Source : Free On-Line Dictionary of Computing

SCSI-3
     
         An ongoing standardisation effort to
        extend the capabilities of {SCSI-2}.  SCSI-3's goals are more
        devices on a bus (up to 32); faster data transfer; greater
        distances between devices (longer cables); more device classes
        and command sets; structured documentation; and a structured
        {protocol} model.
     
        In SCSI-2, data transmission is parallel (8, 16 or 32 bit
        wide).  This gets increasingly difficult with higher data
        rates and longer cables because of varying signal delays on
        different wires.  Furthermore, wiring cost and drive power
        increases with wider data words and higher speed.  This has
        triggered the move to serial interfacing in SCSI-3.  By
        embedding clock information into a serial data stream signal
        delay problems are eliminated.  Driving a single signal also
        consumes less driving power and reduces connector cost and
        size.
     
        To allow for backward compatibility and for added flexibility
        SCSI-3 allows the use of several different transport
        mechanisms, some serial and some parallel.  The software
        {protocol} and command set is the same for each transport.
        This leads to a layered protocol definition similar to
        definitions found in networking.
     
        SCSI-3 is therefore in fact the sum of a number of separate
        standards which are defined by separate groups.  These
        standards and groups are currently:
     
         X3T9.2/91-13R2   SCSI-3 Generic Packetized Protocol
         X3T9.2/92-141    SCSI-3 Queuing Model
         X3T9.2/92-079    SCSI-3 Architecture Model
         IEEE P1394       High Performance Serial Bus
         X3T9.2/92-106    SCSI-3 Block Commands
         X3T9.2/91-189    SCSI-3 Serial Bus Protocol
         X3T9.2/92-105    SCSI-3 SCSI-3 Core Commands
        		  SCSI-3 Common Command Set
         X3T9.2/92-108    SCSI-3 Graphic Commands
         X3T9.2/92-109    SCSI-3 Medium Changer Commands
         X3T9.2/91-11     SCSI-3 Interlocked Protocol
         X3T9.2/91-10     SCSI-3 Parallel Interface
         X3T9.2/92-107    SCSI-3 Stream Commands
        		  SCSI-3 Scanner Commands
     
        Additional Documents for the Fibre Channel are also meant to
        be included in the SCSI-3 framework, i.e.:
     
        		  Fibre Channel SCSI Mapping
        		  Fibre Channel Fabric Requirements
        		  Fibre Channel Low Cost Topologies
         X3T9.3/92-007    Fibre Channel Physical and Signalling Interface
        		  Fibre Channel Single Byte Commands
        		  Fibre Channel Cross Point Switch Topology
         X3T9.2/92-103    SCSI-3 Fibre Channel Protocol (GPP & SBP)
     
        As all of this is an ongoing effort of considerable
        complexity, document structure and workgroups may change.  No
        final standard is issued yet.
     
        In the meantime a group of manufacturers have proposed an
        extension of {SCSI-2} called {Ultra-SCSI} which doubles the
        transfer speed of {Fast-SCSI} to give 20MByte/s on an 8 bit
        connection and 40MByte/s on a 16-bit connection.
     
        [Hermann Strass: "SCSI-Bus erfolgreich anwenden",
        Franzis-Verlag Muenchen 1993].
     
        (1995-04-19)