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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