Frame Relay

Frame Relay is a logical, low-overhead transport protocol that removes much of the overhead
found in X.25. Frames are marked with a data link connection identifier (DLCI) that provides
direction to the switch regarding frame forwarding. As such, frames in Frame Relay are layer 2
elements. In many companies, setting up Frame Relay services between central locations and
remote offices is very popular. The primary benefit of Frame Relay is that it is traditionally tariffed
to be distance insensitive; this means that a connection that crosses the United States will
be comparable in cost to that of a connection across town. In addition, Frame Relay services are
available internationally from many providers.
Frame Relay, in addition to DSL, is becoming more accepted in the telecommuter workspace.
Telecommuters are finding that connections are required for more than a few hours per day—
a threshold that makes ISDN more costly than the other options. In addition, ISDN is incapable
of expanding beyond 128Kbps without using PRI services or bonding. Frame Relay is available
in a myriad of bandwidths, up to and including DS3. New variations on Frame Relay are
increasing this performance characteristic.

Note:
Note that ISDN cannot scale beyond 128Kbps in user data on a single pair of
B channels. Just as two B channels can be bonded together into a single logical
data conduit, it is possible to bond multiple ISDN BRI circuits into a single logical
data stream. Chapter 24, “Point-to-Point Protocol,” discusses PPP bonding
in greater detail, and Chapter 26, “Integrated Services Digital Network (ISDN),”
discusses ISDN bonding.
For the network designer, there are two factors to consider when deploying Frame Relay:
Frame Relay is available with a
committed information rate (CIR)
, and Frame Relay enables
multiple
permanent virtual circuits (PVCs)
to terminate at a single physical connection point on
the router. A PVC is a previously defined logical path through the network. The DLCI is used
to determine which PVC is to be used.
Switched virtual circuits (SVCs)
are alternatives to PVCs.
SVCs are similar to PVCs, but they are not predefined and static, so before data can be transmitted
by using SVCs, a path must be established dynamically through the network.
The CIR is best thought of as a guaranteed amount of bandwidth available on a PVC. This
figure might be substantially lower than the capacity of the circuit itself. The corporation will
pay for the bandwidth guaranteed by the CIR, and any traffic that exceeds the CIR will be handled
on a best effort basis. Thus, a company can obtain better throughput than that for which
it is being charged.
The capability of Frame Relay to enable multiple PVCs to terminate at a single physical connection
point on the router is a powerful tool. This means that a designer need not purchase
additional interfaces to accommodate multiple connections. In addition, there’s a substantially
lowered lead time for new connections, and such connections can be provisioned without a visit
to the head-end location.
The Frame Relay protocol is primarily designed to encapsulate data on reliable, digital connections.
Its benefits include low overhead when compared to X.25 (X.25 using protocol overhead
for data reliability), lower costs when compared to point-to-point connections, and a single access point on the router that can terminate multiple virtual circuits (each of which can
go to different destinations). This last benefit greatly reduces the costs associated with the router
hardware. The Frame Relay protocol and its benefits are explored in more detail in Chapter 29,
“Frame Relay.”
Due to its relatively low cost and high bandwidth, Frame Relay is better suited for higher
bandwidth demands than other access technologies, including ISDN.