Frame relay is a method of transporting digital information over a network. The data is formatted into frames, which are sent over a network of devices usually under the control of a telecommunications company. Diagrams depicting frame-relay networks often display the network as a cloud, as the end user doesn’t generally know (or care) how the network is actually designed. The end user only needs to know that virtual circuits through the cloud will allow the delivery of frames to other end points in the cloud.
Whatever goes into one end of the virtual circuit should come out the other end. The far end appears as though it is on the other end of a physical cable, though in reality, the remote end is typically many hops away.
The virtual circuits may be either switched or permanent. A frame-relay permanent virtual circuit (PVC) is always up, even if it’s not in use. A switched virtual circuit (SVC) is up only when it’s needed. Most data networking deployments use PVCs.
Figure 22-1 shows a typical simple frame-relay network using PVCs. Router A is connected to Router B with a PVC. Router A is also connected to Router C with a PVC. Router B and Router C are not connected to each other. The two PVCs terminate into a single interface on Router A.
Figure 22-1. Simple frame-relay WAN
Physically, each router is connected only to one of the provider’s telecom switches. These ...