transmission header (SNA) (TH)

InSNA networks, the Transmission Header (TH) is one of three data fields of a Path Information Unit( PIU). It is the header that is added to the Path Control Layer (Layer 3) of the Basic Information Unit ( BIU) and is required by the Network Addressable Unit( NAU) to route the message through the SNA network.

Structure of the Path Information Unit (PIU)

The Transmission Header is used to route the message units through the network. It therefore contains the routing information for the transport network. The SNA architecture knows different TH formats and identifies the different header formats with the Format Identifier( FID). The transmission header has different lengths depending on the FID type. The path control( PC) uses the different FID types to route the data between the nodes.