Office Communications Server 2007 uses SQL Server to store user and configuration data. The Standard Edition uses SQL Server 2005 Express, which is automatically installed by Office Communications Server Setup. The Enterprise Edition uses SQL Server 2005 SP1, which must be installed separately on the back-end server. The back-end server computer must be installed with Windows Server 2003 and cannot be collocated with the front-end server. Encrypted File Systems (EFS) should not be turned on in the %TEMP% directory; if it is, Setup will fail.

For best results, two hard drives with fast access time should be installed on the back-end server. One hard drive is used for the SQL Server database and the other hard drive is used for the log files. A 4-PROC server is the minimum requirement. For larger deployments, an 8-PROC server is recommended. A 1-GB network interface card (NIC) for the back-end server is recommended. Office Communications Server 2007 supports the use of 32-bit and 64-bit servers for the SQL Server back-end server.

You can build redundancy into the back-end server for the Enterprise Edition case. Office Communications Server 2007 supports only single-instance failover, also commonly referred to as active/passive clustering. A two-node cluster is configured to fail over to the standby SQL Server if the primary SQL Server fails. SQL backup and restore remains the supported way for interpool disaster recovery.