[aprssig] Error checking within APRS packets

[Guido Trentalancia]

Hello Keith !

On 19/06/2011 20:10, Keith VE7GDH wrote:
> Guido IZ6RDB wrote...
>
>> APRS itself does not carry out any error checking. However error 
>> checking is done at the AX25 layer.
>
> Right.
>
>> Sounds likely a software problem in the IGate software or a
>> problem with the CRC control.
>
> Ah, but the software is UI-View. If it was a bug in the software, 
> there would be thousands of stations showing these symptoms.
I have never used UI-View, therefore my answer might not be the most 
appropriate.

However, consider that:
- UI-View appears to me as currently unmaintained
- there should be several versions available
- in a certain sense (even if there was only one version available) each 
instance of UI-View is different, in that each ham has a different 
environment, a different configuration and a different TNC connected

And "software problems" include (software) misconfiguration problems.

>> 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).
>
> With all due respect, I would differ from Bob's theory.
No way, that can't be that case. That would happen very sporadically 
(provided that CRC is working as expected, which is a pre-requisite for 
saying "I am running AX25 and on top of that I am running APRS").
> Although unlikely, I might go along with a loose serial cable 
> introducing glitches,
> but I would place my money on the TNC, either with PASSALL
> turned on or RF getting into the TNC and doing something to the
> data after the CRC checking.
I am not sure I can get your point fully here. However, as far as I 
remember PASSALL is an option of the Kenwood TNCs that disables CRC'ing 
(and thus passes everything which can be demodulated from RF to serial 
baseband).

If the operator has any such option enabled, then that is a 
misconfiguration. He/she is not operating in conformance with the AX25 
protocol (nor with the APRS protocol).
> Hopefully the iGate operator will be alerted and he will look into it.
Yes. I think the original message stated that the operator had been 
alerted already. It should be very easy to fix.
> 73 es cul - Keith VE7GDH
73,

Guido IZ6RDB