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<hardware> 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].


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