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Hello Max !<br>
<br>
On 19/06/2011 10:08, Max Harper wrote:
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<div>What type of error checking does APRS have to prevent
corrupt data? </div>
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APRS itself does not carry out any error checking. However error
checking is done at the AX25 layer.<br>
Such error control is based upon the Cyclic Redundancy Check (CRC)
defined in ISO 3309 and by CCITT (sometimes named CRC-CCITT or
CRC-16-CCITT).<br>
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<div>There is an Igate in my area that changes the
callsign of some of the packets that it gates. The Igate
is KG4NVN-13 and I notified him a few days ago about it
via email and was told that he would take it off line to
check it out. I don't know if anything has been done to
try and fix it, but it's still changing the call sign of
some of the Igated packets. He is running UI-View and a
D-700.</div>
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Sounds likely a software problem in the IGate software or a problem
with the CRC control.<br>
So, perhaps the TNC receives (from RF) correct packets (CRC
successful) and it passes them to the IGate software but then the
IGate software introduces errors in the packets due to a software
bug or misconfiguration.<br>
Or otherwise, the TNC receives erroneous packets (from RF), it
should verify them using the CRC but because of some bug or
misconfiguration (in the TNC this time) the CRC check is not
performed (or it's "performed" but always succeeds even for
erroneous packets, i.e. it's not performed correctly).<br>
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<div>Of the packets that I have looked at, it always seems
to be just the call sign that has letters changed, the
rest of the packet looked OK.</div>
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The fact that you are reporting the problem happening on several
packets which are supposedly handled during the same day (rather
than very rarely), in my opinion excludes the hypothesis presented
by Bob (of a second packet stepping on top of the original packet,
producing a "combined packet" which would begin with the first
original packets and then would continue with the second packet and
which coincidentally would have the same CRC as the original
packet).<br>
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<div>Thanks Max KG4PI</div>
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Hope it helps. You are welcome. Keep us informed...<br>
<br>
73,<br>
<br>
Guido IZ6RDB<br>
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