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Old 18-03-2011, 05:36 PM
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M.Arsalan Qureshi

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QoS Mechanisms for Improving VoIP Quality

Implementing QoS mechanisms is another key consideration in order to ensure that Voip traffic is forwarded across a network in a timely manner. A variety of different queuing mechanisms can be used on WAN interfaces to help prioritize voice traffic in order to ensure that it is serviced in this manner, and not delayed by other traffic that is less time-sensitive. While the four main queuing techniques typically implemented on Cisco router serial interfaces were looked at earlier in this chapter, voice traffic is typically prioritized using one of the three queuing methods listed below.
  • Class-Based Weighted Fair Queuing (CBWFQ). Class-based WFQ works in a manner somewhat similar to traditional WFQ, with the exception that “classified” traffic can be placed into reserved bandwidth queues, ensuring that certain types of traffic (such as VoIP) are allocated a guaranteed amount of bandwidth. A scheduler services the queues based on the bandwidth assigned to them, also known as the “weight”. While CBWFQ ensures that all packets are allocated appropriate bandwidth based on their weight (and that all queues are serviced), it does not implement strict priority. In other words, this queuing method can still result in delays for VoIP traffic.
  • Low Latency Queuing (LLQ). The LLQ queuing method is strongly recommended as the queuing method for use on WAN links that need to support time-sensitive traffic like VoIP. While LLQ functions in a manner very similar to CBWFQ, it does implement one very important additional feature, namely a priority queue. The priority queue is allocated a defined amount of priority bandwidth (weight), and is always serviced first as long as it does not exceed this bandwidth. Other types of traffic can be assigned to reserved queues (or a default queue) with pre-defined weights, ensuring that they are not starved of bandwidth.
  • IP RTP Priority. The IP RTP Priority queuing method presents one of the simplest methods to ensure that VoIP packets are serviced with appropriate priority. When this queuing method is implemented, RTP voice packets (only) are automatically placed into a priority queue, while all other traffic is queued according to WFQ methods. IP RTP Priority can be implemented with a single command, which makes it an easy way to prioritize voice traffic, especially in environments where all other traffic can be handled equally. IP RTP priority does not become active until a WAN interface is experiencing congestion.
LLQ and IP RTP Priority are the two most popular queuing methods for prioritizing VoIP traffic.

In order for a queuing mechanism like LLQ or IP RTP Priority to queue voice packets into a priority queue correctly, they must be able to identify the traffic as Voip With IP RTP Priority, packets are matched and priority queued according to the UDP port numbers used by RTP voice traffic, which fall into the range 16384 to 32767 (even port numbers only) in Cisco implementations. Odd UTP port numbers in this range are used for call control information, and are not prioritized – they are serviced by the WFQ method like all other traffic.

With LLQ, VoIP traffic is typically determined based on either port numbers (through the use of access lists), or through traffic classification mechanisms. If you recall from Chapter 4, IP headers includes a field that can be used to designate a service “type”, also known as Type of Service (ToS) or IP Precedence. Based on the value configured in this field, network equipment like routers can be configured to grant certain types of traffic (like VoIP) a higher priority based on the queuing methods in use. For example, on a network that supports voice traffic, all voice packets could be tagged with an IP Precedence value of 5. Because this setting is configured in the IP header, it will stay with a packet all the way from the source to the destination, helping to ensure end-to-end QoS, again assuming an appropriate queuing mechanism that considers this information is implemented on all intermediary routing equipment.

LLQ would be the logical choice in such a scenario. On most networks, VoIP traffic has its IP Precedence value configured at the edge of the network, namely on an IP phone. In some cases, however, the phone might not have this ability, and IP Precedence settings might be added to the packet at the distribution layer according to configured policies.

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