Review the Deployment Models 
chapter in the Cisco Unified Communications System Description for a guide to site models that have been tested and verified for Cisco Unified Communications Release 5.1(1).
The following deployment models were designed and tested as part of testing for the contact center environment:
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Single site—In this model, all the voice gateways, agents, desktops, IP Phones, and call processing servers such as Unified CallManager, Unified ICM, and Customer Response Solutions (CRS) and/or Unified CVP are located at the same site and have no WAN connectivity between any contact center software modules.
•Multisite centralized—In the multisite WAN model with centralized call processing, the Unified CallManager cluster resides at a central (or hub) campus and communication with remote offices occurs over the IP WAN. The central site or data center provides the call processing services and acts as the hub for the remote sites. This model also contains distributed voice gateways for locally dialed calls.
•Multisite WAN distributed—In the multisite WAN model with distributed call processing, typically some sites have their own Unified CallManager cluster and are interconnected with intercluster trunks (ICT Trunking). Similar to the multisite centralized model, sites in the multisite WAN distributed model are deployed with distributed voice gateways. Communication between sites occurs over the IP WAN.
See also the Defining Case Studies topic.