Hi,
On Wed, 4 Nov 2009, Christian Lamparter wrote:
On Wednesday 04 November 2009 02:13:49 Adam Wozniak wrote:
I have two systems under test, both Dell laptops (a Latitude D630 and an
Inspiron 600m) both running Ubuntu 9.10 with the latest updates, and
bleeding edge compat-wireless-2009-11-02. I'm using identical AR9170
based D-Link DWA-160 USB 802.11adapters. I'm using nuttcp to measure
throughput. I'm running in ad-hoc mode. Both machines have the same
ar9170 files in /lib/firmware. The machines are sitting about 5 feet
apart in my office.
I'm having occasional problems where throughput drops through the floor
(0.5Mbps - 1.5Mbps). When I cat
/sys/kernel/debug/ieee80211/*/stations/*/rc_stats, one of the machines
lists the full set of rates, but the other only lists 1M and 54M. After
a period of time, that machine drops 54M and lists only one rate
(1Mbps), and the throughput listed by nuttcp drops accordingly. I
assume that, for whatever reason, the rates drop off the list and
minstrel uses the only one left available to it.
If I modify include/net/mac80211.h and force the inline function
rate_supported to always return 1, this fixes the problem. However, I
think this is a band aid around some other issue.
Any clues or ideas what the real issue might be here?
My guess::
When an adhoc node (call it A) merges with a second adhoc node (call it
B) there is a capability comparison.
Node A looks at the rates supported by B and says,
"I must only transmit at rates supported by B"
Some management frames don't contain a full report of the rates supported
by the sender.
My view is that node A (in this example) is incorrectly determining that B
only supports the 1mb/sec rate. Consequently, node A fills the
rate_supported array with one rate - 1mb/sec.
=====
There is no evidence that Minstrel is doing anything wrong.
The original poster's report of examining
/sys/kernel/debug/ieee80211/*/stations/*/rc_stats
is very important, as it shows exactly what minstrel is doing.
When examining network problems, one should always check the rc_stats
file. This tells you much about what is going on in the radio medium.
Derek.
Derek Smithies Ph.D.
IndraNet Technologies Ltd.
ph +64 3 365 6485
Web: http://www.indranet-technologies.com/
"The only thing IE should be used for is to download Fire Fox"
"My favorite language is call STAR. It's extremely concise. It has
exactly one verb '*', which does exactly what I want at the moment."
--Larry Wall
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