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EA2CMN > AX25 01.10.94 10:17l 190 Lines 8422 Bytes #-11687 (0) @ EU
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Subj: AX.25 Doc : Part 9/9 End
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Sent: 940927/1459Z @:EA2CMN.EAZ.ESP.EU #:41780 [Zaragoza] Bid:41780_EA2CMN FBB5
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A : AX25@EU
AX25.DOC PART 9/9
AX.25 Amateur Packet-Radio Link-Layer Protocol
Version 2.0 October 1984
2.4.4.8 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.
2.4.4.9 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
resume 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 2.4.6, below.
2.4.5 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 2.3.4.3.3, above.
Under these conditions, the DXE will ask the other DXE to
reset the link by transmitting a FRMR response as outlined in
2.4.6.3, below.
After sending the FRMR frame, the sending DXE will enter
the frame reject condition. This condition 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 (2.4.4.9) above. If the FRMR is sent N2
times without success, the link shall be reset.
2.4.6 Resetting Procedure
2.4.6.1
The resetting procedure is used to initialize both
directions of data flow after a nonrecoverable error has
occurred. This resetting procedure is used in the information-
transfer state of an AX.25 link only.
2.4.6.2
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
procedure upon receipt of a FRMR frame. Alternatively, the DXE
may respond to a FRMR by terminating the connection with a DISC
frame.
2.4.6.3
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 initial 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.
2.4.6.4
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.
2.4.7 List of System Defined Parameters
2.4.7.1 Timers
To maintain the integrity of the AX.25 level 2
connection, use of these timers is recommended.
2.4.7.1.1 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.
2.4.7.1.2 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 determine 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 maximum throughput. The
k parameter is defined in 2.4.7.4, below.
2.4.7.1.3 Inactive Link Timer T3
The third timer, T3, is used whenever T1 isn't running to
maintain link integrity. It is recommended that whenever there
are no outstanding unacknowledged I frames or P-bit frames
(during the information-transfer state), a RR or RNR frame with
the P bit set to one be sent every T3 time units to query the
status of the other DXE. The period of T3 is locally defined,
and depends greatly on level 1 operation. T3 should be greater
than T1, and may be very large on channels of high integrity.
2.4.7.2 Maximum Number of Retries (N2)
The maximum number of retries is used in conjunction with
the T1 timer.
2.4.7.3 Maximum Number of Octets in an I Field (N1)
The maximum number of octets allowed in the I field will
be 256. There shall also be an integral number of octets.
2.4.7.4 Maximum Number of I Frames Outstanding (k)
The maximum number of outstanding I frames at a time is
seven.
END AX25.DOC PART 9/9
----------------------
73's Nacho EA2CMN @EA2CMN.EAZ.ESP.EU
QTH: Zaragoza (Spain)
[27-Sep-1994 - 15:58]
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