Specifications

Introduction April 2014

Quality of Service (QoS)

Real-time communication traffic is very sensitive to delay and drop.The network must ensure that this type of

traffic is handled with priority so that the stream of audio or video is not interrupted. QoS is the technology that

answers this need.

QoS allows an organization to define different traffic types and to create more deterministic handling for real-

time traffic. QoS is especially useful in congestion handling where a full communications channel might prevent

voice or video streams from being intelligible at the receiving side. Congestion is common when links are

oversubscribed while aggregating traffic from a number of devices and when the link bandwidth to a device is

steps down from a higher bandwidth uplink causing a delay in delivery of some of the traffic in transit. Rather

than creating bandwidth, QoS takes bandwidth from one class (that is, generally the default traffic class) and

gives it to another class during periods of congestion.

Within this design, the QoS profiles are as simple as possible while ensuring support for applications that need

special delivery. This approach establishes a solid, scalable, and modular framework to implement QoS across

the entire network.

The primary goals of implementing QoS within the network are:

• Expedited delivery service of communications for supported, real-time applications.

• Business continuance for business-critical applications.

• Fairness among all other applications when congestion occurs.

• Deprioritized background applications and non-business entertainment-oriented applications so that

these do not delay interactive or business-critical applications.

• A trusted edge around the network to guarantee that users cannot inject their own arbitrary priority

values and to allow the organization to trust marked traffic throughout the network.

To accomplish these goals, the design implements QoS across the network as follows:

• Establish a limited number of traffic classes (that is, typically one to eight classes) within the network that

need special handling (for example, real-time voice, real-time video, high-priority data, interactive traffic,

batch traffic, and default classes).

• Classify applications into the traffic classes.

• Apply special handling to the traffic classes to achieve intended network behavior.

In this design, QoS configurations are as simple as possible, and are applied only to those applications that

require special handling.

This approach establishes a solid, scalable, and modular framework to implement QoS across the entire network.