Riverstone Networks WICT1-12 - Manual (Page 623)

Riverstone Networks RS Switch Router User Guide Release 8.0 30-19
WAN Configuration WAN Rate Shaping
Next, the number of bits from each rate-shaped Ethernet flow is measured as they pass through the WAN port. These
measurements are taken during equal sampling intervals (Tc), which are some fraction of one second. During a
sampling interval, if the number of bits from a flow exceeds a pre-set value, called the Committed Burst Size (Bc), the
rate shaping algorithm stops the Ethernet flow from sending packets directly through the WAN port. Instead, packets
are sent to a queue, and the queue is emptied at a rate that does not exceed Bc. Because Bc is never exceeded, at the
end of one second, the rate shaped Ethernet flow transmits a number of bits less than or equal to CIR.
Note Queue depth is not unlimited. Each queue for each flow can buffer a maximum of
256 packets.
If the traffic through the WAN port is bursty and there are periods when WAN bandwidth is not 100 percent utilized,
the rate shaping algorithm can allow each rate shaped Ethernet flow to exceed its Bc by a specified amount called the
Excess Burst Size (Be). As with Bc, if during a sampling interval, a particular flow exceeds its allocated Be, packets
are buffered until the bit rate for the flow falls to Bc + Be. However, note that Be is not guaranteed because the amount
of excess bandwidth typically changes over time. Because of this dynamic nature of excess bandwidth, queuing also
can occur if the amount of excess WAN bandwidth falls below Bc + Be.
Figure 30-1 demonstrates how the parameters CIR, Bc, Be, and the sampling rate (Tc) interact to perform rate shaping.
Note that in the example, Bc and Be are set to some number of bits and Tc is equal to 1/16 of a second.
Figure 30-1 WAN rate shaping example
The following numbered list corresponds to the numbered points in Figure 30-1 :
1. The number of bits sent through the WAN port during the sampling period Tc is less than Bc.
TIME
BITS
1
2
4 5
6
7
3
Bc + Be
Committed Burst Size (Bc)
Sampling
interval Tc1
Sampling
interval Tc16
(Be) Excess Burst Size