[aprssig] APRS Digipeter Design Questions

Jason Winningham jdw at eng.uah.edu
Fri Apr 29 07:52:09 EDT 2005

On Apr 28, 2005, at 9:02 PM, John Hansen wrote:

> 2C.  How many digipeters should be allowed in a Beacon path (surely 8 
> is not the correct answer!).

Well, the spec says 8, and you mention later you want it available for 
emergency connected mode communications.  If you're specifying 
individual nodes in a path, 8 hops isn't all that bad.

I would think 3 would be the absolute minimum, so that you'd have room 
for a path that starts out RELAY,WIDE2-2 and ends up as 

> 3. I also have a number of questions with regard to duplicate 
> supression.
> 3A.  UIDigi does this by time interval.  While this is desirable, it 
> is much more complicated to keep track of time than to keep track of 
> the number of transmissions.
> Does anyone see a serious problem with this?

I'd rather see a clock, however rough or inaccurate (no need for an 
external clock chip or a GPS connection here, a counter would do).

If the network load is constant this would not be a problem.  If the 
load varies a great deal, then the user's guess at X may cause packets 
for fixed stations to get dropped unfairly if the network loads drops 
below a given level, or it could cause unnecessary duplicates if X is 
guessed too low or the load increases above normal.

This issue would only affect fixed stations that don't vary their 
packets between transmissions.  Since mobile (and weather) stations' 
data vary, there would be a different packet and a different CRC 
generated to keep them from getting ignored.

X is also another parameter that must be configured for each digi to 
work correctly.  Simpler is better.

> 3B.  If I do implement this as described above, what would be a 
> reasonable maximum value for X?

I haven't measured packets time-wise, but if you assume the smallest 
packet that can be transmitted is 0.5s long, and you want to keep at 
least 30s worth of dupe suppression information, then X would need to 
be 30.

> 3C.  What parts of a packet should I include in the CRC calculation?

Well, I guess it's conceivable that 2 stations could have exactly the 
same payload (2 mobile stations, especially if they're close and/or 
running position ambiguity?), so maybe originating callsign + payload??

I have a TNC-X/Digi_Ned rig currently running as a fill-in digi, if you 
need more beta testers. (:


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