Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 133)

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Cisco IP Telephony Network Design Guide

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Chapter 6 Multisite WAN with Distributed Call Processing

Call Admission Control

Voice calls between sites can use the IP WAN as the primary path and the PSTN

as the secondary path in the event the IP WAN is down or has insufficient

resources to handle additional calls. Whether calls use the IP WAN or the PSTN

can be transparent to both the calling party and the called party.

The primary advantage of this deployment model is that, by using local call

processing, it provides the same level of features and capabilities whether the IP

WAN is available or not. Each site can have from one to eight Cisco CallManager

servers in a cluster, based on the number of users. This is the predominant

deployment model for sites with greater than 50 users, and each site can support

up to 10,000 users. In addition, there is no loss of service if the IP WAN is down.

Call Admission Control

Call admission control is a mechanism for guarantying quality of service to a new

call while still providing quality of service to established calls by ensuring that

network resources are available on a call-by-call basis before the new call is

established.

In a converged network paradigm, all traffic types (voice, video, and data) travel

over a common IP-enabled infrastructure. Because of this mixture of traffic types,

the network must be able to handle the requirements of each individual traffic type

with respect to packet loss, latency, and jitter. In such an environment, two main

tasks gain importance:

• Prioritizing one traffic type over another

• Protecting real-time traffic such as voice or video from oversubscribing the

network bandwidth

The first task is effectively handled by quality of service (QoS), which is

discussed in Chapter 8, “Quality of Service.”

The second task is accomplished by call admission control mechanisms. The need

for call admission control in IP telephony networks is amplified greatly by the fact

that all IP phones have an open IP path to the WAN, whereas toll bypass networks,

in contrast, can limit the number of physical trunks eligible to initiate calls across

the WAN. Figure 6-2 illustrates why call admission control is needed.

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 133