Unified Communications Application Server (UCAS) is a platform introduced in Office Communications Server 2007 R2 that makes it easier to build server-side applications that run on Standard Edition servers or Enterprise pool servers. Each Front-End Server in a pool runs an instance of an Application Server host. Applications developed by using the Unified Communication Managed APIs (UCMA) 2.0 can use the UCAS platform as a common framework that leverages Office Communications Server capabilities such as deployment, trust, administration, load balancing and routing, monitoring, and so on. UCAS is designed to host server applications that act as Session Initiation Protocol (SIP) endpoints.
Similar to Office Communications Server conferencing servers, UCAS is another server role. When you deploy UCAS on an Enterprise Edition server consolidated pool topology, each UCAS application runs on all servers in the pool and together they share the overall workload of the application. UCAS consists of a single Windows service, called Application Host (OCSAppServerMaster.exe), and one or more instances of another Windows service called OCSAppServerHost.exe. Each instance of OCSAppServerHost.exe on a server hosts a UCAS application unique to that server.
Several new features in Office Communications Server 2007 R2 are implemented as UCAS applications, including dial-in conferencing, Response Group Service, and Outside Voice Control. For details, see Voice Components.
For details about Application Server architecture, see Application Server Drilldown.