An architecture for a logical device that specifies the format of the logical data units on the device. The logical data unit is a block. All blocks on the device are the same size (fixed size); the subsystem can access them independently. This format is required for the logical data units for host systems that attach with a Small Computer System Interface (SCSI) or fibre-channel interface using Fibre Channel Protocol (FCP). See also Small Computer System Interface, fibre channel.