Configuring Quality of Service (QoS) Configure Quality of Service (QoS)
Digi TransPort® Routers User Guide
573
WRED Maximum Threshold
The maximum queue length threshold for using the WRED algorithm to drop packets. Once the
queue length exceeds this value, the WRED algorithm causes all packets to be dropped.
WRED Maximum Drop Probability (%)
The maximum percentage probability the WRED algorithm uses to determine whether or not a
packet should be dropped when the queue length is approaching the WRED maximum
threshold value.
Note If the length of a queue is less than the WRED Minimum Threshold value, there is a zero-
percent chance that a packet will be dropped. When the queue length is between the WRED
minimum and maximum values, the percent probability of a packet being dropped increases
linearly up to the WRED maximum drop probability.
WRED Queue Length Weight factor
A weighting factor the WRED algorithm uses when calculating the weighted queue length. The
weighted queue length is based on the previous queue length and has a weighting factor that
may be adjusted to provide different transmit characteristics. The actual formula is:
new_l engt h = ( ol d_l engt h * ( 1- 1/ 2^n) ) + ( cur r ent _l engt h * 1/ 2^n)
Small weighting factor values result in a weighted queue length that moves quickly and more
closely matches the actual queue length. Larger weighting factor values result in a queue
length that adjusts more slowly. If a weighted queue length moves too quickly (small weighting
factor), it can result in dropped packets if the transmit rate rises quickly, but will also recover
quickly after the transmit rate tails off. If a weighted queue length moves too slowly (large
weighting factor), it allows a burst of traffic through without dropping packets, but may result
in dropped packets for some time after the actual transmit rate drops off. Therefore, use care
in selecting the weighting factor to suit the type of traffic using the queue.
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