Compatible Systems 5.4 - Manual (Page 297)

Appendices 291
Frame Relay 101
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 a number of the X.25 protocol’s seldom-used
functions and their associated overhead, the Frame Relay protocol allows
communications at up to T1 speeds (about 1.5 megabits per second).
The generic advantage provided by Frame Relay is its ability to combine
multiple streams of “bursty” data (such as LAN protocol traffic) all of which
have 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 which is often significantly lower than standard leased line rates.
Virtual Circuits
Like X.25, Frame Relay is a connection oriented service requiring circuits to
be configured by your carrier to establish a physical link between two or more
locations. Multiple virtual circuits (which appear as virtual point-to-point
links) can be run through the same physical connection.
There are two types of virtual circuits supported in Frame Relay: Permanent
Virtual Circuits (PVC) and Switched Virtual Circuits (SVC).
PVCs are like dedicated point-to-point private lines. Since the physical
connection is always there in the form of a leased line, call setup and tear
down is done by a carrier via a network management system.
SVCs are analogous to X.25 connections, which require call setup and tear
down.
v Note: SVCs are generally not yet available from Frame Relay carriers.
Virtually all Frame Relay communications is presently being done using
PVCs.
Addressing
A number called the Data Link Connection Identifier (DLCI) identifies each
virtual circuit within a shared physical channel.
Frame relay packets are exchanged between nodes by mapping packets
containing the source node’s DLCI address to the destination DLCI address
at the switch. Each switch contains a table identifying the various DLCIs with
their associated user lines and interface trunks. However, the switch has more
or less work depending on if the DLCI has global or local significance.