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[aprssig] conflicting APRS intructions

Robert Bruninga bruninga at usna.edu
Sat Oct 24 16:06:22 UTC 2009


> This, again, argues in favor of this 
> "viscous digipeating" concept for  
> fill-ins, as the aprx digi would LISTEN 
> before automatically (and blindly) 
> repeating every WIDE1-1 packet...

But I think that is what everyone is missing.  It does not hurt
the network one bit for this *blindly* transmitted WIDE1-1 from
a FILL-IN.  Because it is being transmitted *at*the*same*time as
any other copy of the same packet being digipeated by the Higher
and WIDER WIDEn-N's.  So it does no damage whatsoever, and more
importantly, it does it without any impact on anyone else.

> and if aprx heard the major WIDE2 digi repeating 
> the packet, it wouldn't do anything at all.   
> Granted, it might not hear the major WIDE2 
> digipeating even when it does (due to collisions), 
> but this presents a situation no worse than  
> a dumb WIDE1-1 fill-in that just digi's every 
> single WIDE1-1 it hears.

No, it is MUCH worse, because it is at a DIFFERENT TIME, and
therefore jamming someone else in a new time slot.  In effect,
it can DOUBLES the QRM potential for every packet the "visious"
digipeater does transmit.

EXISTING SYSTEM:

1) If the FILL-IN hears a local mobile AND the WIDEN-N hears the
mobile, then BOTH will transmit it at the SAME TIME (because
D-WAIT is set to 0 in all APRS networks).  The result is, the
HIGH&WIDEn-N is at a much more advnatageous position and
propogates the packet outward to ALL surrounding digis and the
FILL-IN's copy is trashed because it has no where near the range
of the HIGH&WIDEn.   = USER GETS OUT.  = OTHER USERS in the dead
zone also hear a copy.

2) If the FILL-IN hears the local mobile and the WIDEn-N does
not, then the FILL-IN digipeats it on to the WIDEn-N.  Just as
we want, and works perfectly.  = USER GETS OUT = OTHER USERS in
the black hole HEAR IT.

*** There is NO added QRM that can potentially block other users
in other time slots anymore than if the mobile was in a GOOD
area and hit the HIGH&WIDEn direct.

VISIOUS PROPOSAL:

1) Both hear  it at the same time:  HIGH&WIDEn hears it and
propogates it.  FILL-IN waits 5 seconds and hears a copy so does
not transmit.  = USER GETS OUT.  * ANOTHER USER in the same dead
zone does *not* get a copy.

1b) TWO HIGH&WIDEn's hear it at the same time as the FILL-IN.
Both HIGH&WIDEn's transmit at the same time (as designed).
FILL-IN decodes nothing.  Waits 5 seconds, and then regurgitates
a packet that has already propogated.  * FILL-IN QRMS the next
user slot.

2) If the FILL-IN hears the local mobile and the WIDEn-N does
not, then the FILL-IN naturally waits for the channel to clear
and sends it on to the WIDEn-N.  Just as we want, and works
perfectly.  = USER GETS OUT. = OTHER USERS in the blackhole HEAR
IT.

So, no matter how I look at it, the "visious" digipeater works
OK, but then adds additional  potential to detract from overally
channel reliability by potentially adding QRM packets in other
time slots that otherwise would remain clear under the present
system.

TWO-WAY FILL-IN:

The ABOVE is for a WIDE1-1 FILL-IN only.  If "VISIOUS"
digipeating were applied to a general purpose WIDEn-N to provide
dual-directional fill-in, then BY DEFNINTION, the overall QRM
level goes up by a huge factor , since incoming packets from
other digipeaters will more often than not, COLLIDE at this
repeater's input (due to all digi's having DWAIT=0) and
therefore this repeater will NOT decode them, and will
regurgitate a DUPE to a packet that was already heard by
everyone else.  Thus DOUBLING the QRM and taking up additional
time slots.

So again, no matter how I look at it, the "visious" digipeater
concept *detracts* from the overlal performance of the regional
network.  I admit that it can provide a slight advantage to a
small subset of users that are positioned into a narrow 13dB
collision range between two HIGH&WIDE digis.  But the advantage
to those few is more than offset by the overall potential for
additional dupes and time slots added to the system.

The way I see it anyway.  

The concept sounds great on first blush, but it simply overlooks
the concept of DWAIT=0 and FRATRICIDE designed into the APRS
digipeater system to make sure that all packets porpogate
OUTWARD RADIALLY at the same time in one slot, and do not then
start spreading out into adjacent slots so that you hear
muitiple ping,pong,ping,pongs.  In a proper unsaturated network
you should hear one of four possibilities:

PONG  = Your packet digipeated by all surrounding digis at once
(you decode one) and you do not hear the second HOP which is out
at the next tier outward.  This is a GOOD local APRS network.

BLAH  = Your packet digipeated by all surrounding digis (you
hear a collision) but everyone else hears it from their stronger
digi and it goes outward N HOPS.  Still a GOOD local network.

----  = You are out of range of a digi, or you collided with
someone else.

PONG,PONG,PONG = is sometimes heard when some digis are held off
by additional QRM on the channel.  But if you hear this
EVERYTIME, then the settings of the digis in your area do not
have DWAIT=0 and are threfore TRIPLING the QRM on the channel...
A BAD designed network for serving the greatest number of
users... (Though it can be a BETTER network if you have an order
of magnitude fewer users, a clear channel and want better
reliability)

PONG....PONG = Somoene has set up a visious digipeater that can
hear other digipeaters at near equal signal strength, and so it
is not hearing most collisions between those two digipeaters and
so it is doubling QRM on the channel unnecessarily.

MOBILE TIP:  By the way, what this means to mobiles is that if
you see MYPACKET of course, you got out.  But if you see a HIGH
SIGNAL strength response to your mobile Beacon, it also probably
means you got out too, but you are hearing a collision coming
back to you (as you should if you are in range of two digis).
SO don't keep beaconing until you get a MYPACKT if you are
seeing this energy in response because you ARE getting out.  If
you keep beaconing always until you see MYPACKET then you too
are adding QRM.

Enjoy.

Bob, WB4APR




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