Specifications

Sample Scenarios 67
discarded. The DE bit can be set by the router to indicate that some traffic should
be considered discard eligible. If appropriate, the frame relay network will discard
frames marked as discard eligible which may allow frames that are not marked
discard eligible to make it through the network.
When congestion occurs, the FR backbone network is responsible for notifying
the sender and receiver by sending out a FECN or a BECN signal. FECN and
BECN are bits that are set in a frame to notify the DTEs at each end of a VC that
congestion is occurring. FECN indicates that congestion is occurring in the same
direction from which the frame was received. BECN indicates that the frames sent
by this DTE are causing network congestion.
The above diagram shows that there is congestion between switch B and C. The
FR network would notify the sender that the network was congested by setting
the BECN bit. The FR network would also notify the receiver that there was
congestion in the network by setting the FECN bit.
Figure 36. Frame Relay BECN
4.1.6 Tc Value Consideration for VoFR
This is the sample problem where Tc=1 sec = (Bc:16000/CIR:16000), CIR=16
kbps, Bc=16000 bits, Be=0, and Line speed=64 kbps. Data like TCP/IP with Tc=1
sec usually works fine without a problem, because data was not generated in very
short intervals and data will burst up to 6400 bits anytime during that 1-second
period. Data can burst up to 64000 bits during 250 msec. But voice frame
generates very short intervals like 45 mse (for example, Frame packing =3) and a
lot of voice frames cannot be sent and discarded with Tc=1sec.The Problem of
Tc=1sec.
IBM 2212
BECN = ON FECN = ON
IBM 2212
Congestion