Using HDLC/LAP-D Protocol
General Protocol Processing
General Protocol Processing
Application Message Headers
All application data is transmitted (or received) as the I-field of either an 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 data to be transferred is dealt with by the application program, which need not even be aware 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 bit set. These 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 and non-UI terminal configured on a single SAPI/TEI.