the exchange of signals. A PVC is identified by a VPI/VCI pair.
In an ATM network, an ATM switch forwards ATM cells by looking up the switching entries and changing the VPIs/VCIs. In PVC mode, the network administrator configures the switching entries and assigns VPIs/VCIs. Users can use the assigned VPIs/VCIs to configure the PVCs. If the ATM interfaces of two ATM devices are directly connected, they must be configured with the same VPIs/VCIs.
Figure 41 shows a typical ATM switching process:
Router A forwards a cell through PVC 0/100 on interface ATM 2/4/1.
ATM switch B receives the cell through PVC 0/100 on interface ATM 2/4/1.
ATM switch B looks up its switching entries and forwards the cell through PVC 2/101 on interface ATM 2/4/2.
Router C receives the cell through PVC 2/101 on interface ATM 2/4/1.
Figure 41: ATM switching
