AEA PAKRATT PK-232, Technical Reference Manual

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PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87 A-23 Page 68
A.4.6.3
A DXE shall reset the link by sending a SABM frame and starting timer T1. Upon receiving a SABM
frame from the DXE previously connected to, the receiver of a SABM frame should send a UA
frame back at the earliest opportunity, set its send and receive state variables, V(S) and V(R), to
zero and stop T1 unless it has sent a SABM or DISC itself. If the UA is correctly received by the ini-
tial DXE, it resets its send and receive state variables, V(S) and V(R), and stops timer T1. Any busy
condition that previously existed will also be cleared.
If a DM response is received, the DXE will enter the disconnected state and stop timer T1. If timer
T1 runs out before a UA or DM response frame is received, the SABM will be retransmitted and
timer T1 restarted. If timer T1 runs out N2 times, the DXE will enter the disconnected state, and
any previously existing link conditions will be cleared.
Other commands or responses received by the DXE before completion of the reset procedure will
be discarded.
A.4.6.4
One DXE may request that the other DXE reset the link by sending a DM response frame. After the
DM frame is sent, the sending DXE will then enter the disconnected state.
A.4.7 List of System Defined Parameters
A.4.7.1 Timers
To maintain the integrity of the AX.25 level 2 connection, use of these timers is recommended.
A.4.7.1.1 Acknowledgement Timer T1
The first timer, T1, is used to make sure a DXE doesn't wait forever for a response to a frame it
sends. This timer cannot be expressed in absolute time, since the time required to send frames
varies greatly with the signaling rate used at level 1. T1 should take at least twice the amount of
time it would take to send maximum length frame to the other DXE, and get the proper response
frame back from the other DXE. This would allow time for the other DXE to do some processing
before responding.
If level 2 repeaters are to be used, the value of T1 should be adjusted according to the number of
repeaters the frame is being transferred through.
A.4.7.1.2 Response Delay Timer T2
The second timer, T2, may be implemented by the DXE to specify a maximum amount of delay to
be introduced between the time an I frame is received, and the time the resulting response frame
is sent. This delay may be introduced to allow a receiving DXE to wait a short period of time to de-
termine if there is more than one frame being sent to it. If more frames are received, the DXE can
acknowledge them at once (up to seven), rather than acknowledge each individual frame. The use
of timer T2 is not mandatory, but is recommended to improve channel efficiency. Note that, on
full-duplex channels, acknowledgements should not be delayed beyond k/2 frames to achieve max-
imum throughput. The k parameter is defined in
A.4.7.4 , below.