A Frame Relay frame is shown in Figure .
The Flags fields delimit the beginning and end of the frame. Following
the leading Flags field are 2 bytes of address information (Figure ).
Ten bits of these 2 bytes make up the actual circuit ID (called the
DLCI, for data-link connection identifier). The 10-bit DLCI value is
the heart of the Frame Relay header. It identifies the logical
connection that is multiplexed into the physical channel. In the basic
(not extended by LMI) mode of addressing, DLCIs have local
significance; that is, the end devices at two different ends of a
connection may use a different DLCI to refer to that same connection.
Figure
provides an example of the use of DLCIs in non-extended Frame Relay
addressing.
In Figure ,
assume two PVCs, one between Atlanta and Los Angeles and one between
San Jose and Pittsburgh. Los Angeles uses DLCI 12 to refer to its PVC
with Atlanta, whereas Atlanta refers to the same PVC as DLCI 82.
Similarly, San Jose uses DLCI 12 to refer to its PVC with Pittsburgh.
The network uses internal proprietary mechanisms to keep the two
locally significant PVC identifiers distinct.
At the end of each DLCI byte (Figure )
is an extended address (EA) bit. If this bit is 1, the current byte is
the last DLCI byte. All implementations currently use a 2-byte DLCI,
but the presence of the EA bits means that longer DLCIs may be agreed
upon and used in the future.
The command/response bit following the
most significant DLCI byte is currently not used.
Finally, 3 bits in the 2-byte DLCI
provide congestion control. The forward explicit congestion
notification (FECN) bit is set by the Frame Relay network in a frame
to tell the DTE receiving the frame that congestion was experienced in
the path from source to destination. The backward explicit congestion
notification (BECN) bit is set by the Frame Relay network in frames
traveling in the opposite direction of frames encountering a congested
path. The notion behind both of these bits is that the FECN or BECN
indication can be acted on by a higher-level protocol that can take
flow-control measures as appropriate. (FECN bits are useful to
higher-layer protocols that use receiver-controlled flow control,
whereas BECN bits are significant to those that depend on
emitter-controlled flow control.)
The discard eligibility (DE) bit is set
by the DTE to tell the Frame Relay network that a frame has lower
importance than other frames and should be discarded before other
frames if the network becomes short of resources. Thus, it represents
a very simple priority mechanism. This bit is usually set only when
the network is congested.
Note: Frame Relay
troubleshooting is covered in Chapter 8.
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