Quantum Scalar 218FC User Manual page 84

Introduction
Your ADIC Scalar 218FC provides connectivity between Fibre Channel devices and SCSI devices. FCP and
SCSI systems employ different methods of addressing devices. This appendix provides an overview of the
addressing methods of both protocols, and the methods used to translate addresses of one type into addresses
of the other type.
SCSI Device Addressing
The SCSI bus establishes bus connections between devices. Targets on the SCSI bus may internally address
logical units (LUNs). The addressing of a specific SCSI device is represented by the BUS:TARGET:LUN
triplet. The SCSI bus allows for up to 16 devices, with up to 8 LUNs per target. The initiator uses a SCSI ID,
so typical configurations have up to 15 target IDs available for use.
BUS identification is intrinsic to the configuration: a SCSI device is attached to only one bus. Target
selection is handled by bus arbitration. Target addresses are assigned to SCSI devices directly through some
means of configuration, such as a jumper, thumb-wheel, or other setting. LUN selection is by means of the
IDENTIFY message, which occurs after target selection.
Fibre Channel Device Addressing
Fibre Channel devices connected to a fabric—any case where 2 or more ports are directly communicating via
switch or arbitrated loop—are addressed by a unique port identifier. This identifier is assigned to a port
during certain well-defined states of the FC protocol. Individual ports are allowed to arbitrate for a known,
user-defined address. If such an address is not provided, or if arbitration for a particular user address fails, the
port is assigned a unique address by the FC protocol. This address is generally not guaranteed to be the same
value between instances. Various scenarios exist where the AL_PA of a device will change, either after
power cycle or loop reconfiguration.
The FC protocol also provides a logical unit address field within command structures to provide addressing
to devices internal to a port. The FCP_CMD payload specifies an 8 byte LUN field. The FCP_CMD
element is the first element of an exchange in FC protocol. Subsequent identification of the elements of the
operation between devices is provided by the exchange identifier. This includes the FCP_XFER_RDY,
FCP_DATA, and FCP_RSP frames.
74 FC to SCSI Mapping