Using QoS to Improve Voice Quality
Voice features that provide QoS are deployed at different points in the network and
designed for use with other QoS features to achieve specific goals, such as minimization
of jitter and delay.
Cisco IOS Software includes a complete set of features for delivering QoS throughout the
network. Although a complete survey of QoS features is beyond the scope of this book,
Cisco’s recommended QoS mechanism for VoIP queuing, in a router’s output interface, is
LLQ.
LLQ provides strict priority queuing (PQ) in conjunction with CBWFQ. LLQ configures
the priority status for a class within CBWFQ, in which voice packets receive priority over
all other traffic.
For example, consider Figure 2-5. Whereas web traffic receives at least 128 kbps of bandwidth
(if the web traffic needs that much bandwidth), voice traffic receives 256 kbps of
“priority” bandwidth (if the voice traffic needs that much bandwidth), meaning the voice
traffic is transmitted first, ahead of the web traffic. However, the voice traffic will not
starve out the other traffic types, because the voice traffic is also limited to consuming
no more than 256 kbps.
Chapter 2: Considering VoIP Design Elements 65
Web => Allocate 128 kbps of Bandwidth
Voice => Allocate 256 kbps of “Priority” Bandwidth
Figure 2-5 Low Latency Queuing Example