Long distance wireless networking

I have a need to tie in a number (up to say 50) of points spread over
about 4 square miles. There is a centrally located base station, which
could accomodate a larger aerial if needed. I have PCs with USB, ethernet,
and serial ports. The communication is low-bandwidth; each unit sends out
a short string - about 100 bytes - every minute or so. Timing is not
critical.

We have tried using radio modems that are supposed to do collision
detection; the problem is that while all radios can reach the base
station, not all radios can hear each other so they end up stepping on
each other.

I guess we're looking at some sort of polled system or a token-ring
system. It would be nice, but not required, to be able to push updates to
each unit; the updates take about 3-5 minutes at 9600 baud, during which
time the other units should keep quiet.

This would be used in a rural area; the obstructions are hills and trees,
not buildings.

Anyone aware of any existing technology that can do this?

TIA...


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Captain Dondo wrote:
> I have a need to tie in a number (up to say 50) of points spread over
> about 4 square miles. There is a centrally located base station,
which
> could accomodate a larger aerial if needed. I have PCs with USB,
ethernet,
> and serial ports. The communication is low-bandwidth; each unit sends
out
> a short string - about 100 bytes - every minute or so. Timing is not
> critical.
>
> We have tried using radio modems that are supposed to do collision
> detection; the problem is that while all radios can reach the base
> station, not all radios can hear each other so they end up stepping
on
> each other.

Not clear what you are using at the base station, but max power,
unlicensed APs may or may not provide the operating modes(s) you
require, eg., repeater, bridge, and AP simultaneously.

Using licensed or unlicensed spectrum/products? By "radios" do you
mean access points for clients or bridges between lan segments?

Also not clear what "stepping on each other" means. Clarify?

> I guess we're looking at some sort of polled system or a token-ring
> system. It would be nice, but not required, to be able to push
updates to
> each unit; the updates take about 3-5 minutes at 9600 baud, during
which
> time the other units should keep quiet.
>
> This would be used in a rural area; the obstructions are hills and
trees,
> not buildings.
>
> Anyone aware of any existing technology that can do this?

If wireless is the way you need to go, you can try this Cisco
"background" doc and see if it provides any ideas. It is almost
certainly "over kill" for your needs, but covers several
provisioning/topology schemes. I may have some other links laying
about.

http://www.cisco.com/univercd/cc/td/...c/wireless.htm

hth,
prg

In article <(E-Mail Removed) >,
Captain Dondo wrote:

>I have a need to tie in a number (up to say 50) of points spread over
>about 4 square miles. There is a centrally located base station, which
>could accomodate a larger aerial if needed.

The antenna at the central location wants to be an omni with a fairly flat
lobe to concentrate the energy on the horizon. You're talking about 108 dB
of path loss at 2.5 GHz, so you want a good bit of antenna gain, and don't
want to waste that with long cables between antenna and the radio.

>We have tried using radio modems that are supposed to do collision
>detection; the problem is that while all radios can reach the base
>station, not all radios can hear each other so they end up stepping on
>each other.

For two reasons. You either have a directional antenna pointing at the
central site (meaning less signal aimed at peers), or you have low gain
omni antennas which means the signals are down in the mud. On top of
this, you have line-of-sight/curvature of the earth problems - in two
miles, the curvature is only about 2 feet, but it would be about 8 feet
at four miles, and that ignores hills and Fresnel zone path clearance.

>I guess we're looking at some sort of polled system or a token-ring
>system.

That would be my guess too.

>This would be used in a rural area; the obstructions are hills and trees,
>not buildings.

The hills are problems - the trees would be too if they are wet and
moderately dense. Up to about 5 GHz, precip isn't going to be a problem,
but if the path isn't well clear of the earth (example 2.4 GHz, 1.5 mile
path -> desired clearance at mid path = 28 feet), you're going to take a
hit in additional path loss.

>Anyone aware of any existing technology that can do this?

Start by scanning 'alt.internet.wireless' if your news server carries it
(otherwise, I'm pretty sure groups.google.com carries it). They cover the
standard wireless scene pretty well. You can also have a look at the
AX25-HOWTO which is dated (2001) and discusses the AX. 25 packet radio
protocol utilized by Amateur Radio Operators worldwide.

219433 Sep 20 2001 AX25-HOWTO

In the early 1990s, I set up a 915 MHz link for NASA that did about 3 miles.
I'm sorry, but I can't remember the vendor we used. This was on an airfield,
and we got 3 Megabit each way, but the antenna were 11 feet above ground at
the remote, and 40ish feet above ground at the central site.

Old guy