[Click] Using click to build an 802.11a MAC layer on USRP2

[Harald Schioeberg]

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Juan Ramon Gutierrez Agullo wrote:
> Hello everyone
> 
> This is my first message here and, of course, I'm a newbie click user. 
> 
> I have an 802.11a transmitter
> (https://www.cgran.org/wiki/ftw80211ofdmtx) running in the USRP2, but it
> only generates a standard-compliant IEEE802.11a/g/p OFDM frame - PHY
> layer.
> Looking at other projects I found the hydra project, and they used click
> to implement the MAC layer (CSMA/CA and DCF). I have some questions and
> I hope you could help me.

USRP uses either usb (USRP1) or ethernet(USRP2) to communicate with the
host, which probably means delays in the magnitude of 10s of us for a
frame-transmission over the interface (Take Linux' interrupt latency for
ethernet-frame-reception into account!)

802.11 slot time is 4 us, which is the time you have available to

a) receive the last sample at the frontend
b) decode it at the CPU
c) deceide whether to ack it
d) encode the ack at the CPU
e) have the first samples available for transmission at frontend

Click runs on the host as a scheduled kernel process, which brings you
to probably 100s of us worst case reaction on incoming samples,
especially on a Linux machine busy decoding samples.

So I don't see any way to implement 802.11 MAC on USRP on the host side,
no matter whether you use click or any other host side tool, IMHO: the
FPGA is your only choice (maybe some fancy real-time kernels might just
make it...)

Please, please, prove me wrong on that, 802.11 on USRPs would be soooooo
cool.

> - How can click communicate with GNU Radio?

My approach would be:
a) implement the MAC on the FPGA,
b) implement TX-queues on the FPGA that use this MAC,
c) write a driver that accepts frames and meta-data for this MAC,
d) write a click element, that takes Ethernet packets and transforms it
into one of these meta-annotated-frames.
e) implement things like rate control or scheduling algorithms as click
elements and have them set proper packet meta-data.

> - Is there any MAC implementation -by default- with click installation?

Nope, as all Wifi cards implement the MAC in hardware, for above
mentioned reasons.

Now, can sombody please tell this poor guy that I'm a moron and there is
a smooth and simple way to do all this ....

	Harald
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