The Enterprise Edition is the same building block as the Standard Edition Server. In fact, it performs the same functionality as the Standard Edition Server. The primary difference is that it improves the scalability and availability of a Standard Edition Server by separating the logical operations that a Standard Edition Server performs into individual servers. An Enterprise Edition deployment is referred to as an Enterprise pool because such a deployment involves multiple physical servers. An Enterprise pool decouples the database from the back-end server running SQL Server and the SIP proxy and registrar service from the front-end servers. The front-end servers no longer maintain persistent data because that data is stored in the SQL back-end server. This is an advantage, because in the event of a system failure, a new front-end server can be quickly brought up to replace it. The front-end servers can further be expanded into specialized server roles by separating the multipoint control units (MCUs) and Web Components onto separate physical servers.

This results in the following two Enterprise pool models:

An Enterprise Edition pool deployment requires a hardware load balancer (HLB) to load balance client connections to the front-end servers and Web servers in the expanded configuration case. The HLB's virtual IP (VIP) must be manually published in DNS. Clients connect to this VIP when signing in to the Enterprise pool. The HLB is configured to load balance incoming client connections among the available front-end servers. In both configurations, an Enterprise pool can scale up to four front-end servers.

For high availability on the back end, because the back-end server is a SQL Server, an Enterprise Edition pool deployment can be clustered by taking advantage of Microsoft Clustering Service (MSCS). MSCS is a feature of the Enterprise Edition and Datacenter Edition of Windows Server 2003 and Windows Server 2003 R2. Microsoft officially supports only active-passive SQL clustering for Office Communications Server 2007 Enterprise pool deployments. Active-passive SQL clustering means one SQL node is active while the other SQL node is passive. The front-end servers in the pool are connected to the active node. The passive node takes over when the active node fails. The passive node must be an exact replica of the active node in the system configuration.

Office Communications Server 2007 does not support active-active SQL clustering. It does support running the back-end SQL Server on a 64-bit server. The back-end server cannot be collocated on a same physical server with a front-end server. Because only the back-end server stores persistent information about each user homed on the pool, the front-end servers maintain transient information—such as logged-on state and control information for an IM, Web, or A/V conference—only for the duration of a user's session.

All servers belonging to an Enterprise pool must be running on a Windows Server 2003 SP1 or Windows Server 2003 R2 computer joined to an Active Directory domain. Thus a scenario in which half of the front-end servers are deployed in domain A and half in domain B, while the back-end servers are deployed in domain C, is not supported by Microsoft. Also, all servers in an Enterprise pool should be within geographic proximity with 1-GB connectivity between front-end servers and back-end servers.

Table 2-2 lists the system requirements for Office Communications Server 2007 Enterprise Edition.