QoS: What Is It? Why Do We Need It?

Abstract

Quality of Service (QoS) is the ability to treat packets differently as they transit a network device, based on the packet contents. Without QoS, all packets on the network vie for the same pool of resources, and when congestion occurs, any packet can be dropped without regard as to what the packet contains. QoS can be used in converged networks to provide voice packets priority access to resources, or it can be used to differentiate data packets from different application stream and provide access to resources according to policy.

Learn more about Quality of Service policies and why your organization should be implementing them.

Sample

Introduction

"My applications are sometimes slow."

"My video stream is jerky and the voice is out of sync."

"My voice calls sound bad at times."

Are these familiar complaints? Do your users face intermittent application problems that are hard to pin down?

This is an issue that many companies face at one time or another. It is also an issue that many companies are dealing with through the implementation of Quality of Service policies.

The Drivers Behind QoS

There are two primary drivers behind the need for QoS policies.

Convergence of Voice and Data Networks
Differentiation between Data Applications

The most common driver for QoS implementation is the convergence of voice and data onto an existing data network. The corporate PBX has reached end-of-life, and the decision has been made to replace it with a Voice over IP (VoIP) telephony service.

The mistake many companies make is to perform a very basic network analysis (if any at all) and add voice to the network without proper planning. Once the VoIP service is up and running, users call in to the helpdesk if service is not up to expected standards, and troubleshooting begins.

Without QoS, all packets on the network vie for the same pool of resources and when congestion occurs, any packet can be dropped without regard as to what the packet contains.

QoS may be implemented for differentiation of data applications only, even where there is no VoIP present. Some companies require allocation of bandwidth for critical applications.

If the company is a Service Provider, they will want to allocate bandwidth per customer and per customer class based on Service Level Agreements signed with that customer.

QoS policies can be implemented to provide resources based on a specific application.

What Is QoS?

QoS is the ability to treat packets differently as they transit a network device, based on the packet contents.

QoS configuration performs different tasks based on the direction of traffic flow and location of the device performing the QoS functionality.

At the access layer, where the ip packet first enters the network, QoS policies classify and mark each packet. This type of policy is applied in the inbound direction on an access layer interface.

Once a packet has been marked on the inbound path, the marking can be used on the outbound path to give each packet access to the appropriate amount of resources.

As the packet travels through the network, each device simply applies policies based on existing markings and does not need to do an in-depth analysis of the content of each packet.

Why Do We Need QoS?

Without QoS policies, each packet is given equal access to resources. If we cannot tell a voice packet from a data packet, we cannot give voice priority.

In order for a company to efficiently utilize its network resources, it must identify which network traffic is critical traffic and allocate appropriate resources to support those traffic streams. If voice is present in the network, it must get priority over all data streams; otherwise, the result could be intermittent voice quality complaints. Voice and video applications are delay- and jitter-sensitive. A good QoS policy will give the voice packet priority access to the interface queue.

For example, a voice packet and an FTP packet both arrive at an outbound router interface at the same time. Without QoS present, the voice packet may need to wait in a queue until the FTP packet has been processed out the interface. This may inject an unacceptable amount of delay into the voice path (depending on the interface speed).

With QoS configuration, the voice packet could be given priority over the FTP packet and be processed first.

If the interface is of a speed less than T1, the FTP packet may be fragmented to ensure that the voice packet does not have an excessive delay.

Another example will illustrate how QoS is used for application differentiation. In this example, a company relies on customer access to its web server so that customers can place orders from the on-line catalog. While on break, a staff member decides to watch a video trailer on an Internet movie web site, which requires significant bandwidth resources. Without QoS, a customer may experience delays in reaching the online catalog because of congestion caused by the video download. With QoS configuration, priority could be given to http packets that are sent to and from the corporate web server, while other web packets could be given lower priority. This would maintain a high level of resources for customer access while still providing some bandwidth to staff web traffic.