In Office Communications Server 2007 R2, the back-end database stores configuration information, contact lists and presence information for users, and any state information required to resume a conference. Presence data and conferencing data are stored in different tables of the same physical database.
In a Standard Edition configuration, all server components are installed on the same computer, including the back-end database. The back-end database on a Standard Edition server uses Microsoft SQL Server 2005 Express Edition with Service Pack 2 (SP2) or later.
In an Enterprise Edition configuration, the back-end database must be configured as a separate dedicated computer. In an Enterprise pool, all servers in the pool share a central Microsoft SQL Server database. This database runs on Microsoft SQL Server 2008 (32-bit or 64-bit) or SQL Server 2005 with Service Pack 2 (SP2) or later (32-bit or 64-bit), and you can cluster it in an active-passive configuration for higher availability.