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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) | SCSI Standards | SCSI-3 ]

SCSI-3 Architecture

Since SCSI-3 defines a number of different standards, each covering different aspects of SCSI, it is necessary to organize these into a format that defines how they relate to each other, and the goals of the interface as a whole. This structure is called the architecture of SCSI-3. SCSI-3 architecture is defined by a document called the SCSI-3 Architecture Model or SAM, which has been approved as ANSI standard X3.270-1996. The T10 technical committee is also currently working on a revision to this document, called the SCSI-3 Architecture Model - 2 or SAM-2. (SAM-2 is actually on its 14th revision at the time of this writing; it's been in progress for over four years!)

The SCSI-3 Architecture Model has several functions. An important one is to organize and categorize the various other standards that fall under SCSI-3. This serves to structure these standards in a way that makes sense to SCSI standards developers, hardware designers and users. The structure defines broad, generic requirements at a high level, which are refined to more specific low-level requirements through the use of particular implementation standards. Most of the different SCSI-3 documents fall into the following three general categories:

  • Commands: These are standards that define specific command sets for either all SCSI devices, or for particular types of SCSI devices.
  • Protocols: These standards formalize the rules by which various devices communicate and share information, allowing different devices to work together. These standards are sometimes said to describe the transport layer of the interface.
  • Interconnects: These are standards that define specific interface details, such as electrical signaling methods and transfer modes. They are sometimes called physical layer standards as well.

The protocols and interconnects are often closely related, with a particular interconnect document typically being associated with a specific protocol standard. In some cases, especially lately, the protocol and interconnect standards are being combined into a single document. All of these standards are tied together by the architecture model, and also by the Common Access Method or CAM, which defines software services for host systems (computers) to interface with SCSI devices.

The architecture model is also responsible for providing much of the "foundation" for the other standards. This is important, because while the standards are developed and enhanced independently of each other, they must share certain common features if SCSI-3 is to remain a coherent standard as a whole. The SAM documents define high-level models of how SCSI works, requirements that apply to all SCSI implementations, and some of the fundamentals of how SCSI devices should be identified and addressed. The architecture model documents also serve as a single, unified place where common terms and concepts are defined. This is useful from a practical standpoint, to avoid the confusion that would result if the various SCSI-3 standards used inconsistent definitions for key terms and acronyms.

For a list and brief description of the standards defined in the SCSI-3 architecture, see the next page.

Next: Overview of SCSI-3 Standards

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