Re: Performing a USRP Packet Loopback

On 01/20/2021 12:12 AM, Jada Mariano Berenguer wrote:
> Hi, I realized that in my packet loopback example, I didn't connect
> any of the QT GUI sinks after the USRP source. The updated flow graph
> is attached in screenshot0. After I connected them, I ran the packet
> loopback example on my MacBook Air using one B210 board with TX/RX and
> RX2 antennas attached and got the results attached in screenshot1, but
> still received underruns as printed out in the terminal in the bottom
> left corner. I also ran the example with two B210 boards with a TX/RX
> antenna on one and a RX2 antenna on the other and got slightly
> different results. Instead, the third graph was just a straight
> horizontal line and still ran into underruns. I also ran the same
> program with a Raspberry Pi 4 with one B210 board and got
> similar results. So I don't think it's an issue concerning computer
> power.
>
> Also, I was wondering what you meant by "for a loopback flow at low
> rates you may need to alter the internal buffer sizes in your
> flow-graph to prevent TX starving for samples because the RX buffers
> are still filling. But this is not a distinctly USRP problem and has
> more to do with GnuRadio". How would I alter the internal buffer sizes
> and what would I need to change it to?
>
> Are there any other suggestions you have for me to successfully get
> this loopback example running?
>
> Thanks!
>
My comments on buffer sizes were driven by an incomplete understanding
of what you were doing--however, having said that,
at low sample rates, the default buffer sizes used by Gnu Radio may
become an issue.

See the Options block to set a global buffering policy:

https://wiki.gnuradio.org/index.php/Options

I'm not that familiar with the blocks you're using in your flow-graph.
If the packet-tx block doesn't use start-of-burst/end-of-burst tagging,
the the UHD sink block will underrun between bursts, since it will be
expecting a continuous flow, but only receiving a bursty flow. This may
not be a problem here--I just don't know how that packet-tx block works.