Frame Relay, also known as 'fast packet' was the next protocol developed after X.25 came into widespread use on the telecommunications networks and certain limitations (namely speed) were recognized as a problem. Frame relay was originally designed to run over ISDN. The CCITT recommendations I.122, Q.922 and Q.931 specify relay, routing and call control functions in frame relay.


X.25 is a store-and-forward technology, whereas Frame relay is cut-through. X.25 verifies the integrity of all data at each node on the network, further slowing communications. Frame relay can support speeds of up to 2 Mbps, unlike X.25 which is limted to 19,200 bps.

Like X.25, frame relay uses virtual circuits over the physical network connections, but has provisions for congestion control in both the upstream and downstream paths. Individual connections are identified by a Data Link Connection Identifier (DLCI).

Frame relay also supports the Local Management Interface (LMI) which is a set of extensions to frame relay to provide additional functionality to support special services.

When around telecom people, pronounce DLCI as del-see; saying dee-el-see-eye tends to confuse them. Note that LMI is still L-M-I.

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