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Re: [Discuss-gnuradio] Understanding flow control

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] Understanding flow control
Date: Fri, 15 Jan 2010 15:18:22 -0800

On Fri, Jan 15, 2010 at 15:08, Tom Gross <address@hidden> wrote:

> yes of course we have two separate gig-e cards.   if the usrp2 is
> sending us "pause" commands then it seems evident the usrp2 is having
> trouble keeping up, not the computer.

The host software, when creating a data stream to be sent to the USRP2
for TX, will create the data as fast as the processor allows, and TX
on the GbE at full wire rate.  The USRP2, however, is "consuming" data
at a fixed rate proportional to the configured TX RF baseband sample
rate.  Even at the fastest sample rate, 25 Msps (interpolation rate
4), this is only 800 Mbps + framing overhead.  So the USRP2 doesn't
have problems "keeping up", it's just that the host can create the
digital sample stream faster than real time, so the USRP2 pauses it
periodically to keep the average data rate down to what is needed.


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