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Chapter 6 Multisite WAN with Distributed Call Processing

Call Admission Control

6-4

Cisco IP Telephony Network Design Guide

78-11103-03

Figure 6-2 Why Call Admission Control is Needed

For distributed call processing systems, you can implement call admission control

with an H.323 gatekeeper. In this design, Cisco CallManager registers with the

Cisco IOS gatekeeper, also known as Multimedia Conference Manager (MCM),

as a Voice over IP (VoIP) gateway and queries it each time it wants to make an IP

WAN call. The Cisco IOS gatekeeper associates each Cisco CallManager with a

zone that has specific bandwidth limitations. Thus the Cisco IOS gatekeeper can

limit the maximum amount of bandwidth consumed by IP WAN voice calls in or

out of a zone.

In brief, when Cisco CallManager wants to place an IP WAN call, it first requests

permission from the gatekeeper. If the gatekeeper grants permission, the call is

placed across the IP WAN. If the gatekeeper denies the request,

Cisco CallManager places the call across the secondary path, the PSTN.

This is effectively a call accounting method of providing admission control, in

which the gatekeeper simply keeps track of the bandwidth the IP WAN calls

consume. The maximum bandwidth setting for a zone should take into account the

limitation that voice traffic should not consume more than 75% of the WAN link.

Figure 6-3 illustrates the process used by this call admission control mechanism.

Note In this scheme, IP phones are not mobile between sites. Should an IP

phone register across the WAN, call admission control would not

operate as designed.

WAN bandwidth can only support 2 calls.

What happens when 3rd call attempted?

causes poor quality for all calls

VoIP data

network

Call #1

Call #2

Call #3

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 134