On 02/16/2016 10:56 AM, William Healey wrote:
Hello,
I am trying to make a simple FFT flow chart using a RFNoC FFT block
and a signal source as input. I am finding that the maximum FFT size I
can use is limited by the MTU of my Ethernet connection. While
connected to the 1GB Ethernet I had a MTU of 1500 and was able to use
an FFT size of 1024 but if I tried to use 2048 I would get a timeout
on chan0 error message, similar to this case
http://comments.gmane.org/gmane.comp.hardware.usrp.e100/13915 . I was
able to increase the FFT size to 1024 because I stayed in the RFNoc
layer to display data instead of moving back up to the host. If I move
up to the host layer then I am limited to FFT size of 512 like the
previous case.
I switched to a 10GB connection which supports a MTU of 9000. This
gave me a new max FFT size of 2048 if I stay in the RFNoC layer and
1024 if I move back up to the host layer. To do this I had to edit the
code for the RFNoC block like in this case
http://comments.gmane.org/gmane.comp.hardware.usrp.e100/16564. In that
example it is mentioned that it may be possible to break up larger
FFTs into multiple packets and recombine them.
I am wondering if there is a way to increase the FFT size past 2048
with a limited MTU. It aslo seems that the max packet size for the
USRP is 8000 so increasing the MTU further may be of little use.
FInally I am wondering if it is possible to have an FFT size up to
8192 and if not, will it be in the future?
I have attached my flowchart below
The RFNOC cross-bar exchanges VITA frames, from what I understand. These
then end up unmolested going out the ethernet port. I think
that there are long-term plans to have some kind of fragmentation
scheme, but it isn't implemented. On 1GiGe/10GiGe, the ethernet
standards limit you to jumbo frames of no more than 9000 bytes, which
means FFTs of no more than 2048 bins (4 bytes per FFT bin).
MANY host ethernet cards either don't support jumbo frames, or only
support a limited size of jumbo frames, and not up to 9000 bytes.
_______________________________________________
Discuss-gnuradio mailing list
Discuss-gnuradio@gnu.org
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
