Frame Relay

Frame Relay

Frame Relay was designed to support the Broadband Integrated Services Digital Network (B-ISDN), which is discussed in the follow-ing section. The specifications for Frame Relay address some of the limitations of X.25. As with X.25, Frame Relay is a packet-switching network service, but Frame Relay was designed around newer, faster fiber-optic networks.

Unlike X.25, Frame Relay assumes a more reliable network. This enables Frame Relay to eliminate much of the X.25 overhead required to provide reliable service on less reliable networks. Frame Relay relies on higher-level protocol layers to provide flow and error control.

Frame Relay typically is implemented as a public data network and, therefore, is regarded as a WAN protocol. The relationship of Frame Relay to the OSI model is shown in Figure 7.9. Notice that the scope of Frame Relay is limited to the Physical and Data Link layers.

Frame Relay provides permanent virtual circuits, which supply permanent virtual pathways for WAN connections. Frame Relay services typically are implemented at line speeds from 56 Kbps up to 1.544 Mbps (T1).

Customers typically purchase access to a specific amount of bandwidth on a frame-relay service. This bandwidth is called the committed information rate (CIR), a data rate for which the customer is guaranteed access. Customers might be permitted to access higher data rates on a pay-per-use, temporary basis. This arrangement enables customers to tailor their network access costs based on their bandwidth requirements.

To use Frame Relay, you must have special, Frame Relay compatible connectivity devices (such as frame-relay-compatible routers and bridges).