[aprssig] Error checking within APRS packets

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Mon Jun 20 17:32:01 EDT 2011

On 6/20/2011 2:38 PM, Gregg Wonderly wrote:
> Do we have any real examples of how the quiet TX 1/2 through packet 
> followed by loud TX complete packet can result in a "packet" too long 
> that would corrupt internal TNC buffers?
> Will the start of the "loud" packet result in an end of packet 
> recognition of the "quiet TX" packet.

AX.25 packets on the air not only carry a CRC-16 (which is the error 
checking that PASSALL ignores), but also carry framing bits that 
positively identify the start and end of a packet.  "Bit-stuffing" is 
used to ensure that a framing bit sequence doesn't occur within a 
packet.  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 (hence the name FCS for 
Frame Check Sequence).

AX.25 bit streams are documented for those that really want to know the 
nitty gritty details.  APRS is transmitted as UI AX.25 packet, so ALL of 
the AX.25 packet framing and error checking applies.  APRS doesn't use 
any of the connection-oriented AX.25 packet types, but only the single 
UI packet type.

> How does all of this play out to indicate that there are failure modes 
> that we've seen but not been able to identify the cause of?

If you are observing these "corrupted" and/or "concatentated" packets 
directly on RF, then this discussion makes sense.  If you are seeing 
apparent concatenations of two packets on APRS-IS, there seems to be a 
server out there at times that has been doing this for over a month 
now.  Those concatenated packets are happening on the -IS only and are 
not actually going over RF unless a message packet happens to get 
concatenated which was my first discovery of those particular errors.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

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