APPENDIX
Protocol Data Mode
104
2.10
•
19.01.2007
5.1.7.6 Receiver
Procedures
A)
Receiving I-frames
When an I-frame is received the following procedure applies:
If the self busy condition is set, the frame is discarded. Otherwise the receiver checks if N(S)
is equal to V(R). In this case the frame is accepted and its content is passed to the higher
layer. The reject exception condition is cleared and V(R) is incremented.
If N(S) is different from V(R) (N(S) sequence error) the frame is discarded and the reject
exception condition is set.
B)
Sending acknowledgements
When the last I-frame was accepted the receiver need not acknowledge it immediately. If
there is data to be sent in the reverse direction the acknowledgement may be carried in an I-
frame. Otherwise the acknowledgement is sent either as RR-response (if the self busy
condition is cleared) or as RNR-response (if it is set).
When the last I-frame was discarded due to N(S) sequence error the receiver must
acknowledge it immediately. If the reject exception condition is newly set, it sends a REJ-
response. If the reject exception condition was already set before, it sends a RR-response.
In any acknowledgement the receiver sends its actual value of V(R), after possible
incrementing, in the N(R) field.
C)
Setting and clearing the self busy condition
If the higher layer is ready to accept data from the LAP receiver, it clears the self busy
condition. The receiver informs the sender by sending a RR-command frame.
If the higher layer is busy, i.e. does not accept data from the LAP receiver, it sets the self
busy condition. The receiver informs the sender by sending a RNR-command frame.
D)
Responding to polling requests
When the receiver receives a RR- or RNR-command frame with P bit set, it immediately
respond to it with a RR-response (if the self busy condition is cleared) or a RNR-response (if
it is set) which has the F bit set.