Mapping is a set of rules that is defined to move information from the Upper Level Protocol interfaces to the lower Fibre Channel levels. the ANSI SCSI committee and the Fibre Channel committees are currently defining these rules.
Currently the rules provide for transforming information units into Fibre Channel sequences and exchanges and back again. When fully developed, these mapping rules are intended to provide clear mapping instructions between the Upper Level Protocol (ULP) and the FC-3 and FC-2 levels to enhance interoperability between applications.
To send data, the FC-4 level takes a unit of information (this is the payload in a frame and is the actual data that is being transmitted, it is referred to as an Information Unit (IU)) from a ULP and transforms it into sequences for the FC-3 and FC-2 levels. To receive data, the FC-4 level takes a sequence from the FC-3 and FC-2 levels and transforms it into an IU for the ULP.