Understanding the network.pdf - Back to Home

This topology is limited to the capabilities provided by the OSI-RM Layer 3 protocol
used to provide delivery, and it is commonly used as a cost savings dodge. With this
configuration, you only need to have the multipoint host contract for a large
bandwidth Frame Relay connection, and the single PVC sites can contract for just a
percentage of the supportable bandwidth of the multipoint site.
Frame Relay Congestion Control
This latter approach works well because Frame Relay supports congestion control
mechanisms. These mechanisms are known as forward and backward explicit
congestion notification (FECN and BECN). To enable FECN or BECN, a notification bit
is set in the Frame Relay frame address field. BECN is used by DTEs to indicate
congestion problems. If a BECN bit is enabled on a received frame, the receiving
DTE reduces CFR for the interface. If consecutive BECN messages are received, the
CFR is reduced further. When the host begins to receive, BECN "clears" messages;
the DTE can begin to increase its CRF. FECN messages are implemented by the
DCEs to indicate to the destination DTE that the frame encountered congestion over
its delivery path through the Frame Relay network.
If the BECN and FECN mechanisms are not adequate to accommodate network
congestion in the Frame Relay cloud, the DCE will implement further congestion
recovery by discarding frames based on the Discard Eligibility (DE). A DE setting of
1 indicates the frame can be discarded in times of congestion. A setting of 0
indicates that the frame should be spared unless there is no alternative. The DE can
be set by the DTE based on the type of traffic contained in the frame or depending
on the DTE's CFR. If the DTE is in excess of its standard usage CFR, the frames will
be tagged with a DE of 1.
The Frame Relay Frame Format
The standard Frame Relay frame has five fields, described in the following list. In
addition to the standard frame, an LMI Frame Relay frame is used when the LMI
extensions are enabled. The standard Frame Relay frame is illustrated in Figure
5.16 and described in the following bulleted list.
Figure 5.16. The standard Frame Relay frame format.