AEA PK-232 Technical Reference Manual page 65

PK-232 TECHNICAL MANUAL
A.4.4.1 Sending I Frames
Whenever a DXE has an I frame to transmit, it will send the I frame with N(S) of the control field
equal to its current send state variable V(S). Once the I frame is sent, the send state variable is in-
cremented by one. If timer T1 is not running, it should be started. If timer T1 is running, it should
be restarted.
The DXE should not transmit any more I frames if its send state variable equals the last received
N(R) from the other side of the link plus seven. If it were to send more I frames, the flow control
window would be exceed, and errors could result.
If a DXE is in a busy condition, it may still send I frames as long as the other device is not also
busy.
If a DXE is in the frame-rejection mode, it will stop sending I frames.
A.4.4.2 Receiving I Frames
A.4.4.2.1
If a DXE receives a valid I frame (one with a correct FCS and whose send sequence number equals
the receiver's receive state variable) and is not in the busy condition, it will accept the received I
frame, increment its receive state variable, and act in one of the following manners:
1. If it has an I frame to send, that I frame may be sent with the transmitted N(R) equal to its re-
ceive state variable V(R) (thus acknowledging the received frame). Alternately, the device may
send a RR frame with N(R) equal to V(R), and then send the I frame.
2. If there are no outstanding I frames, the receiving device will send a RR frame with N(R) equal
to V(R). The receiving DXE may wait a small period of time before sending the RR frame to be
sure additional I frames are not being transmitted.
A.4.4.2.2
If the DXE is in a busy condition, it may ignore any received I frames without reporting this condi-
tion other than repeating the indication of the busy condition.
If a busy condition exists, the DXE receiving the busy condition indication should poll the sender of
the busy indication periodically until the busy condition disappears.
A DXE may poll the busy DXE periodically with RR or RNR frames with the P bit set to one.
The reception of I frames that contain zero-length information fields shall be reported to the next
level but no information field will be transferred.
A.4.4.3 Reception of Out of Sequence Frames
When an I frame is received with a correct FCS, but its send sequence number, N(S), does not
match the current receiver's receive state variable, the frame should be discarded. A REJ frame
shall be sent with a receive sequence number equal to one higher (modulo 8) than the last cor-
rectly received I frame if an uncleared N(S) sequence error condition has not been previously es-
tablished. The received state variable and poll bit of the discarded frame should be checked and
acted upon, if necessary, before discarding the frame.
PK232TM Rev. A 5/87
APPENDIX A – AX.25 LEVEL 2 PROTOCOL
A-20 Page 65