[Xastir] Feature bloat?

[Henk de Groot]

Hello Steve,

At 09:49 15-6-03 -0400, Steve Dimse wrote:
>Actually, it'd probably be better to do this on a packet by packet basis...if
>any particular packet is heard but not digied, then digi it. I think digi-ned
>has this capability, but not sure.

No, it has not. This has been suggested a number of times but the problem 
is, I don't know how to do it! There is no solid way to detect if the 
nearby digi digipeated the signal since its station call is not included in 
the packet most of the time. For example:

Assume I only receive K4HG>APRS,RELAY*,WIDE3-2 but nothing else. It does 
not tell me much. The backup digi may have missed the original packet, 
which is likely if the backup has a smaller footprint than the main-site. 
Our backup can not see if it got this packet from a far-away digi or from 
the main-digi since there is no identification which station altered 
WIDE3-3 into WIDE3-2.

If the backup digi has the same range as the main-digi then I could receive 
K4HG>APRS,RELAY*,WIDE3-2 and then K4HG>APRS,RELAY*,WIDE3-1. Does that mean 
that my nearby main-digi heard it? Since the digi should see a few of the 
surrounding digis it could just as well be a second copy from another 
distant digi.

The only packets I can positively identify as comming from the "main" digi 
are the ones in which its call is included. This is however only a small 
portion of the packets, I think this is one of the huge diadvantages of 
WIDEn-N; you can't see who transmitted the copy.

What is missing is a signal strenght indication in the packet. If it was 60 
dB over S9 then I would know that a very local station send that packet, 
e.g. the main digi.

There are 3 other solutions I can think of that technically work:

The most advanced solution is that the main and backup-digi would have some 
background communication with eachother to tell about the packets received. 
When one goes down, the other completely takes over. But with a background 
link between the stations a much easier solution is to have 2 receivers 
coupled via the digipeater to one transmitter! Of course that is very easy 
to do with DIGI_NED, just make a crossband configuration with 2 receivers 
on the same band and only 1 transmitter. If you want redundancy include a 
signalling wire to activate the transmitter in the backup digi when the 
main-site's transmitter is down.

A second solution is to equip the main-digi with a 70cm second band. Then 
accros town erect a recever station that transmits all received packets via 
70cm to the main-digi. The main digi will ignore packets received on 70cm 
as duplicate if it got the original direct on 2 meters and digipeat the 
packet when it missed the original. You can configure it such way that you 
don't loose an extra hop. You can even make the extra receiver station 
analog, just pass all received data from 2m onto 70cm in audio...

The last option I can think off is just to have enough digipeaters in the 
grid, if you miss a packet then you will see another copy from another digi 
in your view. I think this was the original idea. With overlapping digi's 
you don't loose much coverage if one goes down. Of course this eats extra 
bandwidth...

There is no free lunch in any of these solutions. All these three 
alternative solutions are technially feasible however.

For a client program which originates packes, like Xastir or even a HamHud 
device, it is much easier to detect if they got digipeated. They just 
broadcast the original themselfs and only have to look for a single echo of 
the packet to see if they got digipeated.

If somebody comes up with a solid algotithm how to do make a working backup 
digi I will be happy to implement it to try it out. Up to now I didn't find 
a working method.

Kind regards,

Henk.