Re: [Discuss-gnuradio] USRP2 modulated signal tx error

[Eric Blossom]

On Tue, Mar 24, 2009 at 04:03:36PM -0300, Ronaldo Nunez wrote:
> Hi folks,
> 
> I tried to TX a FM modulated signal using USRP2 BasicTX.
> The signal was a 440 Hz sinusoid, and the carrier 1 MHz.
> I used the signal generator and fm modulator blocks provided by GNURadio.
> 
> When I tx it the follow message appeared on the screen:
> 
> USRP2::Tx_RAW: FIXME: short packet: 1 item (32 bytes)
> 
> Sometimes the item number changes to 4 or 8...
> What that mean?
> 
> I took a look at the usrp2_impl.cc (last revision) and i couldn't
> identify any problem...
> 
> Any suggestion is welcome.
> Regards,
> 
> Ronaldo

Hi Ronaldo,

There's a bug in the on-the-wire format for the USRP2 where we're not
currently transmitting the valid length of the packet.  With ethernet,
the minimum packet size is 64-bytes, and there are some times where
the host code is naively fragmenting the received samples in a way
that produces a packet that is too short for ethernet.

I've opened ticket:378 on this.  It will be properly fixed when we
rework the on-the-wire format as part of the VRT work, but I think
there's an intermediate step that can avoid most cases of the problem
happening.  I'm not sure that I can find time to fix this in time for
3.2.0, but if you'd like to take a look at it, we'd love a patch for
it.  What needs to happen is that the loop in usrp2_impl.cc(tx_raw)
that fragments the nitems items needs to be a little smarter about
breaking the items into ethernet frames.  Each piece needs to be
between 9 and U2_MAX_SAMPLES samples long.  (9 works out to the
smallest number of samples that plus the headers ends up being >= 64
bytes).

Eric