[B.A.T.M.A.N.] BW degradation on p2p links?

Guido Iribarren guidoiribarren at buenosaireslibre.org
Fri Mar 30 06:11:35 CEST 2012


Simon, you saved my day,

On Thu, Mar 29, 2012 at 9:51 AM, Simon Wunderlich
<simon.wunderlich at s2003.tu-chemnitz.de> wrote:
> the problem using one radio is explained further here (from another
> company, but this applies to any WiFi mesh technology)
>
> http://revolutionwifi.blogspot.com/2012/02/mesh-network-performance-impact.html

I was fully aware of this problem with single-radio nodes, and that's
why I was trying to build cheap two-radio nodes using mr3220 + usb
dongles.

>
> Also, if you use two-radio nodes,
> they may interfere with each other if the antennas
> are near to each other. There are quite a few ppl who can confirm this,
> e.g. check this:
>
> https://hackerspace.be/Wbm2009v2/TestInterferenceResults

Ah! you hit the nail right in the head!
I walked around the battlemesh wiki a couple of weeks ago, interested
into past events results, but only came across an rsync daemon serving
a tree with videos, photos and results, mostly in raw data form,
collected during the last (before athens) WBM edition.

I think i was a bit short-sighted, since the Wbm2009v2 link (and the
rest of that wbm wiki) was a wonderful read!

I did a couple of tests, and indeed increasing the distance between
the dongle and the router makes the whole difference.

I googled more on the subject and found a paper with thorough testing results
http://edoc.hu-berlin.de/oa/conferences/reSNAVwf8bA/PDF/28TjAjjEtU7ts.pdf

and even finer-grained testing here
http://userver.ftw.at/~valerio/files/wons.pdf

None of them use 802.11n equipment, so i set out to make my own experiment.
In my case, i needed about 1.5 meter between omni antennas to achieve
full throughput, placing both omni antennas perpendicular to the
ground and on the same plane.
On the other hand, if i place them one over the other (that is, align
their vertical axis, while holding them at different altitudes) a
distance as little as 20cm was enough to overcome interference.
(Sounds reasonable given the omnis gain spatial pattern)

I was able to get 20mbps from A to C (and viceversa), hopping on B.
[A ch=11] -- [ ch=11 B(2 radio) ch=1] -- [ch=1 C]
This was done using either BATMAN or static routing, giving almost
equal results (~1mbps less using batman)

If i put the two radios in B really close to each other (<5 cm)
throughput fell down to 8 mbps.
It's a relief it was such a simple issue!

Thanks a lot Simon and Marek for getting me into the right path,

So far we haven't heard of anyone implementing OpenWRT+batman-adv on
mr3220+usb to build two-radio-nodes mesh network, which means we're
mostly in a "solo adventure"
so your help and attention was even more appreciated!

Guido


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