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?
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