System information

Troubleshooting Frame Relay Connections 18-399
Frame Format
Figure 18-1 The Frame Relay Frame
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 the 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 18–2 provides an example of the use of
DLCIs in nonextended Frame Relay addressing.
Figure 18-2 Frame Relay Addressing
In Figure 18–2, 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 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 on and used in the future.
The bit marked C/R 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 from frames encountering a congested path. The notion behind both of these bits
is that the FECN or BECN indication can be promoted to a higher-level protocol that can take flow
control action 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.
Field length,
in bytes
12 21Variable
Flags FlagsAddress Data FCS
San Jose
DLCI = 12
Pittsburgh
DLCI = 62
Los Angeles
DLCI = 12
Atlanta
DLCI = 82
Switch Switch
Switch Switch
WAN
Router Router
RouterRouter