3Com 3032 Configuration Manual page 263

Configure Frame Relay QoS 259
Figure 82 Frame Relay traffic shaping
Data flow direction
128 kbps
64 kbps
S0 S0
Frame-relay network
Router B Router A
DTE DTE
CIR ALLOW = 64kbps
CIR = 32kbps
If the Frame Relay traffic shaping is applied on the outgoing interface Serial 0 on
Router B, the interface will be able to transmit packets at 64 kbps, a relatively
average rate, so as to avoid the network congestion. Even if the congestion occurs
to the network, Router B can still transmit packets at 32 kbps.
Frame Relay traffic shaping is applied on the outgoing interface on a router. It can
provide for the users the parameters like Committed Information Rate Allowed
(CIR ALLOW), Committed Information Rate (CIR), Committed Burst Size (CBS) and
Excess Burst Size (EBS).
When the network is in normal, the Frame Relay PVCs can transmit packets at the
rate of CIR ALLOW. In this case, the packet traffic transmitted at an interval of Tc is
CBS. Furthermore, the Frame Relay traffic shaping allows the PVCs to transmit
packets at a rate exceeding CIR ALLOW in case of the burst, and the traffic
exceeding the CBS can be EBS at maximum.
When the network congestion occurs, if the Frame Relay switch device has been
configured with the function of congestion management, it will notify the router
of network congestion. Upon receiving the notification, the router will eventually
slow down the transmit rate to the CIR, so as to ease the congestion, then users
can transmit data at the rate of CIR. After this, if no notifications of network
congestion are received within a certain period of time, the router will eventually
raise the transmit rate from the CIR back to the CIR ALLOW.
Figure 83 Fundamentals of Frame Relay traffic shaping
R ATE
C I R ALLO W +EI R £ º 128 Kbps
C I R ALLO W £ º 64 kbps
C I R £ º 32 kbps
TI M E
0s 1s 2s 3s 4s 5s 6s
As shown in Figure 83, the parameters of Frame Relay traffic shaping are
respectively set to be: CIR ALLOW= 64 kbps, CIR = 32 kbps, CBS = 64000 bit, EBS
= 64000 bit, and interval Tc = CBS / CIR ALLOW = 1s. Within the first interval Tc,
the PVC-transmitting burst traffic size equals to CBS+EBS. Beginning from the
second Tc, the transmitted traffic size within each interval Tc becomes CBS. At the
3s, the router receives the Frame Relay packet whose backward explicit congestion