[Server-devel] Large groups of XO-1 do not work with access points

James Cameron quozl at laptop.org
Thu Feb 6 16:45:23 EST 2014


Anna, Ben, Jon, Martin, Adam, Tim, TK, and Tom wrote:
> Very confounding Wifi: while connecting initially, large numbers of
> XO-1s fail to (re?)associate with Village Telco / Mesh Potato and
> other AP’s.  Even while larger numbers of XO-1s connect very
> successfully to TP-Link 3020 AP; and can see Android phone AP’s.
> 802.11s mesh networking cannot be the only root cause, but might
> turning off mesh and/or setting the channel to 9 mitigate the worst
> problems?

You would need to test at least ten TP-Link 3020 APs before you could
reliably claim that XO-1s connect very successfully, because of the
manufacturing variances in the APs.

The behaviour described sounds like a normal response to a dense
networking environment.  After my investigation yesterday, and
reproducing with a group of six XO-1, I can give you some advice.

When designing a classroom network, the XO-1 has a special feature
to consider.

The XO-1 will consume more of the available bandwidth than an XO-1.5
or later, because each XO-1 continually transmits mesh beacons even if
mesh is not in use.  Each XO-1 also responds to every scan (probe
request) by every other laptop.

As the number of laptops in a classroom grows, the available bandwidth
will be depleted much sooner with an XO-1 than with an XO-1.5 or later.

How to deal with it in the field?  Make a scoring system for each
channel, 1, 6, and 11.

Give each classroom AP on a channel a score of 12.

Scan, and give each AP outside the classroom on the same channel a
score of 12.

Give each XO-1 a score of 4.

Give each other device, such as XO-1.5, XO-1.75, or XO-4, desktop
computers with wireless, other laptops with wireless, tablet
computers, Android phones, and iOS phones, a score of 1.

Work to minimise the score.

Here's some examples:

1.  A group of 10 XO-1.5 and one AP will have a score of 22,

2.  A group of 30 XO-1.5 and one AP will have a score of 42,

3.  A group of 10 XO-1 and one AP will have a score of 52,

4.  A group of 30 XO-1 and one AP will have a score of 132.

5.  A group of 30 XO-1, two tablets, two mobile phones, two APs, two
APs next door, will have a score of 172.


Technical details:

"Give each AP on a channel a score of 12."  An AP transmits
a beacon every tenth of a second (102.4ms), and responds to every
probe request sent.

"Give each XO-1 a score of 4."  An XO-1 transmits a beacon every four
tenths of a second (409.6ms), responds to every probe request, and
transmits probe requests every 30 seconds for scanning.

"Give each other device a score of 1."  Each other device will
transmit probe requests for scanning.


Configure a nearby Linux system to act as a wireless monitor, for example;

ifconfig mlan0 down
iwconfig mlan0 mode monitor
ifconfig mlan0 up
tcpdump -i mlan0 -s 0 -w mlan0.tpcdump
(^C after a minute)
wireshark mlan0.tcpdump

Measure the transmission rates for beacons and probe requests.

James Cameron

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