Digi TransPort WR31 User Manual page 509

Configure Quality of Service (QoS)
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_length = (old_length * (1-1/2^n)) + (current_length * 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.
Digi TransPort WR Routers User Guide
Queue Profiles parameters
509