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Problem: Random Early Detection (RED)

RED is a technique used in routers to apply congestion control. When the queue in the router starts to fill then a small proportion of packets are discarded. This is intended to trigger TCP sources to reduce their window sizes and hence throttle back the data rate. This can cause low rates of packet loss in Voice over IP streams. There have been reported incidences in which a series of routers apply RED at the same time, resulting in bursts of packet loss.


Packet loss due to RED can cause intermittent or periodic call quality problems, including "pops".


Check the packet loss rate and discard rate or jitter level at the receiving VoIP end system. If there is a significant rate of bursty packet loss associated with a high level of jitter then it is possible that the loss could be due to RED. If you are able to examine a trace on an IP network analyzer then you can check to see if the loss occurs at regular intervals - if so then RED is a likely suspect.


Network Analyzer, Router statistics, VQmon

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