[aprssig] Error checking within APRS packets

Guido Trentalancia iz6rdb at trentalancia.com
Wed Jun 22 17:23:55 EDT 2011

On 20/06/2011 20:47, Jason KG4WSV wrote:
> On Mon, Jun 20, 2011 at 1:38 PM, Gregg Wonderly<gregg at wonderly.org>  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.
> I am not convinced this can happen.  AX.25 includes a frame delimiter.
>   In order for this scenario to occur, the second packet's start frame
> delimiter couldn't be present at the TNC, else it would simply signal
> the (premature) end if the first interrupted packet to the TNC.  Once
> this vanishingly unlikely event happens, then the checksum would have
> to coincidentally match.
> I haven't done the math, but my guess is you're more likely to be
> struck by lightning than see this occur on VHF.

Should be about 1/2^L, where L is the length of the CRC in bits. 
Therefore for 16 bits should be 1/65536=15*10^-6=1.5*10^-5.

It is low.

Also, for this specific case, we shall not forget that the original 
reporter explicitly mentioned that it occurs only within the CALLSIGN 
which further excludes that possibility.

> -Jason
> kg4wsv

Guido, IZ6RDB

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