Multisite Deployment with Distributed Call Processing

As illustrated in Figure 2-3, the model for a multisite WAN deployment with distributed call processing consists of multiple independent sites, each with its own CUCM cluster, connected to an IP WAN that carries voice traffic between the distributed sites.

CUCM, applications, and DSP resources may be located at each site. The IP WAN carries signaling traffic only for intersite calls; signaling traffic for calls within a site remains local to the site. This way, the amount of signaling traffic between sites is reduced compared to a Centralized Call Processing model.

With the use of gatekeepers, a Distributed Call Processing model can scale to hundreds of sites. It also provides transparent use of the PSTN in the event that the IP WAN is unavailable.

Unified CM Cluster

Unified CM Cluster

Network Diagram Multisite Call Manager
Unified CM Cluster

Unified CM Cluster

Unified CM Cluster

The Multisite with Distributed Call Processing model has the following design characteristics:

■ Maximum of 30,000 SCCP or SIP IP phones or SCCP video endpoints per cluster.

■ Maximum of 1100 MGCP gateways or H.323 devices (gateways, MCUs, trunks, and clients) per CUCM cluster.

Multisite Deployment with Distributed Call Processing 41

■ PSTN for all external calls.

■ DSP resources for conferencing, transcoding, and MTP.

■ Voice mail, unified messaging, and Cisco Unified Presence components.

■ Capability to integrate with legacy PBX and voice-mail systems.

■ H.323 clients, MCUs, and H.323/H.320 gateways that require a gatekeeper to place calls must register with a Cisco IOS Gatekeeper (Cisco IOS Release 12.3(8)T or later). CUCM then uses an H.323 trunk to integrate with the gatekeeper and provide callrouting and bandwidth management services for the H.323 devices registered to it. Multiple Cisco IOS Gatekeepers may be used to provide redundancy. Cisco IOS Gatekeepers may also be used to provide call-routing and bandwidth management between the distributed CUCM clusters. In most situations, Cisco recommends that each CUCM cluster have its own set of endpoint gatekeepers and that a separate set of gatekeepers be used to manage the intercluster calls. It is possible in some circumstances to use the same set of gatekeepers for both functions, depending on the size of the network, the complexity of the dial plan, and so forth.

■ MCU resources are required in each cluster for multipoint videoconferencing. Depending on conferencing requirements, these resources may be either SCCP or H.323 or both, and they may all be located at the regional sites or may be distributed to the remote sites of each cluster if local conferencing resources are required.

■ H.323/H.320 video gateways are needed to communicate with H.320 videoconferencing devices on the public ISDN network. All these gateways may be located at the regional sites, or they may be distributed to the remote sites of each cluster if local ISDN access is required.

■ High-bandwidth audio (for example, G.711, G.722, or Cisco Wideband Audio) between devices in the same site, but low-bandwidth audio (for example, G.729 or G.728) between devices in different sites.

■ High-bandwidth video (for example, 384 kb/s or greater) between devices in the same site, but low-bandwidth video (for example, 128 kb/s) between devices at different sites. The Cisco Unified Video Advantage wideband codec, operating at 7 Mb/s, is recommended only for calls between devices at the same site. Note that the Cisco VT Camera wideband video codec is not supported over intercluster trunks.


The Multisite WAN with Distributed Call Processing model provides the following benefits:

■ PSTN call cost savings when using the IP WAN for calls between sites.

■ Use of the IP WAN to bypass toll charges by routing calls through remote-site gateways, closer to the PSTN number dialed (TEHO).

■ Maximum utilization of available bandwidth by allowing voice traffic to share the IP WAN with other types of traffic.

■ No loss of functionality during IP WAN failure because there is a call-processing agent at each site.

■ Scalability to hundreds of sites.

■ Gatekeeper networks can scale to hundreds of sites, and the design is limited only by the WAN topology.

Best Practices

A multisite WAN deployment with distributed call processing has many of the same requirements as a single-site or a multisite WAN deployment with centralized call processing. Follow the best practices from these other models in addition to the ones listed here for the Distributed Call Processing model.

Among the key elements of this Multisite WAN model are the SIP or gatekeeper proxy servers. They each provide dial plan resolution, with the gatekeeper also providing CAC. A gatekeeper is an H.323 device that provides CAC and E.164 dial plan resolution.

The following best practices apply to the use of a gatekeeper:

■ Use a Cisco IOS Gatekeeper to provide CAC into and out of each site.

■ To provide high availability of the gatekeeper, use Hot Standby Router Protocol (HSRP) gatekeeper pairs, gatekeeper clustering, and alternate gatekeeper support. In addition, use multiple gatekeepers to provide redundancy within the network.

■ Size the platforms appropriately to ensure that performance and capacity requirements can be met.

■ Use only one type of audio codec on the WAN, because the H.323 specification does not allow for Layer 2, IP, User Data Protocol (UDP), or Real-Time Transport Protocol (RTP) header overhead in the bandwidth request. Using one type of codec on the WAN simplifies capacity planning by eliminating the need to overprovision the IP WAN to allow for the worst-case scenario.

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