
APPENDIX
Protocol Data Mode
2.10
•
19.01.2007
103
5.1.7.5 Sender
procedures
A)
Sending I-frames
When an I-frame is sent, its sequence number (i.e. the value of V(S)) is transmitted in the
N(S) field of the control byte. V(S) is incremented directly after the transmission. Timer
DLU.04 is started.
B)
Receiving acknowledgements
Acknowledgements are contained in the N(R) field, which is included in any received I-, RR-,
RNR- or REJ-frame. When an acknowledgement is received it is first checked whether N(R)
is valid, i.e. V(A)
≤
N(R)
≤
V(S) modulo 8. Reception of an invalid N(R) is a severe error
which can only be recovered through a link re-establishment. The following procedure
applies to valid N(R) only.
If N(R) = V(A) then N(R) does not acknowledge any new frames and nothing happens.
If N(R) > V(A) then N(R) acknowledges all frames with sequence numbers up to but not
including N(R). V(A) is set to N(R) and timer DLU.04 is stopped.
If N(R) < V(S) then there are still pending frames left. In this case DLU.04 is started again.
C)
Reacting on flow control
A RNR-frame from the receiver indicates to the sender that it must stop transmitting I-
frames. The sender sets the peer busy condition and stops transmitting I-frames. However it
may still send supervisory frames. Upon entering the peer busy condition the sender starts
timer DLU.04 regardless the processing of the acknowledgement.
A RR- or REJ-frame from the receiver indicates to the sender that it may resume sending I-
frames. The peer busy condition is cleared. The timer DLU.04 is treated according the
processing of the acknowledgement.
D)
Repeating I-frames
A REJ-frame from the receiver indicates to the sender that it must repeat all I-frames with
sequence numbers from and including the received value N(R). The sender sets its value
V(S) to N(R) and repeats the I-frames accordingly.
E)
Timer
Expiry
When timer DLU.04 expires, the timer recovery condition is set. The sender sends an
appropriate supervisory frame (RR- or RNR-frame depending on the self busy condition of its
own receiver) with the P bit set. It then restarts timer DLU.04.
While in timer recovery condition the sender does not transmit I-frames, but waits for a
supervisory response frame with the F bit set. Any acknowledgement with F bit cleared will
only be used to update V(A) but does not effect the timer.
When a supervisory frame with F bit set is received, this clears the timer recovery condition.
V(A) is set to N(R). Also V(S) is set to N(R) which may imply repetition of I-frames.
When timer DLU.04 expires again while the sender is already in timer recovery condition, a
repetition count variable RC is incremented and the timer is restarted. If RC exceeds N250
(value: 3), this is treated as a severe error situation and the link is re-established.