User's Manual

Chapter 3 27
Using HDLC/LAP-D Protocol
General Protocol Processing
GeneralProtocolProcessing
Application Message Headers
Allapplicationdataistransmitted(orreceived)astheI-fieldofeitheran
information transfer frame or an unnumbered information frame. All
headers and/or trailers are supplied by the protocol for outgoing
messages and removed by the protocol for incoming messages. Only the
I-field d ata to be transferred is dealt with by the application program,
which need not even be awa re that HDLC/LAP-D is being used for the
communications.
Unnumbered Information Frames
HDLC/LAP-D can transmit and receive I-fields within either numbered
information frames (I frames) or unnumbered information frames (UI
frames). To transmit and receive UI frames, a terminal must be defined
that has the UI bit set in the terminal’s Poll Word; see “Poll Word” on
page 47. All data sent to a UI terminal will be sent on the configured
SAPI/TEI as a UI frame. When a UI frame is received, and there is an
enabled UI terminal configured with a matching SAPI/TEI, the I-field of
the UI frame will be sent to that UI terminal.
To transmit and receive I frames, multiple frame acknowledged
operation must be established. Multiple frame acknowledged operation
is handled by terminals configured without the UI b it set. T hese
terminals handle all frames that are not associated with a configured
and enabled UI terminal.
The UI and non-UI terminals are capable of coexisting with each other
on a given SAPI/TEI as well as being able to independently exist. In
other words, there may be no terminals, just a UI terminal, just a non-UI
terminal or both a UI a nd non-UI terminal configured on a single
SAPI/TEI.