Small Computer System Interface (SCSI), an ANSI standard, is a parallel interface standard used by Apple Macintosh computers, PCs, and many UNIX systems for attaching peripheral devices to computers. SCSI interfaces provide for faster data transmission rates than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port. There are many variations of SCSI: SCSI-1, SCSI-2, SCSI-3 and the recently approved standard Serial Attached SCSI (SAS).
SCSI-1
SCSI-1 is the original SCSI and it is obsolete so far. Basically, SCSI-1 uses an 8-bit bus, and supports data rates of 4 MBps.
SCSI-2
SCSI-2 is an improved version of SCSI-1. SCSI-2 is based on CCS which is a minimum set of 18 basic commands all manufacture's hardware would work together. SCSI-2 also provided extra speed with options called Fast SCSI and a 16-bit version called Wide SCSI. A feature called command queuing gave the SCSI device the ability to execute command in an order that would be most efficient. Fast SCSI delivers a 10 MB/sec transfer rate. When combined with the 16-bit bus, this doubles to 20 MB/sec. This is called Fast-Wide SCSI.
SCSI-3
SCSI-3 has many advances over SCSI-2 such as Serial SCSI. This feature will allow data transfer up to 100MB/sec through a six-conductor coaxial cable. SCSI-3 solves many of the termination and delay problems of older SCSI versions. SCSI-3 eases SCSI installation woes by being more plug-and-play in nature, such as automatic SCSI ID assigning and termination. SCSI-3 also supports 32 devices while SCSI-2 supports only 8.
SCSI-3 change the document structure, SCSI-3 is not one document with all the different layers and electrical interfaces, but a collection of documents that cover the physical layer, the basic protocol specific to that electrical interface, the primary command set layer (SPC) and the specific protocol layer. The specific protocol layer contains the Hard Disk interface Commands in the Block Commands (SBC), Steam Commands for tape drives (SSC), Controller Commands for RAID arrays (SCC), Multimedia Commands (MMC), Media Changer Commands (MCC) and enclosure services commands (SES) for example. There is an overall architectural model (SAM).
Elements of SCSI-3 are in use today in the forms of Ultra-Wide and Ultra SCSI drives. Ultra SCSI delivers 20MB/sec over the 8-bit bus. Ultra-Wide SCSI incorporates the 16-bit bus, and the speed raises to 40MB/sec.
SAS - Serial Attached SCSI
Serial Attached SCSI (SAS) is an evolutionary replacement for the Parallel SCSI physical storage interface. Serial Attached SCSI offers much faster communication and easier configuration. In addition, Serial Attached SCSI provides device compatibility to Serial ATA and uses similar cabling.
Serial Attached SCSI (SAS) is a point-to-point connection and allows multiple ports to be aggregated into a single controller, either built onto the mother board or as an add-on. Its technology is built upon the robust and tested parallel SCSI communication technology. Serial Attached SCSI (SAS) uses Serial ATA (SATA) cables which are a thin point-to-point connection allowing easy cable routing within a computer system, without the need for daisy-chaining. The first implementation of Serial Attached SCSI provides 1.5 Gb/sec (150MB/sec) of performance for each drive within an array.
Protocol Structure
SCSI: Small Computer System Interface
Related protocols: iSCSI, iFCP, FCP, FCIP, iSNS
Sponsor: SCSI standards are defined by ANSI (http://www.ansi.org)
Reference: http://www.danbbs.dk/~dino/SCSI/: SCSI-2 Specification