APPENDIX
Protocol Data Mode
102
2.10
•
19.01.2007
5.1.7.3 Timers
LAP requires two timers:
•
the LAP-establish timer DLU.02 with timeout period of 2.0 seconds
•
the retransmission timer DLU.04 with timeout period of 1.0 seconds
Since only one timer is active at a time, an implementation may use a single instance of a
timer object to realise both timers.
The following procedures apply to timers:
•
When a timer is started, it begins running until it is stopped or it expires.
•
When a running timer is again started, this has no effect, i.e. the timer carries on running.
•
When a timer is stopped it is automatically reset. When it is then started again, the full
timeout period applies.
•
Restarting a timer is equivalent to stopping it and immediately starting it again.
When a timer expires it generates an event to the LAP state machine.
5.1.7.4 Sequence
variables
While in active state and while both the Peer Busy and the Timer Recovery conditions are
cleared, the sender performs multiple frame operation.
LAP labels each I-frame with a sequence number (see section 5.1.6.1). The sequence
number is also used in acknowledgements.
The sender maintains two sequence variables V(S) and V(A). The receiver maintains a
sequence variable V(R).
•
V(S) contains the sequence number of the next frame to be transmitted.
•
V(A) contains the sequence number of the next frame to be acknowledged.
•
V(R) contains the sequence number of the next frame to be received.
All sequence variables lie within the range 0 to 7. All operations on sequence variables are
modulo 8.
The sequence of I-frames can be separated into three sections:
•
Frames which have been sent and an acknowledgement was received
•
Frames which have been sent but no acknowledgement has bee received yet (pending
frames).
•
Frames which shall be sent
A sender may transmit 8
≥
k
≥
1 I-frames before it requires an acknowledgement from the
receiver.
The maximum number of pending I-frames k is an implementation constant of the sender.
The firmware uses k=4. It is recommended that the LAP implementation on the host also
uses k=4.