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Re: [Discuss-gnuradio] Re: BBN 802.11 code: long preamble only?

From: George Nychis
Subject: Re: [Discuss-gnuradio] Re: BBN 802.11 code: long preamble only?
Date: Mon, 30 Nov 2009 10:43:59 -0500

Hi Doug,

Thanks for the clarifications.  I missed the order that the bits are transmitted in, so that makes sense now.

I took a 1 second capture of 802.11 traffic, that I know has 1Mbps beacons in it and probe responses.  The machine was able to decode them with its Atheros card, and if I look at the magnitude of the raw samples captured by the USRP2, I can see several transmissions:

The lower peaks around 5, 7, and 9x10^6 are beacons at 1Mbps with a short preamble.  I am trying to decode these. I captured the trace at 25Msps and did not experience any overruns.

- George

On Mon, Nov 30, 2009 at 9:53 AM, Doug Geiger <address@hidden> wrote:
George Nychis wrote:
I also think that the decoder is improperly looking for synchronization...

According to the 802.11 spec, the long preamble uses an SFD that is 0xF3A0, and the short is 0x05CF (verified by a quick google (http://www.google.com/search?hl=en&safe=off&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=zoI&q=802.11+0xF3A0+0x05cf&aq=f&oq=&aqi= <http://www.google.com/search?hl=en&safe=off&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=zoI&q=802.11+0xF3A0+0x05cf&aq=f&oq=&aqi=>)

The definition of SFD and RSFD seem to be backwards:

For those a little unfamiliar with the preamble, RFSD = reverse SFD, as the 802.11 spec states that the short preamble uses the reverse of the SFD... but these defines seem to be backwards.

Still trying to get the decoder to pickup a short preamble though...

- George
I believe this is correct - the standard specifies that the SFD is 0xF3A0, MSB to LSB, but the LSB shall be transmitted first in time. So if you reverse the ordering: 0xF3A0 (1111001110100000b) becomes 0x05CF (0000010111001111b). So, while the #define's appear to be reversed (since they are), functionally this is correct. Re: your other question, on d_shift - it appears correct to me as well: take a look at the same loop above on line 191: sfd_search is the descrambled word being checked, it is bit-masked and compared to a shifted SFD (or RSFD), if the comparison fails, sfd_search gets shifted, and the loop continues (up to eight shifts). If the comparison never came back true, the state machine reverts to the preamble search state.

Douglas Geiger
Code 5545
U.S. Naval Research Laboratory
Washington, DC 20375
(202) 767-9048

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