< 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].
(1995-04-19)