Avaya 555-245-600 manual Cb-Wfq / Llq / Cbq

Queuing methods

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.

Issue 6 January 2008 325