[aprssig] Digi Question (viscous QRM)

Robert Bruninga bruninga at usna.edu
Wed Mar 20 08:21:31 CDT 2013


> use a fillin setup with viscous delay
> That way, the digipeater will only digipeat if no other digi hears the
packet,
> but does not add extra traffic to RF if the packet is received by
another digi.

This is bad for APRS. This is a common fallacy and only ADDS QRM to the
channel!

APRS is a congestion limited system which depends on single slot
fratricide for maximum throughput.  This means it is *expected* that all
digipeaters that hear a packet all respond *at-the-same-time* so that only
one slot-per-tier is used to propagate a packet.  This limits the load on
the network of a 2 hop packet to two time slots instead of maybe SIXTEEN
time slots on a "wait-and-see" conventional packet network.

Please understand how any attempt to use "decoded" packets as a measure of
"success" is meaningless on APRS.  Reemmber that  seeing "no decoded
packet" can mean either the channel is clear, or the channel is totally
saturated.  Completely black and white answers to the same question.   I
say again, *never* use the presence of a "decoded" packet as telling you
anything about the loading on an APRS channel!

In all likely hood, a VISCOUS digipeater only ADDS QRM to a channel
because in all likelihood, when it thought it id did not hear any other
digipeater digipeat a packet, in fact, it heard a collision between two
other digipeaters that did, and so then deciding to add its own delayed
copy to the fray only ADDS QRM instead of solving it.

No, any fill-in digi should use the same DWAIT = 0 as all other digis, so
that its packet is transmitted at the SAME time as any other possible
digipeats.  This way this fill-in does *not* add any additional QRM when
the packet was otherwise successful, but then does in fact contribute if
no other digi heard it.

Viscous digipeating can be made to work, but *only* if the decision engine
has access to the receiver singnal strength meter data so that it can make
the decision based *not* on decoded packets, but on channel-not-busy for
the entire time that a packet should havqe been there.  But since no one
takes this extra step, and since the result is *no better* than just
letting it transmit with DWAIT=0 in the first place, why bother.

APRS is not bothered that all the digis transmit at once, since most users
in range of each of those digis will in fact get a copy of the packet from
its strongest digipeater. (the goal of APRS).  Someone who is equidistant
between the two digis is the only person that might hear a collision, but
that is only a few percent of the total area covered.  And even this can
be mitigated by fixed stations by simply placing one's antenna so that one
digi is always at least 10% stronger than the other.  Then one digi will
always win and the station will get ALL packets.

Hope that helps.
Bob, Wb4aPR


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