SCSI (Small Computer System Interface) (Linktionary term)

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SCSI (Small Computer System Interface)

Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

SCSI (pronounced "scuzzy") is a family of disk interface systems that provide access services for peripheral I/O devices such as disk drives, CD-ROM discs, optical disks, tape drives, scanners, and other devices. Theoretically, you can plug any vendor's SCSI device into any SCSI controller. Each I/O device is called an LU (logical unit) and each LU is assigned an LUN (logical unit number).

SCSI is normally implemented in host adapters, which provides a shared bus to which peripheral devices attach. Applications make service requests to SCSI, and SCSI issues commands to the logical units that execute the commands. All commands and data cross the SCSI shared bus, which is a parallel interface. Some of the features of the SCSI interface are described here:

The original SCSI standard supports up to 7 devices on a single host adapter, but new standards support high-speed operation with up to 16 devices and bus lengths of up to 12 meters.

SCSI devices are "smart" devices with their own control circuitry. They can "disconnect" themselves from the host adapter to process tasks on their own, thus freeing up the bus for other transmissions.

The bus can handle simultaneous reads and writes.

SCSI has gone through several upgrades, as outlined in Table S-1. New generations are being defined through the joint efforts of the ANSI (American National Standards Institute) X3T10 (SCSI) Committee and the SCSI Trade Association.

SCSI Trade Association Terms	Maximum Bus Speed [MB/Sec]	Bus Width [Bits]	Maximum Bus Length [Meters] LVD: [low-voltage differential] signaling HVD: [high-voltage differential] signaling			Maximum Device Support
			Single Ended	LVD	HVD
SCSI-1^[2]	5	8	6	^[3]	25	8
Fast SCSI^[2]	10	8	3	^[3]	25	8
Fast/Wide SCSI	20	16	3	^[3]	25	16
Ultra SCSI^[2]	20	8	1.5	^[3]	25	8
Ultra SCSI^[2]	20	8	3			4
Wide Ultra SCSI	40	16		^[3]	25	16
Wide Ultra SCSI	40	16	1.5			8
Wide Ultra SCSI	40	16	3			4
Ultra2 SCSI^{[2, 4]}	40	8	^[4]	12	25	8
Wide Ultra2 SCSI^[4]	80	16	^[4]	12	25	16
Ultra2 SCSI or Ultra160^[6]	160	16	^[4]	12	^[5]	16
Ultra320	320	16	^[4]	12	^[5]	16

Table S-1: SCSI Trade Association-endorsed terminology for SCSI parallel interface technology

[1] The listed maximum bus lengths may be exceeded in Point-to-Point and engineered applications.

[2] Use of the word "Narrow" preceding SCSI, Ultra SCSI, or Ultra2 SCSI is optional.

[3] LVD was not defined in the original SCSI standards for this speed. If all devices on the bus support LVD, then 12-meter operation is possible at this speed. However, if any device on the bus is single-ended only, then the entire bus switches to single-ended mode and the distances in the single-ended column apply.

[4] Single ended is not defined for speeds beyond Ultra.

[5] HVD (Differential) is not defined for speeds beyond Ultra2.

[6] After Ultra2 all new speeds are wide only.

The original SCSI was standardized in the late 1980s. It is characterized by a 50-pin connector. By the 1990s, enhancements were made, such as changing the physical connector to a 68-pin connector. New bus widths (more data lines) were devised. These enhancements are called Wide SCSI. They include bus widths of 16 bits (2 bytes) or 32 bits (4 bytes), allowing higher data transfer rates and addressing of up to 16 devices instead of the original 8. In addition, fast transfer rates are achieved by using synchronous data transfers instead of asynchronous data transfers.

Ultra SCSI (also called Fast 20) was the next advancement. It uses new SCSI chip sets with internal clock speeds that are doubled, thus doubling the megabyte-per-second transfer rates. Basically, Ultra SCSI doubles the transfer rate independent of the bus width. Thus, the original 8-bit SCSI-1 is boosted to 10 MB/sec by applying the Fast enhancements.

