I remember I had the same problem with the crc error on the short preamble when I wrote the code, but I never figured out what the problem was. However, I did receive an email from someone who claimed to have fixed the problem (I think) a couple of years ago. If you want I can email him. Also, I don't know how much the code has changed. but there is a bug in the time synchronization code that will cause some packets to be dropped.
-Dan On Nov 30, 2009, at 4:14 PM, George Nychis wrote: > Hi all, > > I'm in the progress of implementing the 802.11 short preamble on the TX side. > There is code on the RX side (based on yesterday's discussion) to handle a > short preamble, but I am positive there is no code on the TX side to transmit > using a short preamble. Of course, I'd like to implement it as an option. > > The main crux of it is ensuring that only the preamble is modulated at 1Mbps, > and that the PLCP header is actually modulated at 2Mbps. This is unlike the > long preamble where both are modulated at 1Mbps. > > Right now, I have the loopback detecting the short preamble, but the CRCs > fail. If you want to try it: > svn co https://www.cgran.org/svn/projects/bbn_80211/branches/usrp2_version > > Build, etc, and then go to gr-bbn/src/examples and run ./bbn_80211b_sptest.py > ... you will see this, for example: > Recieved (short) header! > signal: 0xCA > service: 0xA0 > length: 0x52FA > crc: 0xB86C > Calculated crc: 0xE491 > *** BAD CRC *** > > To explain my changes a bit, I implemented a small option (which we can add > to the command line) here: > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L166 > > If using a short preamble, we use 7 bytes of scrambled 0's and the reversed > SFD: > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L175 > > During modulation, I make sure to split the preamble and the PLCP header, and > modulate them according to the preamble option: > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L257 > > xpsk_mod_header_1 is to modulate the header at 1Mbps and xpsk_mod_header_2 at > 2Mbps: > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L103 > > Again, when encoding, ensure we modulate based on the preamble option: > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L272 > > One last time, for prepending to the payload: > https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py?rev=383#L331 > > ... and that's it. If anyone pokes at it to pass the CRC with any success, > let me know... I'll keep working on it. > > - George > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
