Storage Area Network: Architecture

Bridges

A bridge is a device that connects two similar or dissimilar segments. Typically, bridges are used in LAN/WAN environments to connect multiple LAN segments. A Fibre Channel-to-SCSI bridge is used to provide connectivity between a Fibre Channel environment and a SCSI environment.

In order to use SCSI tape drives in a SAN environment, a bridge will need to be attached. The bridge can either be internal to the tape device or an external unit. Generally, an optical cable is used to connect the Fibre Channel switch to the bridge. The bridge has two or more SCSI channels connecting to a single or multiple SCSI tape drives. The bridge simply acts as another drive along the path.

Processing SCSI Information

The following describes how a bridge processes SCSI information:

A Fibre Channel host issues a command. The host encapsulates the command in the Fibre Channel protocol and sends the packet to the bridge.
The Fibre Channel port in the bridge receives the packet, interprets the Fibre Channel information, and places the packet in buffer memory.
The bridge then interprets the information and programs a SCSI controller to process the transaction.
The SCSI controller sends the command to the SCSI target.
The target interprets the command and prepares to either read or write data.
Data flows between the host and target through payload buffers.
Response information flows from the SCSI target back to the Fibre Channel host.