ENTERPRISE NETWORKS II: WIDE AREA NETWORKS
Typical bursty traffic of a frame relay circuit. Note the traffic levels indicated.
1. CIR (committed information rate) is the data rate subscribed to by the user. This
rate may be exceed for short bursts during peak period(s) as shown in Figure 12.9b.
2. We can just pay a flat rate.
3. We can pay per packet (i.e., frame).
Now turn to Figure 12.9b. Note that on the right-hand side of the figure there is the
guaranteed transmission bit rate equivalent to the CIR. Depending on the traffic load
and congestion, during short periods the user may exceed the CIR. However, there is a
point where the network cannot sustain further increases in traffic without severe conges-
tion resulting. Traffic above such levels is arbitrarily discarded by the network without
informing the originator.
Congestion Control: A Discussion
Congestion in the user plane occurs when traffic arriving at a resource exceeds the net-
work's capacity. It can also occur for other reasons such as equipment failure. Network
congestion affects the throughput, delay, and frame loss experienced by the end-user.
End-users should reduce their offered load in the face of network congestion. Reduction
of offered load by an end-user may well result in an increase in the effective throughput
available to the end-user during congestion.
Congestion avoidance procedures, including optional explicit congestion notification,
are used at the onset of congestion to minimize its negative effects on the network and its
users. Explicit notification is a procedure used for congestion avoidance and is part of the
data-transfer phase. Users should react to explicit congestion notification (i.e., optional but
highly desirable). Users who are not able to act on explicit congestion notification shall
have the capability to receive and ignore explicit notification generated by the networks.
Congestion recovery and the associated implicit congestion indication due to frame
discard are used to prevent network collapse in the face of severe congestion. Implicit
congestion detection involves certain events available to the protocols operating above
the core function to detect frame loss (e.g., receipt of a REJECT frame, timer recovery).
Upon detection of congestion, the user reduces the offered load to the network. Use of
such reduction by users is optional.
Network Response to Congestion
. Explicit congestion signals are sent in
both the forward direction (toward the frame destination) and in the backwards direction