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

[Doug Geiger]

George Nychis wrote:
> The other thing that is odd, is that the beacon's in my trace use a 
> short preamble (based on flags) but the transmission is at 1Mbps.  
> From looking at the 802.11 spec, short preambles can only be followed 
> by 2, 5.5, or 11Mbps PSDUs.  Here is a pcap of a beacon frame:
> http://cyprus.cmcl.cs.cmu.edu/~gnychis/mfilter/beacon.pcap 
> <http://cyprus.cmcl.cs.cmu.edu/%7Egnychis/mfilter/beacon.pcap>
>
> - George
George,
That is a little bit odd for beacon frames, although my reading of the 
spec doesn't disallow that (at least not in 802.11-2007): 19.3.2.2 Short 
preamble PPDU format says only that the short preamble is appropriate 
for use with 2, 5.5, and 11Mb/s modes. By the way, the short 
preamble/long preamble may be something you can configure on your access 
point - although I suppose I wouldn't be surprised if many recent AP's 
came with it set to short by default (i.e. they assume they're only 
taking to recent hardware). It increases the effective throughput the 
channel is capable of, at the (at this point - probably minimal) cost of 
not being able to talk to pre-802.11g equipment.
Three suggestions: the first is a repeat of the one to enable the 
debugging output from the PLCP block (I think I've even added debugging 
output when digging down into it: e.g. adding lines to output when it 
switches between states (found complete SFD/RSFD, back to searching for 
preamble, etc.). Second - disable the CRC check in the PLCP block (when 
you instantiate the block you can pass a boolean to perform the checks 
or not). You end up with errors in your ouput, but you at least see that 
it saw something resembling an 802.11 frame. Finally: there is no 
automatic gain control in the  receive script(s) - so careful playing 
with the gain setting for the USRP2 for any given TX/RX location can 
improve the FER. There also is no frequency/phase offset correction in 
the receiver code - really, this is a very simplistic receiver that just 
gets the job done (and when they wrote it for the USRP1 - only in very 
good SNR environments). Lots of room for improvement here.
Doug

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