[aprssig] Cross-Band Digi or Gate?

[Stephen H. Smith]

Lynn W. Deffenbaugh (Mr) wrote:
> Greetings,
>
> I'm working on more advanced capabilities for my APRSISCE/32 APRS 
> client software and am wondering what the proper approach is for 
> gating (or is it digipeating?) HF traffic onto local VHF frequencies?  
> Is this an ok thing to do, or just gate the received packets to 
> APRS-IS and be done with it?
>
> If I do cross-band them, do they go out over VHF with just used 
> callsign insertion into the path as a digipeat or does it go back out 
> as a third-party packet?
>
> Are there any good (read: current) guidance on paths for HF APRS?
HF propagation is constantly changing and erratic.  You normally will 
only have a reliable path between any two specific locations (like two 
stations trying to converse with each other) for a few hours a day. On 
the other hand, the band is open to SOMEWHERE hundreds or throusands of 
miles away virually 24/7.  Digipeating on HF (i.e. between HF stations) 
is severely discouraged since WHERE you would be digipeated to is 
completely unpredictable. 

Because of the erratic nature of digipeating on bands where propagation 
is constantly changing, the success rate to reach a given station is 
very low.   In an interactive messaging scenario, you wind up with 
endless missed acks and re-transmissions that will reduce the 
already-low 300 baud throughput of the shared channel even more.  You 
can easily have 8-10 transmissions to get a single message string from 
station A to station B on HF.   [Bear in mind that when you transmit on 
HF, you are potentially occupying the channel over HALF OF NORTH AMERICA 
at once!]

The KAM TNCs treat the crossband digipeat as a normal digi hop triggered 
by the name  "GATE" in the path.  A typical HF path  would be something 
like:

      WA8LMF > APRS,GATE,WIDE2-1    


This would use NO digis on HF, cross over to VHF at various random and 
unknown locations over an area of potentially 2000 miles radius or more 
(depending on propagation) and then do a normal one-hop home station VHF 
path in each of those locations. 


The real question, though,  is: Will the locals in these various remote 
VHF areas appreciate having random "DX" stations from hundreds (or 
thousands) of miles away suddenly appearing on their local 2-meter 
channel???

Given that HF is far more suited to one-way beaconing from remote 
locations to random fixed station igates (rather than interactive 
two-way communications between specific users; i.e. messaging) , 
probably just gating HF receive into the Internet would be more useful 
than retransmitting HF posits onto VHF.      This causes HF fixed 
stations anywhere to become part of an enormous diversity receiving 
system, ensuring that mobile posits from anywhere will always hit the 
APRS-IS regardless of propagation or band conditions.  (Since you don't 
care WHAT station injected you into the Internet system; only that you 
got to the Internet.)


See my discussion on HF APRS at:

     <http://wa8lmf.net/aprs/HF_APRS_Notes.htm>   


for more details on HF operations.  

------------------------------------------------------------------------

--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

NEW!    Universal HF/VHF/UHF Antenna Mounting System
  http://wa8lmf.net/mobile/UniversalAntMountSystem.htm

"APRS 101"  Explanation of APRS Path Selection & Digipeating
  http://wa8lmf.net/DigiPaths

Updated "Rev H" APRS            http://wa8lmf.net/aprs
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