WDS problems observed in today's testing

Ricardo Carrano carrano at ricardocarrano.com
Sat Mar 1 10:21:20 EST 2008


Just to add that:

- The access point Javier mentions is the one I bought yesterday (Linksys
WRT54G)

- Most of this traffic is retransmission (3606):
(wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4 and wlan.ta[4-5] == ce:6e) &&
(wlan.fc.retry == 1)

- It is also interesting to detect other wds peers this AP identified (one
is 00:0b:85:53:27:50 and got ).
((wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4 and wlan.ta[4-5] == ce:6e))
&& (wlan.ra == 00:0b:85:53:27:50)

I believe we should compare this with the previous capture from the Netgear
AP, just to confirm that this is (agian) specific to WDS issues on the
Linksys.

On Sat, Mar 1, 2008 at 5:23 AM, Javier Cardona <javier at cozybit.com> wrote:

> Michail, Chris,
>
>
> This afternoon I captured some traffic while Chris was running tests
> for Peru.  The test setup consisted on ~25 laptops associated to a
> WRT54 access point.  When the laptops were on, associated and (not
> sure about this) idle, we observed a high volume of wireless traffic.
> The spectrum analyzer showed close to 50% duty cycle utilization of
> the channel.
> We also observed that a few xo's could not associate, and some seemed to
> intermittently lose and recover association.
>
> Turning off the WRT54 (and therefore stopping all the infra traffic)
> freed up most of the bandwidth on that channel.
>
> In my 50 second capture (taken before turning off the AP) we observe:
>
> Total traffic:                  15081 frames (100%)
> All WDS traffic (1):             6023 frames ( 40%)
> WDS, xo is source addr (2):      4343 frames ( 29%)
>  (96% of the above xmitted at 1 Mbps (3) and 100% sent by a single AP(4))
>
> Compare that with
>
> xo originated infra frames (5):   401 frames ( 3%)
>  (77% of the above xmitted at rates higher than 2 Mbps (6))
>
> What does all this mean?
>
> 1. Multicast traffic gets replicated and retransmitted.
> 2. The ratio of original frames to AP generated multicast
> retransmissions is 1:11
> 3. Taking into account the data rates this means that for 1 airtime
> unit used to transmit useful traffic, over 200 units are wasted
> transmitting useless WDS traffic.
> 4. All this is done by a single Cisco AP, MAC: 00:1e:7e:44:ce:6e
>
> Michail, is that one of OLPC APs?
> Chris, we should see a big improvement if we can disable that
> "feature" on the AP... or put it under water.
>
> I've posted my capture here:
>
> http://dev.laptop.org/~javier/captures/cisco-wds-traffic-around-xo-testbed.cap<http://dev.laptop.org/%7Ejavier/captures/cisco-wds-traffic-around-xo-testbed.cap>
>  in case someone wants to double check my analysis (Ricardo?).
>
> Cheers,
>
> Javier
>
> (1) wlan.fc.ds == 3
> (2) wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4
> (3) wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4 and radiotap.datarate ==
> 0x2
> (4) wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4 and wlan.ta[4-5] == ce:6e
> (5) wlan.fc.ds == 1 and wlan.sa[0-2] == 00:17:C4
> (6) wlan.fc.ds == 1 and wlan.sa[0-2] == 00:17:C4 and radiotap.datarate > 4
>
> --
> Javier Cardona
> cozybit Inc.
>
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