Cisco Unified Communication systems can be deployed in 3 basic models:
The single-site model for Cisco Unified Communications consists of a call processing agent cluster located at a single site, or campus, with no telephony services provided over an IP WAN. An enterprise would typically deploy the single-site model over a LAN or metropolitan area network (MAN), which
carries the voice traffic within the site. In this model, calls beyond the LAN or MAN use the public switched telephone network (PSTN).
Multisite - Centralized Call Processing
The model for a multisite deployment with centralized call processing consists of a single call processing agent cluster that provides services for many remote sites and uses the IP WAN to transport Cisco Unified Communications traffic between the sites. The IP WAN also carries call control signaling
between the central site and the remote sites. The remote sites rely on the centralized Unified CM cluster to handle their call processing. Applications such as voicemail, presence servers, interactive voice response (IVR) systems, and so forth, are typically centralized as well to reduce the overall costs of administration and maintenance.
Multisite - Distributed Call Processing
The model for a multisite deployment with distributed call processing consists of multiple independent sites, each with its own call processing agent cluster connected to an IP WAN that carries voice traffic between the distributed sites.
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