Re: [Discuss-gnuradio] USRP hardware purchase

[George Nychis]

I've got to go with Dan on this one, there is no chance of 802.11 with 
the USRP and GNU Radio.  However, we are working on something 
802.11-like with the in-band signaling work and the MAC we are building.

I think a big first step for the platform is getting a solid contention 
based MAC protocol, which the platform lacks as a whole.  From this, 
some 802.11-like MAC can be built, and other protocols.

To give you a heads up Dan, we are working on doing carrier sense in the 
FPGA and supporting it with the in-band signaling work.  It's actually 
what we're working on at this second.  Building it in the FPGA avoids 
the FPGA->host->FPGA latency.  The FPGA is going to read carrier sense 
related parameters from registers, and the host will set a flag in the 
packet if it wants the FPGA to carrier sense before transmitting it.  I 
think very time sensitive functionalities like this need to be pushed in 
the FPGA.... though we're also considering building it in the host too 
so we can evaluate the tradeoffs.

- George


Dan Halperin wrote:
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> The USRP has a maximum bandwidth of about 16 MHz (using 8-bit samples
> instead of the usual 16), which is not even enough to cover one
> 802.11b/g channel (22 MHz Nyquist). Thus you'll have a hard time
> decoding b/g packets at faster than 1,2 Mbit rates (BBN has done some
> work facilitating the recovery of these packets).
>
> On top of that, an (extremely, I think) optimistic lower bound for
> latency in a GNU radio system is over 1 millisecond from the RX antenna
> through the FPGA over the USB to the computer to your software back over
> the USB and out the TX antenna. This makes it _reallllllly_ hard to get
> the 10 _microsecond_ turnaround required for an 802.11b ACK, and carrier
> sense is meaningless with such a latency.
>
> Also, see Tom Rondeau's email of 12/05/2006 for more comments, but the
> short of it is that you're unlikely to be able to get any sort of
> reasonable interoperation between the USRP and commercial 802.11b/g
> hardware.
>
> - -Dan
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