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Frame relay is a metropolitan and wide area networking solution that implements data link switching techniques. Data frames are sent across permanent or switched virtual circuits that are defined within a network of frame relay switches. Frame relay networks are operated by local and long-distance carriers such as Sprint, MCI, AT&T, and most of the RBOCs (Regional Bell Operating Companies).

Frame relay is an outgrowth of work done on ISDN (Integrated Services Digital Network). Basically, frame relay is the frame-switching component of ISDN, and it is now sold as a separate service. While ISDN is primarily a circuit-based service that mimics a telephone call, frame relay was designed into the service to provide reliable any-to-any connections across a switched network.

Subscribers contract for a virtual circuit between two points. The circuit has some guaranteed level of service. What the subscriber sees is a private data pipe with that level of service from one point to the other. Inside the network itself, the path of the pipe may vary, depending on traffic conditions, downed lines, and so on. However, because the service is switched, the network will quickly establish new circuit paths and maintain the levels of service guaranteed to subscribers. If the carrier cannot maintain the contracted service levels, the subscriber will obtain some form of credit.

Frame relay is an any-to-any service that is shared by many users. Virtual circuits can be established between any two devices attached to the nework. In contrast, leased TDM circuits such as T1 are set up between two points with a specific bandwidth. The advantage of frame relay is that many users share the network, which means that carriers can offer lower monthly rates. If one subscriber is not using its share of the bandwidth, that share is available for another subscriber to use. The data rate is more flexible. Instead of one fixed rate, bursts are allowed if the network has available capacity.

The downside of a shared network is that when traffic increases, service levels may drop. Carriers will write this into their contracts. For example, a carrier will add together the bandwidth requirements of all of its subscribers and then ensure that the network has enough capacity to deliver that bandwidth. This is the committed rate that it guarantees to subscribers. Figure F-10 illustrates the advantage of frame relay when compared to a network of private dedicated circuits such as T1 lines. There are four sites in different cities. The customer wants to inter- connect all sites. To do so with T1 circuits would require six long-distance links. With frame relay, subscribers simply connect each site via a short-distance circuit into a carrier's local frame relay cloud. The frame relay network then provides the long-distance component.