network protocols handbook.pdf

238Protocols GuideSAN ProtocolsProtocol NameSCSI: Small Computer SystemInterfaceProtocol DescriptionSmall Computer System Interface (SCSI), an ANSI standard,is a parallel interface standard used by Apple Macintosh computers,PCs, and many UNIX systems for attaching peripheraldevices to computers. SCSI interfaces provide for faster datatransmission rates than standard serial and parallel ports. In addition,you can attach many devices to a single SCSI port. Thereare many variations of SCSI: SCSI-1, SCSI-2, SCSI-3 and therecently approved standard Serial Attached SCSI (SAS).SCSI-1SCSI-1 is the original SCSI and is now obsolete. Basically,SCSI-1 used an 8-bit bus, and supported data rates of 4 MBps.SCSI-2SCSI-2, an improved version of SCSI-1 based on CCS, is aminimum set of 18 basic commands which work on all manufacture’shardware. SCSI-2 also provides extra speed with optionscalled Fast SCSI and a 16-bit version called Wide SCSI. Afeature called command queuing gives the SCSI device the abilityto execute commands in the most eficient order. Fast SCSIdelivers a 10 MB/sec transfer rate. When combined with a 16-bitbus, this doubles to 20 MB/sec (Fast-Wide SCSI).SCSI-3SCSI-3 has many advances over SCSI-2 such as Serial SCSI.This feature will allow data transfer up to 100MB/sec through asix-conductor coaxial cable. SCSI-3 solves many of the terminationand delay problems of older SCSI versions. It also easesSCSI installation woes by being more plug-and-play in nature,as by automatic SCSI ID assigning and termination. SCSI-3 alsosupports 32 devices while SCSI-2 supports only 8.SCSI-3 changed the document structure. It is not one documentdealing with all the different layers and electrical interfaces,but a collection of documents covering the physical layer, thebasic protocol specific to that electrical interface, the primarycommand set layer (SPC) and the specific protocol layer. Forexample, the specific protocol layer document contains theHard Disk interface Commands in the Block Commands (SBC),Steam Commands for tape drives (SSC), Controller Commandsfor RAID arrays (SCC), Multimedia Commands (MMC), MediaChanger Commands (MCC) and enclosure services commands(SES). There is an overall architectural model (SAM).Elements of SCSI-3 are in use today in the forms of Ultra-Wideand Ultra SCSI drives. Ultra SCSI delivers 20MB/sec over the8-bit bus. Ultra-Wide SCSI incorporates the 16-bit bus, and thespeed rises to 40MB/sec.SAS - Serial Attached SCSISerial Attached SCSI (SAS) is an evolutionary replacement forthe Parallel SCSI physical storage interface. Serial AttachedSCSI offers much faster communication and easier configuration.In addition, Serial Attached SCSI provides device compatibilitywith Serial ATA and uses similar cabling.Serial Attached SCSI (SAS) is a point-to-point connection andallows multiple ports to be aggregated into a single controller, eitherbuilt onto the mother board or as an add-on. Its technologyis built upon the robust and tested parallel SCSI communicationtechnology. Serial Attached SCSI (SAS) uses Serial ATA (SATA)cables which are a thin point-to-point connection allowing easycable routing within a computer system, without the need fordaisy-chaining. The first implementation of Serial Attached SCSIprovides 1.5 Gb/sec (150MB/sec) of performance for each drivewithin an array.