Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

196

|

Chapter 9: Quality of Service

There’s also an aggressive suppressor algorithm available—but you’ll need to uncom-

ment this header in /usr/src/zaptel/zconfig.h and recompile the Zaptel driver module

in order to use it:

#define AGGRESSIVE_SUPPRESSOR

CoS

CoS, or Class of Service, systems work to prioritize traffic on a single data link.

While QoS refers to the greater network, CoS refers to only a single data link. So, an

Ethernet switch might provide a CoS packet prioritization to/from a single host, but

a group of routers might participate in a more elaborate, end-to-end QoS solution.

The key difference is this: CoS is a single-link approach, while QoS is an end-to-end

approach.

Class of Service systems define per-hop behavior, so they cannot guarantee a service

level in terms of capacity or speed. Instead they give their “best effort” to deliver pri-

ority traffic according to those per-hop behaviors, which are established by you, the

administrator. Class of Service solutions are great on data links where less than 30%

of the traffic is voice, which is probably a majority of enterprise networks today. But

they tend not to be as effective when priority traffic like voice represents the lion’s

share of the packets on the data link—as is often the case when the data link is a

slow one. ...