Добавил:
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз: Предмет: Файл:
DQOS Exam Certification Guide - Cisco press.pdf
Скачиваний:
68
Добавлен:
24.05.2014
Размер:
12.7 Mб
Скачать

QoS: Tuning Bandwidth, Delay, Jitter, and Loss 7

In networking, QoS describes a large array of concepts and tools that can be used to affect the packet’s access to some service. Most of us think of queuing features when we think of QoS— reordering the output queue so that one packet gets better service than another. But many other QoS features affect the quality—compression, drop policy, shaping, policing, and signaling, to name a few. In the end, whichever mechanism you use, you improve the behavior for one type of packet over another. Just like at the bank, implementing QoS is “managed fairness,” and at the same time it is “managed unfairness”—you purposefully choose to favor one packet over another.

All of us can relate to the frustration of waiting in lines (queues) for things in our daily lives. It would be great if there were never any people in line ahead of us at the tollbooths, or waiting to get on a ride at Disneyland (or any other place). For that to be possible, however, there would need to be a lot more tollbooths, Disneyland would need to be 20 times larger, and banks would need to hire a lot more tellers. Even so, adding more capacity would not always solve the prob- lem—the tollbooth would still be crowded at rush hour, Disneyland would still be crowded when schools are not in session, and banks would still be crowded on Friday afternoons when everyone is trying to cash his or her weekly paycheck (at least where I live!). Making Disneyland 20 times larger, so that there are no queues, is financially ridiculous—likewise, the addition of 20 times more bandwidth to an existing link is probably also financially unreasonable. After all, you can afford only so much capacity, or bandwidth in the case of networking.

This chapter begins by taking a close look at the four traffic characteristics that QoS tools can affect:

Bandwidth

Delay

Jitter

Packet loss

Whereas QoS tools improve these characteristics for some flows, the same tools might degrade service for other flows. Therefore, before you can intelligently decide to reduce one packet’s delay by increasing another packet’s delay, you should understand what each type of application needs. The second part of this “Foundation Topics” section examines voice, video, and data flows in light of their needs for bandwidth, delay, jitter, and loss.

QoS: Tuning Bandwidth, Delay, Jitter, and Loss

Different types of end-user traffic require different performance characteristics on a network. A file-transfer application might just need throughput, but the delay a single packet experiences might not matter. Interactive applications might need consistent response time. Voice calls need low, consistent delay, and video conferencing needs low, consistent delay as well as high throughput.

Users might legitimately complain about the performance of their applications, and the performance issues may be related to the network. Of course, most end users will believe the network

8 Chapter 1: QoS Overview

is responsible for performance problems, whether it is or not! Reasonable complaints include the following:

My application is slow.

My file takes too long to transfer now.

The video freezes.

The phone call has so much delay we keep talking at the same time, not knowing whether the other person has paused.

I keep losing calls.

In some cases, the root problem can be removed, or at least its impact lessened, by implementing QoS features.

So, how do voice, video, and data traffic behave in networks that do not use QoS? Well, certainly the performance varies. Table 1-2 outlines some of the behaviors in a network without QoS.

Table 1-2 Traffic Behavior with No QoS

Type of Traffic

Behavior Without QoS

 

 

Voice

Voice is hard to understand.

 

 

 

Voice breaks up, sounds choppy.

 

 

 

Delays make interacting difficult; callers do not know when other party has finished

 

talking.

 

 

 

Calls are disconnected.

 

 

Video

Picture displays erratically; jerky movements.

 

 

 

Audio not in sync with video.

 

 

 

Movement slows down.

 

 

Data

Data arrives after it is no longer useful.

 

 

 

Customer waiting for customer care agent, who waits for a screen to display.

 

 

 

Erratic response times frustrate users, who may give up or try later.

 

 

QoS attempts to solve network traffic performance issues, although QoS is not a cure-all. To improve network performance, QoS features affect a network by manipulating the following network characteristics:

Bandwidth

Delay

Jitter (delay variation)

Packet loss