SCSI-3
SCSI-3
(hardware)In SCSI-2, data transmission is parallel (8, 16 or 32 bitwide). This gets increasingly difficult with higher datarates and longer cables because of varying signal delays ondifferent wires. Furthermore, wiring cost and drive powerincreases with wider data words and higher speed. This hastriggered the move to serial interfacing in SCSI-3. Byembedding clock information into a serial data stream signaldelay problems are eliminated. Driving a single signal alsoconsumes less driving power and reduces connector cost andsize.
To allow for backward compatibility and for added flexibilitySCSI-3 allows the use of several different transportmechanisms, some serial and some parallel. The softwareprotocol and command set is the same for each transport.This leads to a layered protocol definition similar todefinitions found in networking.
SCSI-3 is therefore in fact the sum of a number of separatestandards which are defined by separate groups. Thesestandards and groups are currently:
X3T9.2/91-13R2 SCSI-3 Generic Packetized ProtocolX3T9.2/92-141 SCSI-3 Queuing ModelX3T9.2/92-079 SCSI-3 Architecture ModelIEEE P1394 High Performance Serial BusX3T9.2/92-106 SCSI-3 Block CommandsX3T9.2/91-189 SCSI-3 Serial Bus ProtocolX3T9.2/92-105 SCSI-3 SCSI-3 Core CommandsSCSI-3 Common Command SetX3T9.2/92-108 SCSI-3 Graphic CommandsX3T9.2/92-109 SCSI-3 Medium Changer CommandsX3T9.2/91-11 SCSI-3 Interlocked ProtocolX3T9.2/91-10 SCSI-3 Parallel InterfaceX3T9.2/92-107 SCSI-3 Stream CommandsSCSI-3 Scanner Commands
Additional Documents for the Fibre Channel are also meant tobe included in the SCSI-3 framework, i.e.:
Fibre Channel SCSI MappingFibre Channel Fabric RequirementsFibre Channel Low Cost TopologiesX3T9.3/92-007 Fibre Channel Physical and Signalling InterfaceFibre Channel Single Byte CommandsFibre Channel Cross Point Switch TopologyX3T9.2/92-103 SCSI-3 Fibre Channel Protocol (GPP & SBP)
As all of this is an ongoing effort of considerablecomplexity, document structure and workgroups may change. Nofinal standard is issued yet.
In the meantime a group of manufacturers have proposed anextension of SCSI-2 called Ultra-SCSI which doubles thetransfer speed of Fast-SCSI to give 20MByte/s on an 8 bitconnection and 40MByte/s on a 16-bit connection.
[Hermann Strass: "SCSI-Bus erfolgreich anwenden",Franzis-Verlag Muenchen 1993].