[aprssig] Error checking within APRS packets

[Guido Trentalancia]

Hello Max !

On 19/06/2011 10:08, Max Harper wrote:
> What type of error checking does APRS have to prevent corrupt data?
>
APRS itself does not carry out any error checking. However error 
checking is done at the AX25 layer.
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).
> 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.
>
Sounds likely a software problem in the IGate software or a problem with 
the CRC control.
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.
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).
> 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.
>
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).
> Thanks Max KG4PI
>
Hope it helps. You are welcome. Keep us informed...

73,

Guido IZ6RDB
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