Multisite Deployment with Centralized Call Processing
The multisite deployment 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 2-4 illustrates a typical centralized call-processing deployment, with a CUCM cluster at the central site or data center and a QoS-enabled IP WAN to connect all the sites. The remote sites rely on the centralized CUCM cluster to manage their call processing. Applications such as voice mail and interactive voice response systems are typically centralized as well to reduce the overall costs of administration and maintenance.
Figure 2-4 Centralized Multisite Deployment
The Cisco Unified Survivable Remote Site Telephony (SRST) and E-SRST features that are available in Cisco IOS gateways provide call-processing services to remote IP phones during a WAN outage. When the IP WAN is down, the IP phones at the remote branch office can register to the local Cisco Unified SRST router. The Cisco Unified SRST router can process calls between registered IP phones and send calls to other sites through the PSTN. Figure 2-5 gives an overview of remote site SRST/E-SRST deployment with centralized call processing. The same arrangement however, will work if there are different CUCM clusters (distributed call processing or clustering over WAN) with one or more remote sites.
Figure 2-5 Cisco Unified SRST/E-SRST Deployment with Centralized Call Processing
To avoid oversubscribing the WAN links with voice traffic, causing deterioration of the quality of established calls, Call Admission Control (CAC) is used to limit the number of calls between the sites.
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.
Design Guidelines for Multisite WAN Model with Centralized Call Processing
Consider the following best practice guidelines when implementing a multisite WAN model with centralized call processing:
Use a maximum of 2000 locations per CUCM cluster.
Use a maximum of 2100 H.323 devices (gateways, multipoint control units, trunks, and clients) or 1100 MGCP gateways per CUCM cluster.
Minimize delay between CUCM and remote locations to reduce voice cut-through delays.
Use enhanced locations CAC mechanism in CUCM to provide CAC into and out of remote branches. Locations can support a maximum of 40,000 IP phones per cluster when CUCM runs on the largest supported cluster. Another option is to use Resource Reservation Protocol (RSVP)-based CAC between locations.
Choose appropriate platform for SRST support. There is no limit to the number of IP phones at each individual remote branch. However, the capability that the Cisco Unified SRST feature provides in the branch router limits remote branches to a maximum of 1500 Cisco IP phones on a Cisco 3945E Integrated Services Router during a WAN outage or failover to SRST. Other platforms have different (lower) limits.
Use high-bandwidth audio (for example, G.711 or G.722) between devices in the same site (intrasite), but low-bandwidth audio (for example, G.729) between devices in different sites (intersite).
Use high-bandwidth video (for example, 1.5 Mbps with 4CIF or 720p, to 2 Mbps with 1080p) between devices in the same site, but low-bandwidth video (for example, 384 kbps with 448p or CIF) between devices at different sites.
Use a minimum of 1.5 Mbps or greater WAN link speed. Video is not recommended on WAN connections that operate at speeds lower than 1.5 Mbps.
If a distributed call-processing model is more suitable for the business needs of a customer, the choices include installing a CUCM cluster at the remote branch or running CUCM Express on the branch router.
Benefits of Multisite Deployment with Centralized Call Processing Model
A multisite deployment with centralized call processing saves PSTN costs for intersite calls by using the IP WAN instead of the PSTN. The IP WAN can also be used to bypass toll charges by routing calls through remote site gateways that are closer to the PSTN number that is dialed. This practice is known as Tail End Hop Off (TEHO). TEHO is not permitted 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. Deploying QoS and CAC ensures voice quality. AAR reroutes calls over the PSTN if CAC denies the calls because of oversubscription.
Cisco Extension Mobility can be used within the CUCM cluster, allowing roaming users to use their directory numbers at remote phones as if they were at their home phones.
When the multisite WAN with centralized call-processing deployment model is used, CUCM administration is centralized, and therefore simpler, compared with a multisite WAN with distributed call-processing model where multiple clusters must be separately administered.