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Queuing methods
Issue 6 January 2008 325
Round-robin queuing is not particularly suited to IP Telephony. It does not ensure strict enough
priority to voice packets, so they may still wait behind other traffic flows in other queues. Latency
and jitter can be at unacceptable levels.
CB-WFQ / LLQ / CBQ
Class-Based Weighted Fair Queuing (CB-WFQ) with Low-Latency Queuing (LLQ), which is
sometimes called Class-Based Queuing (CBQ), combines the above-mentioned queuing
mechanisms. Generally, there is one strict-priority queue, several round-robin queues, and
weighted fair queuing for the remainder. This queuing mechanism works very well for
converged networks. IP Telephony bearer and signaling packets receive the priority they need,
while there remains an equitable mechanism for distributing remaining bandwidth. In addition,
limits can be set on the high-priority queue to prevent it from using more than a specified
amount of bandwidth. Bandwidth that is reserved for the high-priority queue will be given to
other queues if insufficient traffic enters the high-priority queue.
RED / WRED
Although they are not queuing methods per se, Random Early Detection (RED) and Weighted
Random Early Detection (WRED) are important queue management techniques. RED and
WRED work by randomly discarding packets from a queue. RED takes advantage of the
congestion control mechanism of TCP. By randomly dropping packets prior to periods of high
congestion, RED causes the packet source to decrease its transmission rate. Assuming that the
packet source is using TCP, it will decrease its transmission rate until all the packets reach their
destination, which indicates that the congestion is cleared. Some implementations of RED,
called Weighted Random Early Detection (WRED), combines the capabilities of the RED
algorithm with IP Precedence. This combination provides for preferential traffic handling for
higher-priority packets. It can selectively discard lower-priority traffic when the interface begins
to get congested, and provide differentiated performance characteristics for different classes of
service.
RED and WRED are useful tools for managing “data” traffic, but should not be used for “voice.”
Because IP Telephony traffic runs over UDP, because IP Telephony protocols do not retransmit
lost packets, and because IP Telephony transmits at a constant rate, the IP Telephony queue
should never be configured for WRED. WRED only adds unnecessary packet loss, and
consequently reduces voice quality.