Re: [Discuss-gnuradio] gnuradio transmission delay

[Josh Blum]

On 06/13/2012 05:15 PM, Phelps Williams wrote:
> I have a simple bpsk transmitter setup with a custom udp source block
> which outputs a fixed pattern if no data is available from the socket.
>  I have instrumented the block to indicate when data is read from the
> socket inside of work().
> 
> My radio looks like this:
> Custom UDP Source -> DPSK Mod (DBPSK, 10 samp/sym) -> Multiply Const
> (0.3) -> UHD: USRP Sink
> 
> My sample rate is 1e6.
> 
> I can tell from my instrumented UDP Source that udp traffic is handled
> by work and passed on almost immediately.  However I can tell from
> both monitoring the spectrum and the receiver output on the other side
> that the data takes between 7 and 10 seconds before I see the expected
> output from the N200.  I thought this could be improved by reducing
> the maximum socket buffer size (sudo sysctl -w
> net.core.wmem_max=5000000) but that didn't seem to make a difference.
> I went as low as wmem_max=10000 with no difference in delay.  My goal
> is to get the total delay to < 1sec.
> 
> What else could be adding delay to my transmission path?  Are there
> other buffers I can tweak?  Where else should I be looking?
> 

Well the usrp only has a megabyte of buffering, as does the socket. At
that rate, you couldnt not be buffering more than half a second's worth
in the post-gnuradio part of the chain.

I bet you have produced a lot of data all at once, filled up all of the
gnuradio buffering, and you are simply measuring the time it takes to
drain these buffers at a rate of 1Msps.

If thats the case, you might pace your inputs. Or not transmit when you
have nothing to transmit, this lets things flush out and idle.

-josh