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

Simon Wunderlich simon.wunderlich at s2003.tu-chemnitz.de
Thu Mar 29 14:51:26 CEST 2012


Hey Guido,

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

Also, if you use two-radio nodes, make sure that you use one module
on 2.4 GHz and one 5 GHz - if you use two modules in the same 
frequency band with omni antennas, even if you use separate channels
(like 1 and 11), 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

Possible Solutions:
 * use different bands
 * use directional or sector antennas
 * have a reasonable distance between your omni antennas

Cheers,
	Simon

On Wed, Mar 28, 2012 at 07:41:05PM -0300, Guido Iribarren wrote:
> (this time i'll keep the mail short, insane details are attached as text :) )
> 
> Well, luckily for batman-adv devs, but unfortunately for me, the
> problem is in fact related to relaying between wlan interfaces in
> mr3x20 hardware (at least)
> 
> I understand this is not anymore batman related, but maybe someone has
> experience on this, or can point me into the right direction /
> maillist ?
> Given you have a good deal of equipments at the BattleMesh, maybe
> someone has already tried adding a wifi dongle and can confirm seeing
> this behaviour?
> (in a line of 2-radio nodes with static routing, packet throughput is
> halved on each hop)
> 
> ========
> 
> I've repeated the iperf tests in a controlled environment, this time
> using different subnets on each interface and static routes built with
> "route add". iptables flushed with -F and policy -P FORWARD ACCEPT
> no batman at all.
> 
> Packets coming in through wlan1 and out through wlan0 (or viceversa)
> experience (unexpected) "bandwidth degradation". Throughput drops from
> 30mbps to 15mbps (rough avg)
> The speed is closely comparable to using only 1 radio (packet comes in
> through wlan0, goes out through wlan0), so adding the dongle makes no
> net difference. (besides alternating the channel for transmission,
> throughput is halved over one hop anyway)
> 2 consecutive hops, alternating wifi radios, yield 1/3 throughput.
> (like it happens in a 1-radio mesh)
> 
> this does not happen if the packet comes in through eth0 and goes out
> through wlanX (or viceversa) . Throughput is maintained high at
> 30mbps.
> 
> =========
> 
> So the hardware (or kernel??) is uncapable of keeping up with the
> transfer speed *when two radios are involved*.
> 
> Has anyone bumped into a similar case, or has experience with these
> equipments? Maybe (again!) i am doing something wrong?
> 
> If this is confirmed, my only alternative to maintain good throughput
> along the mesh will be to install 2 x mr3220 on each node, connected
> back-to-back with ethernet cable. This will definitely work but the
> cost for each node owner evidently rises ;(
> 
> Thanks a lot!
> 
> Guido


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