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[aprssig] APRS low-power-local ALT input channel

Robert Bruninga bruninga at usna.edu
Mon Sep 27 15:12:03 UTC 2004


>>> mwrobertson at comcast.net 9/26/04 12:11:35 AM >>>
Regarding APRS alt-input on 144.99 and output on 144.39:

>I hope this never takes place on a wide scale....
>To advocate listening on one frequency and transmitting 
>on another without some kind of CD, is asking for some 
>serious kind network failures!

Not really, every single user of ANY voice repeater does it
all the time.   It is such a standard pratice that most people
appear to not even think about it. That is what the +600 or 
-600 shift is doing on your radio.  Users are transmitting in 
the *blind* on the input of the voice repeater (without 
listening there first) every time they key their radio to talk.

>I am not aware of any trackers or base stations or 
>digipeaters that do this now. 

Try about 10,000 repeaters in the ARRL directory.

>But to just blatantly come up and digipeat a low power 
>signal from another frequency with out CD on 144.390, 
>is "intentional interference". 

Depends on which part of your sentence above does
the "without CD" apply?  If you are referring to TXing
onto 144.39 "without CD" then you are missunderstanding
the concept.  No one ever said that.  You are being led 
astray by naysayers and obfuscators.  The packet like any
other packet will be introduced on 144.39 only after
the TNC there hears a clear channel by using its CD.

But if you are talking about TXing a packet "from another
freqeuncy without CD" such as the proposed  144.99
(if avail in  your area) with a TX offset of +600, then this 
is no different than keying up ANY 2m or 440 voice radio 
with a TX offset.  

Why is it ok to use TX offsets on 2m voice and
cause collisions (doubles) that last for minutes at a time
while then saying it is "bad practice" and "poor network"
design when we do it on APRS with a +600 offset?

Ah, one might claim that a repeater is full duplex real time
and so a user can tell if the channel is busy or not.  Well
this simply is not so.  Here are 4 examples of why not:

1) If it was, we would not hear people *doubling*
2) ALL modern repeaters have 1 second KERCHUNK
    delays, that delay the repeater coming up by
    about a second.  This is NO-DIFFERENTto what 
    happens with a 1 second APRS packet between the 
    input and output of an alt-input channel digipeater.
3) Many repeaters have delay lines that further delay 
    the output again, making the output not a true 
    copy of what is on the input.
4) Many repeaters use PL on the input.  Thus guaranteeing
    that they are NOT hearing other users on the input
    channel.

None of these techniques which support the concept of
operating SPLIT frequencies are "bad practice" and in fact
they benefit the "design" of the networks on which they
are implemented.   After 12 years of APRS growth it is time
to consider adding the split frequency concept to APRS 
to drastically improve local performance.

>It is not Aloha. Isn't Aloha "Transmit if you don't hear another 
>station" and the digipeaters... will sort it out?

No.  That is CSMA.  Aloha is where you cannot hear the
other users.  Which is what is the case between users
of APRS who cannot hear but a tiny-tiny percentage
of the other users direct in real time...

>Even in a busy area like Chicago, I can track several 
>5 watt trackers when they are on.

Sure but with what reliabiilty?  We frequently see un-
quantified comments in HAM radio say "it works", but 
the question is how well and compared to what?

Anyway, just some thoughts...

de WB4APR, Bob





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