Ultra2 SCSI (also called Fast 40) was one of the most significant advancements. It uses LVDS (low-voltage differential signaling), which improves the signal-to-noise ratio and allows cable lengths up to 12 meters. Up to this point, SCSI implementations used the single-ended bus, which is a signaling scheme that has limited cable lengths. These lengths must be strictly adhered to since the Fast and Ultra SCSI enhancements were achieved by doubling the clock rate, and thus the fundamental frequency at which data is transferred. Doing so required halving the cable lengths to prevent signal degradation.

The LVDS signaling scheme used with Ultra2 SCSI improves performance, but the differential bus pushes down the number of devices supported on the bus. LVDS uses extremely low voltage levels, which translates to low radiation and less power consumption. Noise is reduced by sending signals across two wires with opposing voltage levels. The receiver reads the difference in voltage levels and rejects the noise. LVDS is an important signaling technology used in other interconnection schemes such as InfiniBand. See "LVDS (Low-Voltage Differential Signaling)" and "Switch Fabrics and Bus Design."

The current generation, Ultra3 SCSI, was defined in the late 1990s. It defines several new generations of SCSI, including Ultra160 SCSI (160 MB/sec), Ultra320 SCSI (320 MB/sec), and Ultra640 SCSI (640 MB/sec). Ultra 160 SCSI products started shipping in 1999 and Ultra320 SCSI products were appearing in 2001. Adaptec and QLogic are popular sources of SCSI products. Ultra3 SCSI adds features such as the ability to choose the highest possible data transfer rate; packetization (transfer multiple commands and messages at once); and QAS (quick arbitrate and select), which provides faster arbitration to reduce connect/disconnect on the bus.

Other recent SCSI developments include VHDCI (Very High Density Cable Interconnect), LUN bridging, and SCSI switching. VHDCI provides high-speed interconnections where space is limited. With LUN bridges, 960 devices can be connected, while expanders allow SCSI cables to extend up to 75 meters. SCSI switching provides the same benefits as other switching technologies.

New SCSI drives that rotate at 15,000 rpms have the potential to deliver a sustained data rate of over 40 MB/sec. A typical server will have four drives, and those drives can produce a combined data rate of 160 MB/sec or more. Clearly, the new Ultra3 SCSI standards are needed. But these new systems must be installed in 64-bit PCI systems since the older 32-bit, 33-MHz PCI bus has a maximum data rate of 133 MB/sec.

Similar technologies include USB (Universal Serial Bus), FireWire (IEEE 1394), Fibre Channel, and InfiniBand. USB is considered a bus for printers, mice, and keyboards. Firewire is competitive with SCSI, but its market has not developed. Fibre Channel is considered a SAN interconnect. In the SAN environment, SCSI is still used for in-the-box drive connections. InfiniBand is a potential PCI replacement, as well as a cluster and SAN interconnection technology.

SCSI-FCP (Small Computer System Interface-Fibre Channel Protocol) is an implementation of Fibre Channel that transports SCSI protocols. SCSI is a disk interface technology that normally runs over a parallel connection. SCSI-FCP is a serial SCSI that allows SCSI-based applications to use an underlying Fibre Channel connection. SCSI-FCP is widely used to connect high-performance servers to storage subsystems, especially in the SAN environment. It provides higher performance (100 Mbits/sec), supports cable lengths up to 10km, and can address up to 16 million devices. Data is transferred in frames rather than blocks.

A new specification called iSCSI (Internet SCSI) is a mapping of SCSI into the TCP/IP protocol. This scheme allows clients to directly connect with a SCSI device across a network and perform block transfer (as opposed to file-level transfers) directly over a network. The communication between an initiator and target occurs over one or more TCP connections. This technology and a similar technology for Fibre Channel are being developed by the IETF IP Storage (ips) Working Group. See "IP Storage" for more information.