Re: [Discuss-gnuradio] USRP Delay Measurements

[Eric Blossom]

On Wed, Nov 08, 2006 at 02:06:11PM -0800, Thomas Schmid wrote:
> Hi all,
> 
> I am currently trying to measure delays on the USRP and found some
> interesting things. I wanted to see if others have similar results:
> 
> - First I measured the round trip time of my IEEE 802.15.4
> implementation. I have to computers, one runs a ping application, the
> other one a pong. The average delay in this case is 26.8ms.
> 
> - Next, I measured the round trip time of my FSK implementation (also
> based on packets). Here, the mean RTT is 14.9ms.
> 
> - Next, I tried to find out the one way delay, i.e., how much time
> does it take between generating one sample at the computer, until that
> sample gets sent out at the USRP output. For this, I changed the code
> of the sig_gen class such that it generates a square wave instead of a
> cosine and I added code that toggles the parallel port pins according
> to the state of the square wave. Then I used an oscilloscope to
> measure the delay between the two signals, i.e., the one generated at
> the parallel port and the one from the USRP (LFTX). The signal is at 2
> Hz.
>  The delays in this case depend a lot on what I choose for the
> decimation (while keeping a constant square wave frequency). If I use
> decimation=500, then I find an average delay of 65.8ms. When I change
> the decimation to something else,  I get values ranging from 30ms up
> to 150/200ms (I didn't do exact measurements for these yet).
> 
> What I am wondering is, why is there such a huge difference? Is it
> because of the GNU Radio scheduler? Or is there some other problems?
> 
> Thomas

Have you tried explicity setting the fast usb buffering?
Take a look at the examples in gnuradio-examples/python/digital/*.py

You want to pass the fusb_* args to the usrp constructor.

Eric