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-21 Page 66
A.4.4.4 Reception of Incorrect Frames
When a DXE receives a frame with an incorrect FCS, an invalid frame, or a frame with an improper
address, that frame shall be discarded.
A.4.4.5 Receiving Acknowledgement
Whenever an I or S frame is correctly received, even in a busy condition, the N(R) of the received
frame should be checked to see if it includes an acknowledgement of outstanding sent I frames.
The T1 timer should be cancelled if the received frame actually acknowledges previously unac-
knowledged frames. If the T1 timer is cancelled and there are still some frames that have been
sent that are not acknowledged, T1 should be started again. If the T1 timer runs out before an ac-
knowledgement is received, the device should proceed to the retransmission procedure in
A.4.4.9 .
A.4.4.6 Receiving Reject
Upon receiving a REJ frame, the transmitting DXE will set its send state variable to the same value
as the REJ frame's received sequence number in the control field. The DXE will then retransmit
any I frame(s) outstanding at the next available opportunity conforming to the following:
1. If the DXE is not transmitting at the time, and the channel is open, the device may commence
to retransmit the I frame(s) immediately.
2. If the DXE is operating on a full-duplex channel transmitting a UI or S frame when it receives a
REJ frame, it may finish sending the UI or S frame and then retransmit the I frame(s).
3. If the DXE is operating in a full-duplex channel transmitting another I frame when it receives a
REJ frame, it may abort the I frame it was sending and start retransmission of the requested I
frames immediately.
4. The DXE may send just the one I frame outstanding, or it may send more than the one indi-
cated if more I frames followed the first one not acknowledged, provided the total to be sent
does not exceed the flow-control window (7 frames).
If the DXE receives a REJ frame with the poll bit set, it should respond with either a RR or RNR
frame with the final bit set before retransmitting the outstanding I frame(s).
A.4.4.7 Receiving a RNR Frame
Whenever a DXE receives a RNR frame, it shall stop transmission of I frames until the busy condi-
tion has been cleared. If timer T1 runs out after the RNR was received, the waiting acknowledge-
ment procedure listed in
A.4.4.9 , below, should be performed. The poll bit may be used in con-
junction with S frames to test for a change in the condition of the busied-out DXE.
A.4.4.8 Sending a Busy Indication
Whenever a DXE enters a busy condition, it will indicate this by sending a RNR response at the
next opportunity. While the DXE is in the busy condition, it may receive and process S frames, and
if a received S frame has the P bit set to one, the DXE should send a RNR frame with the F bit set
to one at the next possible opportunity. To clear the busy condition, the DXE should send either a
RR or REJ frame with the received sequence number equal to the current receive state variable,
depending on whether the last received I frame was properly received or not.