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Understanding Frame Relay 257
Foundation Topics
Understanding Frame Relay
Frame Relay is a connection-oriented, Layer 2 networking technology. It operates at speeds
from 56 kpbs to 45 Mbps. It is very flexible and offers a wide array of deployment options.
Frame Relay operates by statistically multiplexing multiple data streams over a single physical
link. Each data stream is known as a virtual circuit (VC).
Frame Relay VCs come in two flavors, permanent and switched. Permanent Virtual Circuits
(PVCs) are, just as the name implies, permanent, nailed up circuits. They don't tear down or
reestablish dynamically. Switched Virtual Circuits (SVCs) are just the opposite. With SVCs, a
data connection is made only when there is traffic to send across the link. Frame Relay SVCs
are established dynamically and can reroute around network failures.
NOTE
Frame Relay SVCs are beyond the scope of this book (as well as the exam) and are not covered
in detail.
Each VC is tagged with an identifier to keep it unique. The identifier, known as a Data Link
Connection Identifier (DLCI), is determined on a per-leg basis during the transmission. In other
words, it is locally significant. It must be unique and agreed upon by two adjacent Frame Relay
devices. As long as the two agree, the value can be any valid number, and the number doesn't
have to be the same end to end (that is, from router to router across a telco network).
Valid DLCI numbers are 16­1007. For DLCI purposes, 0­15 are reserved, as are 1008­1023.
The DLCI also defines the logical connection between the Frame Relay (FR) switch and the
customer premises equipment (CPE). DLCIs are a constant subject of discussion throughout
this chapter.
Device Roles
Frame Relay devices fall into one of two possible roles, data terminal equipment (DTE) or data
circuit-terminating equipment (DCE). DCE is sometimes known as data communications
equipment as well. Both terms are correct.
It is important to understand that the DTE/DCE relationship is a Layer 2 (data link) layer
relationship. DTE and DCE relationships are normally electrical (that is, Layer 1). The DTE/
DCE relationship at Layer 1 is independent of that at Layer 2. In other words, just because a
router is a Layer 1 DCE doesn't mean it is also the Layer 2 DCE.