Switching to VoIP

This is the Title of the Book, eMatter Edition

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

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information. Many vendors call ToS IP precedence. 802.1p and ToS tend to be used

together when TCP/IP is the networking protocol.

If your VoIP network will be more than 70% data-to-voice and

unlikely to reach capacity, packet prioritization techniques like LAN-

oriented 802.1p and its WAN cousin DiffServ are adequate.

802.1p is a standard that benefits a single data link as opposed to a group of them

using a single CoS policy. So it tends to be a feature of Ethernet switches, as opposed

to IP routers, which instead support ToS. Since 3 bits are allocated for classification,

802.1p allows for eight classes of service. The names associated with these classes are

arbitrary to the network they are used on, but Table 9-4 lists the suggested, generic

names.

The eight classes’ individual per-hop behaviors are administrator-defined, so you

could define class 5 for voice traffic (which is the prevailing wisdom). This would

give voice traffic higher precedence on the local data link. Each hop along the path of

the traffic would have to support the same per-hop precedence behavior in order to

make the QoS policy consistent across a routed WAN.

Like many low-layer standards, 802.1p is a product of the IEEE.

Project 9.1, later in the chapter, describes how to check all the routers

along a certain ...