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SCSI architectural model
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<p>The SCSI architectural model provides an abstract view of the way that <a href="/facts/SCSI/pxXulrBr">SCSI</a> devices communicate. It is intended to show how the different SCSI standards are inter-related. The main concepts and terminology of the SCSI architectural model are: </p> <ul><li>Only the externally observable behavior is defined in SCSI standards.</li> <li>The relationship between SCSI devices is described by a client-server service-delivery model. The client is called a <a href="/facts/SCSI_initiator/dAuwXvS5">SCSI initiator</a> and the server is called a <a href="/facts/SCSI_target/dAuwXvS5">SCSI target</a>.</li> <li>A SCSI domain consists of at least one SCSI device, at least one SCSI target and at least one SCSI initiator interconnected by a service delivery subsystem.</li> <li>A SCSI device has one or more SCSI ports, and a SCSI port may have an optional SCSI port identifier (SCSI ID or PID).</li> <li>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 <a href="/facts/World_Wide_Name/ffdG9eWj">World Wide Name</a>. Note that the "world" may only consist of a very small number of SCSI devices.</li> <li>A SCSI target consists of one or more logical units (LUNs), which are identified by <a href="/facts/Logical_unit_number/6FkUxbw6">logical unit numbers</a>.</li> <li>A <a href="/facts/Logical_unit_number/6FkUxbw6">LUN</a> may have dependent LUNs embedded within it. This can recur up to a maximum nesting depth of four addressable levels.</li> <li>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.</li> <li>SCSI distributed objects are considered to communicate in a three layer model: <ul><li>The highest level of abstraction is the SCSI Application Layer (SAL) where an initiator and a target are considered to communicate using <a href="/facts/SCSI_command/Ql9f9VrY">SCSI commands</a> sent via the SCSI application protocol.</li> <li>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 <a href="/facts/Fibre_Channel/C7C0yreb">Fibre Channel</a>, <a href="/facts/Serial_Storage_Architecture/lV3127yx"> SSA</a>, <a href="/facts/Serial_attached_SCSI/ekU6NmqQ">SAS</a>, <a href="/facts/USB_Attached_SCSI/e5qK8Lkq">UAS</a>, <a href="/facts/ISCSI/oxaQpYBc">iSCSI</a> and the <a href="/facts/Parallel_SCSI/7dVrf7Lc">SCSI Parallel Interface</a>.</li> <li>The lowest level is the SCSI Interconnect Layer (SIL) where an initiator and a target are considered to communicate using an interconnect. It consists of the services, signaling mechanism and interconnect subsystem used for the physical transfer of data from an initiator to a target.</li></ul></li> <li>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 (identified by Q).</li></ul>