[aprssig] Error checking within APRS packets
bruninga at usna.edu
Mon Jun 20 21:33:39 EDT 2011
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.
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