Exchange/SNA Manual

Operational Features
Introduction
1–10 104700 Tandem Computers Incorporated
Exchange/SNA supports the following function management headers:
Sends and receives type 1 function management headers
Receives type 3 function management headers
For information about the BIND requirements of Exchange/SNA, see Appendix C.
Function Management
Protocols
SNA type 1 function management headers (FMH1) are used to begin and end data
streams to and from the host, except for console data (commands to the host RJE
subsystem). Console data is sent to the host system without any function management
headers. If Exchange/SNA receives data from the host that does not begin with an
FMH1, the data is directed to the console subdevice.
Each data stream begins with an FMH1 in which Begin Destination Selection (BDS) is
specified. The media field in the FMH1 specifies the type of Exchange/SNA subdevice
(printer, reader, or punch) that is sending or receiving the data. The media subaddress
field specifies the individual subdevice by number. In addition to the subdevice
specification, the FMH1 also specifies the record length and type of data compression
(or compaction) that is used in the transmitted data.
An FMH1 with BDS specified does not actually start the transmission of data; rather, it
is a request to begin transmission to or from the specified subdevice. The transmission
of data does not begin until the BDS is acknowledged by the recipient.
Each data stream ends with an FMH1 in which End Destination Selection (EDS) is
specified. The EDS functions as an end-of-file indicator for Exchange/SNA and as an
end-of-job indicator for the host RJE subsystem.
For a description of type 1 function management headers, see Appendix D.