format identifier (SNA) (FID)

A Format Identifier Field (FID) is used in the SNA architecture to transmit encapsulated messages between SNA subareas. The format identifier supports virtual and explicit routes as well as transmission groups.

Important Format Indicators are FID0 Transmission Header( TH), which is used for communication between SNA nodes and non-SNA nodes.

The FID1 Transmission Header (TH), which is used for encapsulating messages exchanged between two subarea nodes that do not support virtual and explicit routes.

FID2 is used by the Transmission Header (TH) for communication between a subarea node and a Physical Unit 2( PU).

FID3 is used by the Transmission Header (TH) for communication between a subarea node and a Physical Unit 1 (PU) and uses logical addresses.

FID4 is used by the Transmission Header for encapsulating messages exchanged between two subarea nodes. Unlike FID1, these node hosts supported virtual and explicit routes.