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[aprssig] Someone's Server Mangling Packets

Andre aprs at pe1rdw.demon.nl
Wed Jan 25 14:16:34 UTC 2012


Op 25-1-2012 14:44, Bob Bruninga schreef:
> Concatenated packets:
>
> I have seen these rarely since the beginning of APRS.  I assumed they were
> my own DOS processor taking a few milliseconds break and as a result
> concatenating packets.  Since the TNC would not make such a mistake due to
> the CRC check, I assumed it was in the  APRS client.
>
> However, if we realize that almost every packet heard these days is a
> concatenation of one weaker packet overlayed with the most recent strongest
> packet, and since the ending flag is the same as a beginning flag, there is
> a finite probability that the sum of a few bit errors could result in a
> concatenated packet with a valid checksum at the end...
>
> I think I theorized this once before, and maybe someone shot it down, but
> I'd like to see the probabiltites again...
>
> Bob, Wb4APR
>
CRC is 16 bit so the probabilety if a mangled packet having the correct 
CRC is one in 2^16 or 65536.
I think it is far more likely that a TNC/PC link in converse mode drops 
a lf/cr or in simple kiss mode drops 2 FEND bytes in a row.
Also telnet links can drop characters like a TNC in converse mode.

so while it is posible it is not likely.

73 Andre PE1RDW



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