| 3.3 | Layer 2 Protocol Overview - WAN Protocols | ||
| 3.3.6 | Frame Relay |
|
Frame Relay is often described as a faster
version of X.25. This claim has some merit, because Frame Relay is
quite similar to X.25. It is a packet-switching technology, but
features smaller packet sizes and fewer error-checking mechanisms.
Frame Relay currently supports transfer of packets through PVCs
(permanent virtual circuits) between the endpoint routers of the Frame
Relay cloud. Few manufacturers of Frame Relay DCE equipment presently
support SVC's (switched virtual circuits) - note that X.25 and ATM are
two technologies which actively support SVCs.
Note: Frame Relay is classified as a packet-switched technology even though, as its name implies, it is a data link layer protocol that uses frames! The PVC endpoints are defined by DLCIs (data-link connection identifiers) and are given a committed information rate (CIR) through the Frame Relay network. DLCI pairs are also given a minimum available quantity of bandwidth with the option to temporarily burst beyond that limit under certain circumstances. The figure to the left illustrates the use of DLCIs in Frame Relay networks. Each dashed line between two locations represents a DLCI pair. Also, note that Location D does not use a router to connect to the Frame Relay network. Frame Relay, as with any data link layer protocol, contains native mechanisms for addressing. Such mechanisms are primitive in comparison to comparable network layer mechanisms, but may suffice for interconnecting stub networks. In the figure, Location D connects to only one other location, and thus can dispense with the cost and complexity of a routed connection. Instead, it uses a Frame Relay access device (FRAD) to connect the LAN to the WAN. Frame Relay WANs are built by provisioning a point-to-point private line from the work location to the nearest central office that provides this service. At the central office, this private line terminates in a Frame Relay switch that is either fully or partially meshed with the other Frame Relay switches that comprise that carrier's Frame Relay commercial infrastructure. Much like the central-office voice switches that comprise the Public Switched Telephone Network (PSTN), the Frame Relay switches remain invisible to the user community and its applications. The primary benefit of Frame Relay is that it can reduce the cost of networking locations that are geographically dispersed by minimizing the length of transmission facilities. These circuits are commercially available at 1.544 Mbps, with CIRs used to create logical subrate connections to multiple locations. |