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Emergency comms. Was RE: [aprssig] Igates Are A Fair Weather Solution (was: "Finito")

Jason Winningham jdw at eng.uah.edu
Tue Aug 30 15:56:18 UTC 2005


On Aug 30, 2005, at 10:09 AM, Dave Baxter wrote:

>> Maybe, but if those 150 to 250 stations are transmitting very often,
>> your network is dropping a _lot_ of packets.
>
> So what if a lot of packets are being dropped?

large number of dropped datagrams == overloaded, unreliable network.  
Introduction to Networks 101.

>   It's a UI,

Yes, and two of the assumptions typically made with datagrams are: a) 
the packet has a very good chance of being delivered, and b) if 
reliable delivery is absolutely required, delivery confirmation will 
happen at a higher layer.  In APRS we do _not_ have any sort of 
confirmed delivery (other than some point to point messaging 
conversations, a small fraction of the overall traffic), so a reliable 
APRS network must _not_ drop a large percentage of its packets, else it 
isn't reliable.

> Yes, sometimes it doesn't get through, but for "Emergency" use,
> why use the normal (busy) frequency anyway?

Because that's where the tested infrastructure is located?  That's 
where people are actually listening?

There's FM capture and power settings and antenna factors and mobile 
flutter and band openings and all sorts of things that affect the RF 
signal.  Don't forget, however, that when you start talking about a 
data network you must realize there are several layers to the beast, 
and the RF layer is maybe 1/4 to 1/7 of the picture you must see when 
you're building a network with an RF layer 1.

FM radio is not all there is to APRS.  education, education, and more 
education etc....  (:

I have no idea what CW has to do with the discussion at hand, so I'll 
let it pass.

> and as for most things PC based, well...

Right.  Hence my entry into this discussion with the questions about 
NWS SAME and the usefulness of a non-pc, non-internet means of getting 
some weather data onto APRS.

-Jason
kg4wsv





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