Multisite WAN with Centralized Call Processing

The Multisite WAN with Centralized Call Processing model consists of a centralized CUCM cluster that provides services for many sites and uses the IP WAN to transport IP telephony traffic between the sites.

The IP WAN also carries call-control signaling between the CUCM cluster at the central site and the IP phones at the remote sites.

Figure 2-2 illustrates a typical centralized call-processing deployment, with a CUCM cluster at the central site and an IP WAN with QoS enabled to connect all the sites. The remote sites rely on the centralized CUCM cluster to handle their call processing. In addition, applications such as voice mail and interactive voice response (IVR) systems are typically centralized to reduce the overall costs of administration and maintenance.

The Cisco Unified Survivable Remote Site Telephony (SRST) feature available in Cisco IOS gateways provides call-processing services to remote IP phones during WAN outage. When the IP WAN is down, the IP phones at the remote branch office can register to the SRST router. The SRST router can process calls between registered IP phones and can send calls to other sites through the PSTN. The phone-registration process is explained in further detail in Chapter 7, "Endpoints."

Deterioration of the quality of established calls can occur when WAN links are oversubscribed with voice traffic. To limit the number of calls between the sites, use call admission control (CAC). CUCM has no concept of limited bandwidth without the configuration of CAC. CUCM "believes" that bandwidth is infinite.

Centralized Call Processing models can take advantage of automated alternate routing (AAR) features. AAR allows CUCM to dynamically reroute a call over the PSTN if the call is denied because of CAC.

Multisite WAN with Centralized Call Processing

Figure 2-2 Multisite WAN with Centralized Call Processing

Figure 2-2 Multisite WAN with Centralized Call Processing


When implementing the Multisite WAN model with centralized call processing, consider the following guidelines:

■ Maximum of 1000 locations per CUCM cluster.

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

■ Delay between CUCM and remote locations minimized to reduce voice cut-through delays.

■ Locations mechanism in CUCM used to provide CAC into and out of remote branches. The locations can support a maximum of 30,000 IP phones per cluster when CUCM runs on the largest supported server. Since CUCM Release 5, you can use Resource Reservation Protocol (RSVP)-based CAC between locations.

■ CUCM does not limit the number of devices that can be deployed at a remote branch, but best practice mandates deploying a number of phones equal to the IP phones at a capability that is provided by the SRST branch router. SRST limits remote branches to a maximum of 720 SCCP or 480 SIP Cisco IP Phones and 960 directory numbers during WAN outage or failover to SRST. The number of phones and lines (directory numbers) that are supported is based on the hardware and IOS version used at the branch.

■ Minimum of 768 kb/s or greater WAN link speeds. Video is not recommended on WAN connections that operate at speeds lower than 768 kb/s.

■ CAC is provided by CUCM locations for calls between sites controlled by the same CUCM cluster, and by the Cisco IOS Gatekeeper for calls between CUCM clusters. AAR is also supported for both intracluster and intercluster video calls.

Table 2-1 Survivable Remote Site Telephony Hardware Requirements

Cisco Router

Maximum Cisco Unified IP Phones

























6500 CMM


Multisite WAN with centralized call processing saves PSTN costs for intersite calls by using the IP WAN rather than the PSTN. IP WAN can also be used to bypass toll charges by routing calls through remote-site gateways, closer to the PSTN number dialed. This practice is known as tail-end hop-off (TEHO). TEHO is disallowed in some countries, and local regulations should be verified before implementing TEHO.

This deployment model maximizes the utilization of available bandwidth by allowing voice traffic to share the IP WAN with other types of traffic. Voice quality is ensured by deploying QoS and CAC.

Cisco Unified Extension Mobility can be used within the CUCM cluster, which allows roaming users to use their directory number at remote phones as if they were at their home phone.

When using the Multisite WAN with Centralized Call Processing deployment model, CUCM administration is centralized and therefore simpler compared to a Multisite with Distributed Call Processing model where multiple clusters have to be separately administered.

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