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Scuzzy moving on ... Part I.


The small computer system interface (SCSI) standard, commonly referred to as "scuzzy," is continually evolving. To keep you informed of the latest SCSI changes, we teamed up to provide a follow-up to a SCSI article which appeared in the CHIPS Summer 2004 edition ( This article, Part I, of a two part series, will highlight the latest SCSI technologies and standards. For example, there are new devices available such as the Ultra SPI-3 (SCSI-3 Parallel Interface) and SPI-4, and Ultra160 or Ultra320 parallel SCSI devices. The Ultra160 doubles Ultra2 SCSI's speed by as much as 160 MBps for a 16-bit data bus. It is commonly referred to as the Fast-80.

The Ultra160 uses a SPI-3 third generation parallel SCSI interface, which adds five new features: (1) Fast-80 or a data bus speed running at 80 MHz; (2) Cyclic Redundancy Check (CRC)--a common error checking protocol, which is used to ensure data integrity as a safety measure since transfer speeds were being increased, leading to the possibility of data corruption; (3) Domain Validation, which improves the robustness of the process by which different SCSI devices determine an optimal data transfer rate; (4) Quick arbitration and selection (QAS), which represents a change in the way devices determine which device has control of the SCSI bus. (This feature provides a small improvement in performance.); and (5) Packetization--reduces the overhead associated with data transfer.

The Ultra320 uses SPI-4 fourth generation interface for SCSI and has similar features of the SPI-3 except that it again doubles the speed of data transfer to 320 MBps by running the data bus speed at 160 MHz. The Ultra320 is also referred to as Fast-160.

What's New?

Early in 2003, Ultra640 was issued as a standard by the InterNa-tional Committee for Information Technology Standards (INCITS) and called 367-2003 or SPI-5. The SPI-5 is the fifth generation of the SCSI-3 standard. SPI-5 incorporates Fast-320. Ultra640 required a new transfer mode with a 160 MHz free running clock speed to eliminate Inter-Symbol Interface (ISI) problems. Ultra640 uses paced data transfers or packetized SCSI; a free running clock; ISI precompensation drivers and active adapter filter receivers; skew compensation; training patterns for the adaptive active filters; and expander communications.

The Ultra640 increases speed to 640 MBps and has similar features as the Ultra320, but offers double the speed. With Ultra640, the support for single-ended interfaces has been downplayed in the SPI-5 interface so future devices may not be backward compatible. This was done to keep SCSI devices running at optimum speed rather than using a single-ended speed of 20 MBps. When you have a mixed SCSI environment on the same connection, the speed will drop to the slowest connection standard so it is a disadvantage to mix narrow with wide SCSI devices. The Ultra640 is new and one of the new adapter products in development is Tekram's DC-390U4 series SCSI adapter. It can achieve a maximum data transfer rate of 640 MBps using a 64-bit adapter even though it is advertised as using an Ultra320 adapter.

iSCSI, known as Internet SCSI, embeds SCSI-3 over TCP/IP (Trans-mission Control Protocol/Internet Protocol); some experts say it may eventually replace Fibre Channel. Fibre Channel is a serial data transfer architecture. The most prominent Fibre Channel standard is Fibre Channel Arbitrated Loop (FC-AL). It is designed for new mass storage devices and other peripheral devices that require very high bandwidth. Using optical fiber to connect devices, FC-AL supports full-duplex data transfer rates of 100MBps.

iSCSI can easily address both the low-end and high-end markets by using Fast or Gigabit Ethernet networks or another network medium to transfer data between SCSI devices. IBM and Hewlett-Packard along with other vendors support iSCSI. This new SCSI standard can promote: storage area network (SAN), network-attached storage (NAS), geographic distribution, data integrity, disk farms, use the existing network cable plant and a single technology for connection of storage systems within local-area networks (LANs) and wide-area networks (WANs). iSCSI will work over a WAN using standard TCP/IP to access iSCSI devices. Data can then be distributed over different networks. iSCSI's lack of built-in security is resolved by using the network security proto-cols, which will control data using servers, routers, virtual LANs (VLANs) or firewalls. See Figure 1 for an example of a basic iSCSI network design.


What's Next?

Serial Attached SCSI (SAS) or INCITS 376-2003 is another new standard approved by the American National Standards Institute (ANSI) that boasts a greater transmission distance with point-to-point topology using dedicated connections. SAS lowers the operating cost of SCSI with the added benefits of increased cooling, making it easy to connect devices with simplified cable connections. SAS transfer rates start at 1 GBps or 150 MBps. SAS will currently support 3 GB or 300 MBps. The next generation of SAS promises 6 GBps throughput. SAS offers double the speed of the Serial Advanced Technology Attachment (SATA).

SATA is a serial link, a single cable with a minimum of four wires creating a point-to-point connection between devices. Transfer rates for Serial ATA begin at 150 MBps. SATA offers a high data transfer speed with a lower cost than most parallel SCSI devices. SATA devices are becoming popular because of the low cost and the increased cooling capability of the small narrow, serial cable which replaces the flat 40-pin or 80-pin cable. The new SCSI standard of SAS will surpass the performance, flexibility and connectivity of SATA, but SAS and SATA devices are compatible. This compatibility offers benefits to system builders, integrators and end users. System builders can use SAS high performance features to support enterprise networks while SATA can support desktops and LANS. Integrators will have less worry since the interface standards can be interchanged, and end users will get a faster processing speed with SATA.

Where are SCSI devices used?

Parallel SCSI standards of Ultra160, Ultra320 and Ultra640, coupled with SAS will support enterprise networks and disk farms. While iSCSI devices are designed to support network storage at minimal cost using the existing cable plant, SATA-1 and 2 devices are generally used at the desktop level connecting internal hard drives or other peripherals such as optical drives. Universal System Bus (USB) v2.0 and FireWire (IEEE 1394) are external bus standards, which are primarily designed to support desktop external peripherals such as printers, mice, keyboards and external hard drives. Although they are fast and flexible supporting Plug-and-Play and hot plugging, they are comparatively inexpensive. A 1394 port can support isochronous data, delivering data at a guaranteed speed. This makes it perfect for devices that transfer high levels of data in real-time, such as video devices.

For more information go to the Technical Committee T10 (a technical committee of INCITS) Web site at

Patrick Koehler is a member of the SPAWAR Systems Center Charleston, FORCEnet Engineering and Technology Support Branch. He has a bachelor's degree in computer information systems and holds certifications in A+, CCNA, CCNP, MCDBA 2000, MCP/MCSA/MCSE Windows 2003, MS Outlook/Powerpoint/Access/Word Expert 2002, Network+, Security+, Server+ and Inet+.

Lt. Cmdr. Stan Bush is the Military Faculty and the Program Officer for the Information Technology Management curriculum at the Naval Postgraduate School. He holds a bachelor's degree in computer information systems and a master's degree in computer science. He also holds certifications in MCP, CISSP and CISM as well as certificates in the DoD CIO & NSTISSI No. 4011 Programs. Web site:
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Title Annotation:small computer system interface
Author:Koehler, Patrick G.; Bush, Stan
Date:Jan 1, 2005
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