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This appendix explains some of the major components of Frame Relay.

Frame Relay is a streamlined subset of the X.25 packet switching protocol which has been used by many 
corporations for wide area communications for a number of years. By removing the X.25 protocol error 
correction function and its associated overhead, Frame Relay can be used effectively up to T3 speeds over 
error-free lines (about 45 megabits per second).

The advantage provided by Frame Relay is its ability to combine multiple streams of “bursty” data (such as LAN 
protocol traffic) with relatively low average usage rates into a single channel with a relatively higher average 
usage rate. This “statistical multiplexing” effect allows your Frame Relay carrier to provide high bandwidth wide 
area connectivity to you at a price that is often significantly lower than standard leased line rates.

This section covers the following topics:

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Like X.25, Frame Relay is a connection-oriented ser vice requiring your carrier to configure circuits to establish a 
physical link between two or more locations. A single, physical connection can suppor t multiple vir tual circuits 
(which appear as vir tual point-to-point links). 

There are two types of Frame Relay vir tual circuits: Permanent Vir tual Circuits (PVCs) and Switched Vir tual 
Circuits (SVCs).

A PVC is a permanent association between two Frame Relay devices attached on a Frame Relay network. Once 
a PVC is defined, it requires no setup or disconnection. Each PVC has its own committed information rate, 
committed burst size, and excess burst.

SVCs are analogous to X.25 or ISDN connections, which require call setup and tear down.

Note: SVCs are generally not yet available from Frame Relay carriers. Vir tually all Frame Relay communications 
presently use PVCs.