fc4

FC-4 is the upper-layer protocol mapping in the Fibre Channel (FC) protocol architecture. It defines how higher-level data protocols are transported over Fibre Channel fabrics by mapping their data units into FC payloads and translating them back when received. FC-4 sits above the FC-0 physical layer, FC-1 encoding, and FC-2 framing and link management, and below the specific upper-layer protocols it supports.

The primary function of FC-4 is to provide the interface between the Fibre Channel transport and the data protocols that use it. It does not govern the low-level transport mechanisms themselves; instead, it specifies how protocol data units are packaged for transmission over FC and how FC payloads are interpreted by the corresponding upper-layer protocols.

Protocols commonly associated with FC-4 include SCSI, mapped as SCSI-Open (FCP or Fibre Channel Protocol for SCSI), and FICON for IBM mainframe environments. In addition, FC-4 can accommodate other protocols that require Fibre Channel transport, allowing diverse storage and data services to operate over FC networks. The most widely deployed FC-4 mapping is the one used for SCSI-based storage access, which has driven much of Fibre Channel’s adoption in storage area networks.

Governance and evolution: FC-4 has been part of the Fibre Channel standards since the technology’s early development and has evolved as new upper-layer protocols were defined. It is standardized and maintained within the Fibre Channel standard family, with involvement from standards bodies and industry groups. Today, FC-4 remains a foundational element of Fibre Channel, enabling standardized protocol interoperability across storage and data services implemented over FC networks.