WDS problems observed in today's testing

[Javier Cardona]

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
 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.