The SCSI-3 standard is a collection of other standards. These standards are arranged into a framework based on the SCSI-3 architecture documents. Used in several high-end systems, SCSI-3 often utilizes a MicroD 68-pin connector having thumbscrews. It is also referred to as Mini 68.
The most widely used bus width for SCSI-3 is 16 bits, with a transfer rate of 20 MBps.
In SCSI-2, data is transmitted in parallel (i.e., 8, 16 or 32 bits wide). This can get significantly challenging with longer cables and higher data rates owing to varying signal delays on various wires. In addition, drive power and wiring expenditure grow with higher speeds and wider data words. This has triggered the migration to serial interfacing in SCSI-3.
Issues with delay are eradicated by embedding clock information to serial data stream signals. Also, driving a single signal uses up less driving power and cuts down connector price and size.
In order to permit backward compatibility and increased flexibility, SCSI-3 permits the use of many different transport systems, some parallel and some serial. For every transport, the command set and software protocol are the same. This results in a layered protocol definition that is much like the definitions present in networking.
SCSI-3 therefore is the sum of quite a few independent standards that are based on independent groups.