Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 62)

Chapter 2 Campus Infrastructure Considerations

Quality of Service

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Cisco IP Telephony Network Design Guide

78-11103-03

Quality of Service

In a converged environment, all types of traffic travel over a single transport

infrastructure. Yet all traffic types are not the same. Data is bursty, loss intolerant,

and not latency sensitive. Voice, on the other hand, is nonbursty and has some

tolerance to loss but is latency sensitive. The challenge is in providing the

required level of service for each of these traffic types.

Running both voice and data on a common network requires the proper quality of

service (QoS) tools to ensure that the delay and loss parameters of voice traffic

are satisfied. These tools are available as features in IP phones, switches, and

routers.

See Chapter 8, “Quality of Service,” for information on WAN QoS.

Traffic Classification Types

The goal of protecting voice traffic from being run over by data traffic is

accomplished by classifying voice traffic as high priority and then allowing it to

travel in the network before low priority traffic. Classification can be done at

Layer 2 or at Layer 3 as follows:

• At Layer 2 using the three bits in the 802.1p field (referred to as class of

service, or CoS), which is part of the 802.1Q tag.

• At Layer 3 using the three bits of the differentiated services code point

(DSCP) field in the type of service (ToS) byte of the IP header.

Classification is the first step toward achieving quality of service. Ideally, this step

should be done as close to the source as possible, usually at the access layer of the

network.

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 62