SCSI Architectural Model

The SCSI architectural model is a high-level, abstract view of the way that SCSI devices communicate. It is intended to show how the different SCSI standards are inter-related. The following summary is an overview of the main concepts and terminology of the SCSI archtectural model:

*Only externally-observable behaviours are defined in SCSI standards.
*The relationship between SCSI devices is encapsulated by a client-server service-delivery model. The client is called a SCSI initiator and the server is called a SCSI target.
*A SCSI domain consists of a least one SCSI device, at least one SCSI target and at least one SCSI initiator interconnected by a service delivery subsystem.
*A SCSI device has one or more SCSI ports, and a SCSI port may have an optional SCSI port identifier.
*A SCSI device can have an optional SCSI device name which must be unique within the SCSI domain in which the SCSI device has SCSI ports. This is often called a World Wide Name. Note that the "world" may only consist of a very small number of SCSI devices.
*A SCSI target consists of one or more SCSI logical unit numbers (LUNs).
*A LUN may have dependent LUNs embedded within it. This can recur up to a maximum nesting depth of four addressable levels.
*There are three type of SCSI ports: initiator ports, target ports and target/initiator ports. A SCSI device may contain any combination of initiator ports, target ports and target/initiator ports.
*SCSI distributed objects are considered to communicate in a three layer model:
**The highest level of abstraction is the SCSI Application Layer (SAL) where an initiator and a target are considered to communicate using SCSI commands sent via the SCSI application protocol
**The SCSI Transport Protocol Layer (STPL) is where an initiator and a target are considered to communicate using a SCSI transport protocol. Examples of SCSI transport protocols are Fibre Channel, SSA and the SCSI Parallel Interface
**The lowest level is the SCSI Interconnect Layer (SIL) where an initiator and a target are considered to communicate using an interconnect
*A SCSI task is represented by an I_T_L_Q nexus. This is where one Initiator Port talks to one Target Port, addressing one LUN and together they execute one Task).

External links

* [http://www.t10.org/ftp/t10/drafts/sam4/sam4r05.pdf SCSI archictectural model]
* [http://www.t10.org/ T10 Technical Committee] (list of all SCSI standards)