[aprssig] What sets the max packet length for APRS

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Sat Nov 10 11:28:07 EST 2012

As I understand the history, there are some radios that won't even 
display the 67 characters.  There are/were TNC limits on how long UI 
packets could be overall.  Some TNCs can do longer, and the APRS-IS 
carries a much longer limit. But the specification tells you what should 
work.  Exceeding those limits may result in unexpected behavior, but as 
you've observed, there are lots of packets out there that seem quite long.

BTW, after the colon is the payload.  That particular packet has a 
timestamp, position information, weather, and a final comment.

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

On 11/10/2012 11:21 AM, Bryan Hoyer wrote:
> APRS1.01 states 67 characters for a message and up to 43 for a 
> comment, yet I see packets twice that long on APRS.FI. This ones 109 
> after the :
> *KA8OAD-1 <http://aprs.fi/?c=raw&limit=&call=KA8OAD-1>*>APU25N,K8YSE-1 
> <http://aprs.fi/?c=raw&limit=&call=K8YSE-1>,WIDE1,K8UI-1 
> <http://aprs.fi/?c=raw&limit=&call=K8UI-1>,WIDE2*,qAR,K8LI-1 
> <http://aprs.fi/?c=raw&limit=&call=K8LI-1>:@101613z4108.09N/08136.25W_190/004g005t057r000p020P020b10220h51FWX_http://pages.sssnet.com/kend 
> <http://pages.sssnet.com/kend> Fairlawn,OH
> AX.25 says 256 bytes for the UI payload
> A Tweet is 140 characters
> An SMS is 160 characters
> Is it just the display limit of some radios?
> Just exploring the capabilities and limitations of APRS
> Thanks!
> Bryan Hoyer - K7UDR
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