Application Message Headers |
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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 |
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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”. 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.
Timeout Processing |
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Frame timeouts are set in the Select configuration word. The
T200 timer defines the period within which a response to a Command
frame must be received across the HDLC/LAP-D link. Should this timer
expire before the expected response is received, then the Command
frame will be re-transmitted up to a pre-determined maximum number
of times. The maximum number of re-transmissions (N200) is also
set by the Select configuration word. If the timer expires on that
number of successive occasions, then the LAP-D link will be reset.
An additional timer, known as T203 is used to control the
maximum amount of time that can pass without any frames being exchanged.
It is configured in the first Option word and defaults to being T200*N200*100/16.
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