
APPENDIX
Protocol Data Mode
2.10
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19.01.2007
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5.1.6.1 Information
frames
When data is transmitted over LAP, it is segmented into I-frames. Each I-frame is identified
by a sequence number. This number is incremented for every new I-frame. When an I-frame
is repeated, e.g. due to a detected transmission error, it retains its original sequence
number. The sequence number preserves integrity of the sequence of I-frames.
Sequence numbers may have values 0 to 7. All operations on sequence numbers are
modulo 8. E.g. if the sequence number had value 7, incrementing will result in value 0.
Each I-frame carries also an acknowledgement for the reverse direction. So the information
flow in one direction is multiplexed with the acknowledgement flow in the reverse direction.
5.1.6.2 Supervisory frames RR, RNR and REJ
The receiver informs the transmitter about its conditions (see section 5.1.7.2) by using RR-,
RNR- and REJ-frames.
A RR-frame indicates that the self busy condition is cleared.
A RNR-frame indicates that the self busy condition is set.
A REJ-frame indicates that the reject exception condition changes from cleared to set. It also
indicates that the self busy condition is cleared.
5.1.6.3 Supervisory frames SABM and UA
The SABM command is used for the sole purpose of link re-establishment in certain error
recovery situations.
The UA response is used for the sole purpose of responding to a SABM command.