ATTO Technology 2200R/D, 3200R, 2100R manual SCSI Architecture Model SAM-2 Logical Unit Addressing

ATTO Technology, Inc. ATTO FibreBridge Installation & Operation Manual

SCSI Target Identifier

The SCSI Target Identifier indicates the SCSI ID assigned to the target. Each device on the same physical SCSI bus must be assigned a unique SCSI ID. A Wide (16-bit) SCSI bus will support a maximum of 16 SCSI IDs (including the SCSI ID assigned to the FibreBridge).

SCSI Logical Unit Number

The SCSI Logical Unit Number indicates the logical unit being addressed within the specified target.

SCSI Architecture Model (SAM-2) Logical Unit Addressing

The SCSI Architecture Model – 2 (SAM-2) specification defines a reference model that specifies common behaviors for SCSI devices, and an abstract structure that is generic to all SCSI-3 I/O system implementations. The structure for Logical Unit Addressing is detailed below.

What is a Logical Unit?

A logical unit is the entity that executes SCSI Commands sent by SCSI initiators. A logical unit is addressed by a Logical Unit Number (LUN). A LUN is represented by a 64-bit number in Fibre Channel and a 5-bit number in parallel SCSI. The FibreBridge will map between an 8-byte (64-bit) Fibre Channel LUN and a parallel SCSI LUN.

Hierarchical Addressing Model

SAM-2 defines the concept of hierarchical LUN addressing and the four addressing levels that are supported. The hierarchical LUN address is shown in the table below.

Bit	7	6	5		4	3	2	1	0
Byte
0	(MSB)			First level addressing field
1									(LSB)
2	(MSB)			Second level addressing field
3									(LSB)
4	(MSB)			Third level addressing field
5									(LSB)
6	(MSB)			Fourth level addressing field
7									(LSB)

The FibreBridge implements a single level of hierarchical LUN addressing by using the 16-bit “First Level Addressing Field”. The contents of the second, third and fourth level addressing fields are ignored by the FibreBridge.

Addressing Field Format

Each of the four addressing level fields is structured as a 16-bit value with the following format.

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