[aprssig] Error checking within APRS packets

[Jerry Wyatt]

>>SO then the -next- byte is assumed to be the FCS and it has a one-in-256 chance 
>>of being correct.

Sorry Bob but I must disagree. A CRC-16 has 16 bits or two bytes or a max value 
of FFFF. This gives a one-in-65k chance.

Jerry - N7QVU


----- Original Message ----
> From: Bob Bruninga <bruninga at usna.edu>
> To: TAPR APRS Mailing List <aprssig at tapr.org>
> Sent: Mon, June 20, 2011 6:33:39 PM
> Subject: Re: [aprssig] Error checking within APRS packets
> 
> This is correct, but not entirely the conclusions....
> 
> >> AX.25 packets... not only carry a CRC-16
> >> but also... framing bits that positively 
> >> identify the start and end of a packet. 
> 
> True... BUT The starting and ending FRAME bytes are IDENTICAL.
> 
> >> So, if a TNC is receiving a packet and another 
> >> packet stomps over it, the framing is the first 
> >> line of defense to drop the first packet and restart
> >> receiving with the new packet. When the start/end
> >> framing sequences have been identified, 
> >> then the CRC-16 is used to ensure that everything 
> >> within the frame is good...
> 
> Yes, but since the starting FRAME byte of the second packet looks identical to 
>an "ending" FRAME byte, then the first packet appears complete.  SO then the 
>-next- byte is assumed to be the FCS and it has a one-in-256 chance of being 
>correct.
> 
> Thus this first packet is truncated but can (rarely) appear valid.  Its been 30 
>years since I looked at the details and I may not have this exactly correct, but 
>I'm pretty sure that there is no difference between the starting and ending FRAM 
>byte.
> 
> Bob, Wb4APR
> 
> 
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