2.3 Quality of Service

This topic describes how QoS can solve voice quality issues.

QoS allows certain communication flows to be processed with higher priority than others:

  1. Traffic is identified.
  2. Traffic is divided into classes.
  3. QoS policy is applied per class.

QoS allows voice and video to be prioritized over data.

The primary goal of QoS is to provide better service, including dedicated bandwidth, controlled jitter and latency (required by some real-time and interactive traffic), and improved loss characteristics, by giving priority to certain communication flows. It is also important to make sure that providing priority for one or more flows does not make other flows fail.

You can provide better service by either raising the priority of a flow or limiting the priority of another flow. Some QoS mechanisms are congestion management, congestion avoidance, and link efficiency.

When you implement QoS, the implementation is split into three major steps:

  • Traffic is identified (voice, video, signaling, data, and so on).
  • Traffic is divided into classes (real-time traffic, mission-critical traffic, less important traffic, and so on).
  • QoS policy is applied per-class and specifies how to serve each class.

QoS Advantages

QoS can improve the quality of voice and video calls by giving priority to RTP packets.

With QoS enabled, voice traffic has absolute priority over other traffic.

  • Prevents jitter caused by variable queuing delays
  • Ensures enough bandwidth for signaling
  • Prevents packet loss caused by tail drops in queues

With QoS enabled, voice traffic is given absolute priority queuing (“PQ” in the figure) over all other traffic. This approach prevents jitter, which is caused by variable queuing delays. It also prevents lost voice packets, which are caused by tail drops that occur when buffers are complete. To avoid the complete blocking of other traffic, you should limit voice bandwidth. The number of voice calls should also be limited by CAC so that there is not more voice traffic than there is bandwidth that has been reserved for it.

Finally, to ensure proper service for voice calls, you should configure QoS to guarantee a certain bandwidth for signaling traffic. Otherwise, despite the fact that the quality of active calls may be acceptable, calls cannot be torn down and new calls cannot be established.