AEA PAKRATT PK-232, Technical Reference Manual

Page: 67 / 101

PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87 A-22 Page 67
A.4.4.9 Waiting Acknowledgement
If timer T1 runs out waiting the acknowledgement from the other DXE for an I frame transmitted,
the DXE will restart timer T1 and transmit an appropriate supervisory command frame (RR or RNR)
with the P bit set. If the DXE receives correctly a supervisory response frame with the F bit set and
with an N(R) within the range from the last N(R) received to the last N(S) sent plus one, the DXE
will restart timer T1 and will set its send state variable V(S) to the received N(R). It may then re-
sume with I frame transmission or retransmission, as appropriate. If, on the other hand, the DXE
receives correctly a supervisory response frame with the F bit not set, or an I frame or supervisory
command frame, and with an N(R) within the range from the last N(R) received to the last N(S)
sent plus one, the DXE will not restart timer T1, but will use the received N(R) as an indication of
acknowledgement of transmitted I frames up to and including I frame numbered N(R)-1.
If timer T1 runs out before a supervisory response frame with the F bit set is received, the DXE
will retransmit an appropriate supervisory command frame (RR or RNR) with the P bit set. After N2
attempts to get a supervisory response frame with the F bit set from the other DXE, the DXE will
initiate a link resetting procedure as described in
A.4.6 , below.
A.4.5 Frame Rejection Conditions
A DXE shall initiate the frame-reset procedure when a frame is received with the correct FCS and
address field during the information-transfer state with one or more of the conditions in
A.3.4.3.3 ,
above.
Under these conditions, the DXE will ask the other DXE to reset the link by transmitting a FRMR re-
sponse as outlined in
A.4.6.3 , below.
After sending the FRMR frame, the sending DXE will enter the frame reject condition. This condi-
tion is cleared when the DXE that sent the FRMR frame receives a SABM or DISC command, or a
DM response frame. Any other command received while the DXE is in the frame reject state will
cause another FRMR to be sent out with the same information field as originally sent.
In the frame rejection condition, additional I frames will not be transmitted, and received I frames
and S frames will be discarded by the DXE.
The DXE that sent the FRMR frame shall start the T1 timer when the FRMR is sent. If no SABM or
DISC frame is received before the timer runs out, the FRMR frame shall be retransmitted, and the
T1 timer restarted as described in the waiting acknowledgement section (
A.4.4.9 ) above. If the
FRMR is sent N2 times without success, the link shall be reset.
A.4.6 Resetting Procedure
A.4.6.1
The resetting procedure is used to initialize both directions of data flow after a nonrecoverable er-
ror has occurred. This resetting procedure is used in the information- transfer state of an AX.25
link only.
A.4.6.2
A DXE shall initiate a reset procedure whenever it receives an unexpected UA response frame or
an unsolicited response frame with the F bit set to one. A DXE may also initiate the reset proce-
dure upon receipt of a FRMR frame. Alternatively, the DXE may respond to a FRMR by terminating
the connection with a DISC frame.