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Chapter 1 IP Telephony Deployment Models

Multi-Site WAN with Centralized Call Processing

Note

It is possible to use other WAN technologies that lack some of the QoS features required for converged
voice and data traffic, but these technologies have special design considerations that are beyond the
scope of this document. In addition, those other technologies usually do not maintain good voice quality
due to their lack of QoS features.

Best Practices for the Multi-Site Model with Centralized Call Processing

Follow these guidelines and best practices when implementing the multi-site WAN model with
centralized call processing:

Minimize delay between Cisco CallManager and remote locations to reduce voice cut-through
delays (also known as clipping).

For hub-and-spoke topologies, use the locations mechanism in Cisco CallManager for call
admission control into and out of remote branches. If the WAN uses Cisco IOS Multiprotocol Label
Switching (MPLS), see the section on

Multi-Site MPLS WAN Considerations, page 1-20

The locations mechanism works across multiple servers in Cisco CallManager Release 3.1 and later.
This configuration can support a maximum of 30,000 IP phones when Cisco CallManager runs on
the largest supported server.

The number of IP phones and line appearances supported in Survivable Remote Site Telephony
(SRST) mode at each remote site depends on the branch router platform, the amount of memory
installed, and the Cisco IOS release. (For the latest SRST platform and code specifications, refer to
the SRST documentation at Cisco.com.) Generally speaking, however, the choice of whether to
adopt a centralized call processing or distributed call processing approach for a given site depends
on a number of factors such as:

IP WAN bandwidth or delay limitations

Criticality of the voice network

Feature set needs

Scalability

Ease of management

Cost

If a distributed call processing model is deemed more suitable for the customer's business needs, the
choices include installing a local Cisco CallManager server or running the Cisco IOS Telephony
Service (ITS) on the branch router.

Call Admission Control for Centralized Call Processing

Multi-site deployments require some form of call admission control to ensure the voice quality of calls
transmitted across network links that have limited available bandwidth. Cisco CallManager provides a
simple mechanism know as locations for implementing call admission control in multi-site WAN
deployments with centralized call processing. Follow these guidelines when using locations for call
admission control:

Locations require a hub-and-spoke network topology.

Configure a separate location in Cisco CallManager for each site.