Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 137)

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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

to this solution. In this case, the dial plan is tightly coupled with the gatekeeper

call admission control mechanism because it is the dial plan that ultimately

decides when to place a call across the IP WAN and what to do if the gatekeeper

rejects the call. Dial plan issues are addressed in the “Dial Plan Considerations”

section on page 6-15.

As mentioned before, you can use an H.323 gatekeeper to achieve call admission

control by limiting the number of calls allowed in or out of specified zones. This

effectively limits the amount of bandwidth per site because each site can be

associated with a particular zone. This is the model that Cisco Call Manager uses

with a gatekeeper to perform call admission control in hub-and-spoke topologies.

In addition to call admission control, a second very important function performed

by the gatekeeper is address resolution. At any given site, Cisco CallManager

knows about the extension range it controls and is able to route calls to those

extensions. For anything outside its range, Cisco CallManager can go to a

gatekeeper, which returns the IP address of another Cisco CallManager to which

it should direct the call. This is possible because each Cisco CallManager (or one

Cisco CallManager from a cluster) registers with the gatekeeper that is statically

configured with the address range maintained by that particular cluster.

This address resolution feature greatly simplifies the dial plan in a multisite

distributed call processing environment. The “Dial Plan Considerations” section

on page 6-15 contains a more detailed discussion of address resolution.

In summary, the capabilities of Cisco CallManager for call admission control and

E.164 address resolution are:

• Support for up to 100 sites in a multisite distributed call processing

environment.

• Gatekeeper capability for address resolution of intercluster calls, which

results in a simplified dial plan.

• Cisco CallManager requests 128 kbps of bandwidth for G.711 calls and

20 kbps for G.729 calls.

• Compressed Real-time Transport Protocol (cRTP) is not factored into the

bandwidth calculations for the call Admission Request (ARQ).

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 137