George Nychis wrote:
Hey Brian,
Running through a descrambler and then trying to correlate with 128
ones, I get the following for the 11 packet trace:
http://cyprus.cmcl.cs.cmu.edu/~gnychis/mfilter/descrambled.png
<http://cyprus.cmcl.cs.cmu.edu/%7Egnychis/mfilter/descrambled.png>
George,
Just to give you an update - I did run a couple more tests on
transmitting today. I am able to receive proper 802.11 frames, but
they're mostly filled with garbage. When I fill in some fake (but known)
values in the MPDU header (i.e. frame control, duration, mac addresses,
etc.) and the payload consists of Hello World, and the text from
getty.txt, I'm not getting correct values when I look at in in Wireshark
- but a few words from getty.txt do show up somewhere in the middle. So
the PHY stuff is sort of working, I do wonder if the scrambler isn't the
bug here, but that'd require more checking.
The main (functional) modification I've made to the script (besides
muddling with the payload), is to lock the USRP2's clock to my external
reference clock (a GPSDO I have to provide a 10Mhz reference). Running
the straight usrp2_version script does not work - suggesting the clock
offset may be too large for the commercial card to lock onto with the
USRP2 in free-running mode. Hmmm, my copy of the 802.11 spec says the
transmit center frequency tolerance should be +/- 25ppm, and I recall
the USRP2's clock in free-running mode was ~+/- 20ppm, so I find it odd
that the clock being locked would make or break the (sort-of working)
transmit script. In the meantime, from the behavior I'm seeing on the
captures from the commercial 802.11 card, I suspect it is the scrambler
that isn't working (i.e. lots of garbage, some correct bytes, followed
by garbage).
Certainly needs more investigation,
Doug
--
Douglas Geiger
Code 5545
U.S. Naval Research Laboratory
Washington, DC 20375
(202) 767-9048
[email protected]
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