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Explain SCSI Architecture with neat diagram

SCSI-3 Client-Server Model

• SCSI-3 architecture derives its base from the client-server relationship, in which a client directs a service request to a server, which then fulfills the client’s request. In a SCSI environment, an initiator-target concept represents the client-server model. In a SCSI-3 client-server model, a particular SCSI device acts as a SCSI target device, a SCSI initiator device, or a SCSI target/initiator device.

• Each device performs the following functions:

1. SCSI initiator device: Issues a command to the SCSI target device, to perform a task. A SCSI host adaptor is an example of an initiator.

2. SCSI target device: Executes commands to perform the task received from a SCSI initiator. Typically a SCSI peripheral device acts as a target device. However, in certain implementations, the host adaptor can also be a target device.

• Following Figure displays the SCSI-3 client-server model, in which a SCSI initiator, or a client, sends a request to a SCSI target, or a server. The target performs the tasks requested and sends the output to the initiator, using the protocol service interface.

• A SCSI target device contains one or more logical units. A logical unit is an object that implements one of the device functional models as described in the SCSI command standards. The logical unit processes the commands sent by a SCSI initiator. A logical unit has two components, a device server and a task manager, as shown in Figure 5-4. The device server addresses client requests, and the task manager performs management functions.

  

SCSI Communication Model

• A SCSI communication model is comprised of three interconnecting layers as defined in the SAM-3 and is similar to the OSI seven-layer model. Lower-level layers render their services to the upper-level layers. A high-level layer communicates with a low-level layer by invoking the services that the low-level layer provides. The protocol at each layer defines the communication between peer layer entities.

  

There are three layers in the SCSI communication model:

1. SCSI application layer (SAL): This layer contains both client and serverapplications that initiate and process SCSI I/O operations using a SCSI application protocol.

2. SCSI transport protocol layer (STPL): This layer contains the services andprotocols that allow communication between an initiator and targets.

Interconnect layer: This layer facilitates data transfer between the initiatorand targets. The interconnect layer is also known as the service deliverysubsystem and comprises the services, signaling mechanisms, and inter-connects for data transfer.