Chapter 16: Providing Quality of Service

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Overview

A typical enterprise network topology has multiple local area networks (LANs) connected by bridges or Layer 2 switches. Many times the different LANs are all of one type, typically wired Ethernet, but that is changing. Wireless networks are entering these hallowed grounds. Furthermore, today's networks are converging into a unique network infrastructure that is expected to carry data in the form of voice, video, and mission critical applications. These applications require a certain amount of bandwidth in order to provide the end-user with a quality product, and that's where Quality of Service comes into play. The term "Quality of Service" (QoS) refers to the quality of network services. It is sort of an umbrella term for all the related technologies that are put in place with the objective of providing QoS, so as to supply better network services in order to satisfy network applications' bandwidth needs. QoS is not related to a specific technology or network topology, but transparently crosses all networks—IP, Frame Relay, ATM, Ethernet, cellular, and Wi-Fi.

With the help of "Quality of Service" or "QoS" technologies, networks (and network managers) can give priority to certain data, users, and/or applications. This is typically accomplished via bandwidth management techniques that allow specific types of application to have specified bandwidth and delivery requirements satisfied. In a non-QoS-enabled network all data packets generally receive the same "best effort" service.

When QoS mechanisms are in place, an overburdened WLAN can be optimized so it can sufficiently service mission-critical applications, and queue less important data for transmission as network conditions allow. QoS allows an unresponsive, overburdened, or oversubscribed network resource to be sufficiently responsive so as to handle any necessary prioritized network traffic.

QoS techniques operate by providing not only the necessary bandwidth resources, but also control of latency and jitter (which is required for interactive traffic), while improving a network's data loss characteristics. QoS technologies achieve these objectives by providing a large assortment of tools and protocols for managing and avoiding network congestion, applying bandwidth reservation, and instituting traffic shaping and policing techniques. For example:

• Enhanced queuing tools can differentiate incoming traffic in multiple queues to prioritize interactive applications (e.g. video packets) to provide the end-user with a quality video stream.

• QoS tools can manage and even avoid congestion by slowing bulk traffic applications such as large file transfers.

• QoS mechanisms can shape and police data traffic in order to provide a specific level of service between the data source and its end-users.

• Bandwidth reservation techniques that cross the network can provide a guaranteed end-to-end service for constant bit rate services like voice and video.

Historically, much of the data that networks move around has related to business applications where strict QoS measures are unnecessary, since these applications aren't materially affected by packet arrival latencies. But now, whether the network landscape consists solely of wired LANs, is wireless, or a mix of both, networks deliver a variety of applications, some of which are quite sensitive to QoS. So while delivering text and other relatively simple types of data around a network (wired or wireless) doesn't necessarily require complex Quality of Service mechanisms, network managers find that more and more they must deal with QoS issues, since many of today's applications differ in their network transmission requirements. Thus network managers are seeking quality of service technologies that offer guaranteed bandwidth while maintaining specific link quality, delay, and jitter parameters.

Note, however, that when considering Quality of Service in a networking environment, you also must allow for the human element. That is because, at a fundamental level, QoS services are based on human perceptual considerations.

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