Information Technology Reference
In-Depth Information
Figure18-1 The Frame Relay Frame
Field length,
in bytes
1
2
Variable
2
1
Flags
Address
Data
FCS
Flags
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.
Figure18-2 Frame Relay Addressing
San Jose
Pittsburgh
DLCI = 12
DLCI = 62
Switch
Switch
Switch
Switch
WAN
DLCI = 82
DLCI = 12
Atlanta
Los Angeles
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.)
