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

Guido Iribarren guidoiribarren at buenosaireslibre.org
Thu Mar 29 00:41:05 CEST 2012


(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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