A Microsoft Message Queue (MSMQ) Server that contains a read-only copy of the primary site controller (PSC) or primary enterprise controller (PEC) database. MSMQ sites do not require backup site controllers (BSCs), but at least one backup site controller should be installed in each MSMQ site for load balancing and failure recovery.
A PSC or PEC must be installed before you can install any backup site controllers. The BSC also functions as an MSMQ Routing Server.
In this capacity, the BSC provides intermediate store-and-forward message queuing services and dynamic routing. BSCs must be installed on computers running Microsoft Windows NT Server, Enterprise Edition.