The Office Communications Server 2007 R2, Back-End Database, stores user data for all Enterprise Edition servers within a pool. As a centralized repository, the Back-End Database cannot be installed on the same computer as any other Office Communications Server role. The Back-End Database cannot reside on an Enterprise Edition server in the pool.
The Back-End Database is created automatically when you create the pool, but the computer that you designate as the back end must already be running the Microsoft SQL Server database software in order for installation to succeed. Before you deploy Enterprise Edition server, install SQL Server 2008 or SQL Server 2005 with Service Pack 2 (SP2) on a dedicated computer that meets the hardware requirements described in the topic of the Office Communications Server 2007 R2 Supported Topologies and Infrastructure Requirements documentation. Both 32-bit and 64-bit versions of SQL Server are supported. 64-bit is recommended.
Before you create the Enterprise pool, if you install SQL Server 2008 on a Windows Server 2008 operating system, you must also configure the Windows Firewall to allow SQL Server 2008 access to Transmission Control Protocol (TCP) port 1433. By default, SQL Server uses port 1433 to listen for incoming connections. If the default port is not used, you need to add an exception for the non-default port to Windows Firewall.