[aprssig] Error checking within APRS packets
n7qvu at yahoo.com
Mon Jun 20 22:04:07 EDT 2011
>>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
> 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
> Bob, Wb4APR
> aprssig mailing list
> aprssig at tapr.org
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