CUCM Deployment Models

Best practices for the single-site model include the following:

• The single-site model is suited for enterprises where most calls are made to destinations within the same site or to the public switched telephone network (PSTN).
• Deploy a highly available network infrastructure, with inline power, QoS, and security for phones.
• Use the Media Gateway Control Protocol (MGCP) between CUCM and voice gateways (assuming H.323 functionality is not required).
• Use the G.711 codec.

Best practices for the multisite WAN with centralized call-processing model include the following:

• Consider the factors that typically motivate the decision to deploy this model (or the multisite WAN with distributed call-processing model), including WAN bandwidth, delay limitations, scalability, management, cost, features, and so on.
• Use CUCM locations for call admission control (CAC).
• Consider deploying Survivable Remote Site Telephony (SRST). For Skinny Client Control Protocol (SCCP) phones, use SRST or Cisco Unified Communications Manager Express (CME) in SRST mode, and for Session Initiation Protocol (SIP) phones, use SIP SRST. If you are using MGCP phones, use MGCP gateway fallback.
• Minimize delay between CUCM and remote sites as much as possible.

Best practices for the multisite WAN with distributed call-processing model include the following:

• Follow general guidelines for the single-site and multisite WAN with centralized call-processing models, in addition to other specific best practices for this model.
• Use H.323 gatekeepers for CAC and dial plan resolution. Alternatively, use SIP proxies for dial plan resolution.
• Ensure high availability for gatekeepers by using mechanisms such as Hot Standby Router Protocol (HSRP) gatekeeper pairs, gatekeeper clustering, and redundancy using multiple gatekeepers. Similarly, if you are using SIP proxies, ensure that there is redundancy for these devices.

Best practices for clustering over the IP WAN include the following:

• If you have between two and four sites, consider a local failover deployment model, with CUCM subscriber and backup servers at the same site with no WAN between them.
• If you have up to eight sites, consider a remote failover deployment, with CUCM subscribers backed up by subscribers at another site.
• Ensure that the maximum one-way delay between CUCM server does not exceed 40 milliseconds (80 milliseconds round-trip).
• Minimize jitter, congestion, and packet loss for Intra-Cluster Communication Signaling (ICCS).
• Provision bandwidth between servers in accordance with expected call volume and device types/numbers.

Reference : http://www.ciscopress.com/articles/article.asp?p=1807179