AEA PAKRATT PK-232, Technical Reference Manual

Page: 60 / 101

PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87 A-15 Page 60
A.3.4.3.6 Unnumbered Information (UI) Frame
The Unnumbered Information frame contains PID and information fields and is used to pass infor-
mation along the link outside the normal information controls. This allows information fields to go
back and forth on the link bypassing flow control. Since these frames are not acknowledgeable, if
one gets obliterated, there is no way to recover it. A received UI frame with the P bit set shall
cause a response to be transmitted. This response shall be a DM frame when in the disconnected
state or a RR (or RNR, if appropriate) frame in the information transfer state.
A.3.5 Link Error Reporting and Recovery
There are several link-layer errors that are recoverable without terminating the connection. These
error situations may occur as a result of malfunctions within the DXE, or if transmission errors oc-
cur.
A.3.5.1 DXE Busy Condition
When a DXE becomes temporarily unable to receive I frames, such as when receive buffers are
full, it will send a Receive Not Ready (RNR) frame. This informs the other DXE that this DXE can-
not handle any more I frames at the moment. This condition is usually cleared by the sending of a
UA, RR, REJ, or SABM command frame.
A.3.5.2 Send Sequence Number Error
If the send sequence number, N(S), of an otherwise error- free received frame does not match the
receive state variable, V(R), a send sequence error has occurred, and the information field will be
discarded. The receiver will not acknowledge this frame, or any other I frames, until N(S) matches
V(R).
The control field of the erroneous I frame(s) will be accepted so that link supervisory functions
such as checking the P/F bit can still be performed. Because of this updating, the retransmitted I
frame may have an updated P bit and N(R).
A.3.5.3 Reject (REJ) Recovery
REJ is used to request a retransmission of I frames following the detection of a N(S) sequence er-
ror. Only one outstanding "sent REJ" condition is allowed at a time. This condition is cleared when
the requested I frame has been received.
A DXE receiving the REJ command will clear the condition by resending all outstanding I frames
(up to the window size), starting with the one indicated in N(R) of the REJ frame.
A.3.5.4 Time-out Error Recovery
A.3.5.4.1 T1 Timer Recovery
If a DXE, due to a transmission error, does not receive (or receives and discards) a single I frame
or the last I frame in a sequence of I frames, it will not detect a send-sequence- number error,
and therefore will not transmit a REJ. The DXE which transmitted the unacknowledged I frame(s)
shall, following the completion of time-out period T1, take appropriate recovery action to deter-
mine when I frame retransmission should begin as described in
A.4.4.9 , below. This condition is
cleared by the reception of an acknowledgement for the sent frame(s), or by the link being reset.
See
A.4.6 .