Re: [Discuss-gnuradio] Physical layer for packet-based communication

[David Young]

On Fri, Feb 04, 2005 at 10:21:06AM -0500, Rahul Dhar wrote:
> Ideally, I'd like to see a physical layer API so I can write my own MAC
> layer.  Can GNU Radio handle 802.11 a/b/g?  What I'd like to do is have
> a MAC layer smart enough to detect when the MAC protocol it's using
> isn't optimal given network conditions, and then switch to a protocol
> better suited to the traffic characteristics.  For examle, 802.11 has
> DCF and PCF modes.  For smaller networks, DCF (based on CSMA/CA) is
> normally fine, but when you reach a certain number of nodes, or start
> having nodes that are sending large amounts of data, PCF can be better
> suited because of its contention-free period.
> 
> I'm not sure how feasible that would be, but I'd like to start by
> implementing simple DCF and PCF schemes and eventually work towards an
> 802.11 implementation.  A fake network device would be nice, though.  If
> nothing more, it lets applications take advantage of the radio in a
> standard way.

Rahul,

I have been thinking a little about how to produce a novel MAC, without
spending gobs of time & money.  Here are some ideas:

        1 Use an off-the-shelf 802.11 card.  Change its mode from
          PCF/DCF using software that runs on the host.

        2 Use an off-the-shelf 802.11 card.  Tweak its MAC/PHY parameters
          until the default carrier-sensing and such are effectively
          disabled---this could get hairy!  Use the host CPU or a
          coprocessor to schedule packet transmissions.

        3 Use an existing 802.11 PHY, but replace the MAC
          with a microcontroller [*].  Design a PHY abstraction.
          Implement, in C (say), the PHY abstraction for the PHY of your
          choice [**].  Program your MAC in C.

        4 Use an existing 802.11 PHY.  Replace the MAC with
          an FPGA.  Program your MAC in VHDL.  (I don't see much advantage
          to this over #2.)

All those ideas need a lot of development, but I hope the general ideas
are clear.

Dave

[*] Lots of MACs are ARM u-controllers---TI's and Atmel's, for
    instance---but docs are not freely available.

[**] Docs for PHYs by Intersil/Conexant, RF Microdevices, and others
     are available, if you ask nicely.

-- 
David Young             OJC Technologies
address@hidden      Urbana, IL * (217) 278-3933