FICON (Fibre Connection) is the IBM proprietary name for the ANSI FC-SB-3 Single-Byte Command Code Sets-3 Mapping Protocol for Fibre Channel (FC) protocol.

It is a FC layer 4 protocol used to map both IBM’s antecedent (either ESCON or parallel) channel-to-control-unit cabling infrastructure and protocol onto standard FC services and infrastructure. The topology is fabric utilizing FC switches or directors. Valid rates include 1, 2, 4, and 8 Gigabit data rates at distances up to 100 km. FICON has replaced ESCON in current deployments because of FICON’s technical superiority (especially its higher performance) and lower cost.

Except for some initialization dialogue that requires stronger synchronization, FICON uses Fibre Channel class of service 3.

Each FICON channel port is capable of multiple concurrent data exchanges (currently a maximum of 32) in full duplex mode. Information for active exchanges is transferred in Fibre Channel sequences mapped as FICON Information Units (IUs) which consist of one to four Fibre Channel frames, only the first of which carries 32 bytes of FICON (FC-SB-3) mapping protocol. Each FICON exchange may transfer one or many such IUs.

FICON channels use five classes of IUs to conduct information transfers between a channel and a control unit. They are: Data, Command, Status, Control, Command and Data, and lastly Link Control. Only a channel port may send Command or Command and Data IUs, while only a control unit port may send Status IUs.

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