Cisco Catalyst QoS: Quality of Service in Campus Networks

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Quality of Service: An Overview

The constantly changing needs of networks have created a demand for sensitive applica-

tions (such as voice over IP (VoIP) and video conferencing over IP), and networks are being

asked to support increasingly mission-critical data traffic. Providing predictable service

levels for all of these different types of traffic has become an important task for network

administrators. Being able to provide predictable and differentiated service levels is key to

ensuring that all application traffic receives the treatment that it requires to function properly.

This chapter covers several aspects of quality of service (QoS) and discusses how to provide

QoS in Cisco networks. Specifically, this chapter covers the following topics:

• Understanding QoS

• Deploying QoS in the WAN/LAN: High-Level Overview

• Cisco AVVID (Architecture for Voice, Video, and Integrated Data)

• Overview of Integrated Services and Differentiated Services

• Differentiated Services: A Standards Approach

Understanding QoS

The following section defines QoS in terms of measurable characteristics. It is important,

however, to recognize that fully understanding QoS requires more than a definition. To truly

understand QoS, you must understand the concept of managed unfairness, the necessity for

predictability, and the goals of QoS. In addition to a definition of QoS in measurable terms,

the following section explains each of these things, to provide you with a well-rounded and

practical definition of QoS.

Definition of QoS

QoS is defined in several ways, and the combination of all of these definitions is really the

best definition of all. A technical definition is that QoS is a set of techniques to manage

bandwidth, delay, jitter, and packets loss for flows in a network. The purpose of every QoS

mechanism is to influence at least one of these four characteristics and, in some cases, all

four of these

6 Chapter 1: Quality of Service: An Overview

Bandwidth

Bandwidth itself is defined as the rated throughput capacity of a given network medium or

protocol. In the case of QoS, bandwidth more specifically means the allocation of

bandwidth, because QoS does not have the capability to influence the actual capacity of any

given link. That is to say that no QoS mechanism actually creates additional bandwidth,

rather QoS mechanisms enable the administrator to more efficiently utilize the existing

bandwidth. Bandwidth is sometimes also referred to as throughput.

Delay

Delay has several possible meanings, but when discussing QoS, processing delay is the

time between when a device receives a frame and when that frame is forwarded out of the

destination port, serialization delay is the time that it take to actually transmit a packet or

frame, and end-to-end delay is the total delay that a packet experiences from source to destination.

Jitter

Jitter is the difference between interpacket arrival and departure—that is, the variation in

delay from one packet to another.

Packet Loss

Packet loss is just losing packets along the forwarding path. Packet loss results from many

causes, such as buffer congestion, line errors, or even QoS mechanisms that intentionally

drop packets.

Table 1-1 shows examples of the varying requirements of common applications for

bandwidth, delay, jitter, and packet loss.

Managed Unfairness

Another, more pragmatic definition of QoS is managed unfairness. The best way to explain

this definition is using an analogy to airline service. Sometimes airlines are unable to sell

all of their seats in first class, but they rarely leave the gate with first class seats available,

Table 1-1 Traffic Requirements of Common Applications

Voice FTP Telnet

Bandwidth Required Low to moderate Moderate to High Low

Drop Sensitive Low Low Moderate

Delay Sensitive High Low Moderate

Jitter High Low Moderate