Re: [Discuss-gnuradio] Understanding flow control

[Eric Blossom]

On Fri, Jan 15, 2010 at 01:54:24PM -0500, Tom Gross wrote:
> 
> I guess I could study the firmware source (if it's in the C code where
> this happens) to figure out what happens if RX is OFF.  My assumption
> is that somewhere in the USRP2 code there is some recognition that it
> can't keep up with transmit data, thus causing it to send pause
> signals back to the ethernet controller (is that correct?).  Maybe
> it's not in the firmware but built into some ethernet port controller
> chip.

Actually, PAUSE handling is all handled in the FPGA.  When the FIFO is
getting full, a PAUSE frame is sent on the wire telling the host to
stop sending for a while.

> Or maybe my understanding of what RX ON/OFF does is completely wrong.
> :-)  So, I guess I'm asking: as I understand it, the USRP2 sends pause
> packets (or something) to the ethernet controller when it can't keep
> up with data being sent to it.   RX ON means that the controller will
> acknowledge these pause commands and stop sending data.  Or have I got
> that completely backwards?

In ethtool lingo, "Rx ON" means that the host will listen to the PAUSE
frames.  This is what we want. Otherwise the host will continue
blasting away, and they'll get dropped somewhere along the way.  "Tx
OFF" means that the host does not send PAUSE frames.  This is what we
want.  The USRP2 never listens to PAUSE frames, since it doesn't have
enough buffer to avoid an overrun.

We're using "Asymmetric flow control".  See also:

  http://grouper.ieee.org/groups/802/3/z/public/presentations/nov1996/asym.pdf

Eric