The Digital Fact Book - Quantel

SCSIThe Small Computer Systems Interface is a high data rate, general-purpose parallel interfaceintroduced in 1979 allowing up to eight devices to be connected to one bus (now 16 forWide SCSI). This could comprise a controller and up to seven disks or devices of differentsorts – hard disks, optical disks, tape drives, scanners, etc., and may be shared betweenseveral computers.Since then SCSI has hugely increased in performance but is now used mainly onhigh-performance workstations and RAIDs on servers while other lower cost interfacessuch as USB2 and IEEE1394 connect external devices and SATA is used for hard disks.The original SCSI specified a cabling standard (50-way) that had to be kept short, a protocolfor sending and receiving commands and their format. It is intended as a device-independentinterface so the host computer needs no details about the peripherals it controls. SCSI’scontinued development has resulted in the following range of maximum transfer rates thatkeep ahead of modern disk drive transfer rates:Standard SCSIFast SCSIUltra SCSIUltra SCSI 2 (LVD)SCSI 80SCSI 1605 M transfers/sec. (max)10 M transfers/sec. (max)20 M transfers/sec. (max)40 M transfers/sec. (max)80 M transfers/sec. (max)160 M transfers/sec. (max)There are other SCSI interfaces. Fibre Channel (FC) can be used as the transport mediumwith FC-AL 1Gb to FC-AL 4Gb offering data throughputs of up to 100 – 400 MB/s overdistances of 3km. Longer term, it is believed that iSCSI, Ultra 3 SCSI over TCP/IP, will winout as developments in Ethernet outpace those in FC. The performance of this thoughis network-dependent.Serial SCSI using SSA (Serial Storage Architecture) FC-AL, IEEE1394, and Serial AttachedSCSI (SAS), break away from the parallel cabling to offer data transfers up to 300 MB/s(over SAS). This is an area of further development.See also: Disk drivesSSD, SDHCSee SDTV, Secure DataSDISee Serial Digital Interface151