[Olsr-users] G1 / Linksys WRT demo

[L. Aaron Kaplan]

On Jan 16, 2010, at 10:15 PM, marc fawzi wrote:

> Thanks Aaron,
> 
> Does that mean that OLSR is optimized to work best when having a lot of nodes in the network each with a small radius of 2-way coverage...? and that increasing the radius of 2-way coverage per node (for all nodes) degrades performance?
> 
Roughly you are on the right track, yes. But let's take a look first at layer 1 and 2 (OLSR is on OSI layer 3)...

If you look at physics then the signal decreases with the square root of the distance.
 
  S(x) = 1/(x^2)   ... x = distance in meters.                     (equation 1)

Now you need to know the noise in your area. Let's call this "N". If you now look at the encoding techniques of Wi-Fi  you will see that certain encodings require a minimum "SNR" (or signal to noise ratio). 
Or mathematically speaking:

  SNR = S/N      (at location x).       (see: http://en.wikipedia.org/wiki/Signal-to-noise_ratio)       (equation 2)

Based on my own experience, you can still establish a link with SNR > 10 dB. But it will be a bit crappy. You can achieve pretty good links with an SNR > 30 dB. 
So, to increase the SNR you can now either use a "booster" ( stronger signal) or a better antenna (more directional antenna), or you can reduce the noise. But what happens when everybody in your neighborhood (that includes your own nodes) increases S? Right, the stronger S becomes N for somebody else  :)

So the very best for building good Wi-Fi links is a) to use directional antennas (don't pick up noise from other directions) b) to send softly (in order to keep the overall noise low).
But what happens when you send softly? Well, in order to have good SNR (and not disturb others), you can take a look at equation 1 and see that you need to place the next Wi-Fi hop close to you. Then it will have a good SNR from your signal, but still your node won't disturb others so much. 

q.e.d 

I hope it helped a bit.

In general, getting a few folks together and building an initial Wi-Fi mesh network can be a very rewarding experience.
You do something good for the community, you learn each day and who knows - maybe you might be after some time the only (community-) ISP who will offer decent broadband connectivity in some areas :))

You can get a very good intro to RF engineering from this book:
"The Electronics of Radio", David B. Rutledge, Cambridge University Press 1999.
I personally love that book.

a.



> 
> 
> On Sat, Jan 16, 2010 at 12:54 PM, L. Aaron Kaplan <(spam-protected)> wrote:
> 
> On Jan 16, 2010, at 9:25 PM, marc fawzi wrote:
> 
>> 
>> I just found this list of supported OpenWrt hardware:
>> 
>> http://nuwiki.openwrt.org/toh/start 
>> 
>> But what is the problem with wanting to enhance the reception by using an amplifier like this one? http://www.danets.com/turbotenna/turbotenna%20009xt.php
>> 
> 
> The problem is that you disturb everyone else. And that is against the idea of a mesh. At the end you start a race who sends "louder" and then at some point all stations "scream" at each other. This results in high noise for everybody but no bandwidth.
> 
> 
>> I assume the higher the reception s/n the wider the coverage area per node (assuming you pick hardware that has sufficiently high Tx power)
>> 
> Think: many small mesh nodes which whisper. Much more effective re-use of shared spectrum.
> 
> 
>> In other words, what is wrong with wanting to boost both transmit and receive signals (above and beyond what the hardware is capable of)?
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> On Sat, Jan 16, 2010 at 12:08 PM, Henning Rogge <(spam-protected)> wrote:
>> Am Samstag 16 Januar 2010 21:05:54 schrieb Mitar:
>> > Hi!
>> >
>> > On Sat, Jan 16, 2010 at 8:54 PM, marc fawzi <(spam-protected)> wrote:
>> > > I'm limited to the WRT54-GL because I thought that's the only model that
>> > > OLSR has been ported to .... ?
>> >
>> > Where you got this? If it runs (more or less standard) Linux it runs
>> > OLSR. So for example wherever you can put OpenWrt you can have OLSR.
>> > (Limited by CPU and memory of course and other similar limitations.)
>> Let's not forget that OLSRd runs on BSD, Win32 and OSx.
>> 
>> Henning Rogge
>> 
>> -- 
>> Olsr-users mailing list
>> (spam-protected)
>> http://lists.olsr.org/mailman/listinfo/olsr-users
> 
> 

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