[USRP-users] problem with loopbacks when using USRP

mleech at ripnet.com mleech at ripnet.com
Tue Jul 28 12:06:19 EDT 2015


It means your computer isn't keeping up. USRPs are fundamentally
streaming devices. When a 'U' shows up, it means that the TX FIFO has
become empty, with no samples to send. Higher-level "packets" are just
that, a higher-level concept that the USRP knows nothing about. It
streams samples, it has zero clue what they mean or any kind of internal
syntax or framing or anything. It has no way to handle underruns
"nicely", because packets are a layer of abstraction significantly above
what it's dealing with. 

You'd have the same issues regardless of which USRP you're using--if
your computer can't "keep up" with producing samples at the desired
rate, it will be unable to keep up no matter which USRP you're using
(modulo subtle driver differences between USB and Ethernet, for

On 2015-07-28 12:01, Jason Matusiak wrote: 

>> No, the U's aren't OK to ignore. That means you're under-running on the TX side, which means missing samples. If you're getting a lot of them, it means you aren't likely getting *any* good frames over the air.
> OK, looks like I totally misunderstood that... I was thinking that it
> meant that I the transmitter was ready to send but I hadn't prepared a
> new packet yet, which would be fine. What is the cause/solution to the
> underrun? I tried reducing the sample rate from 10MHz to 2MHz, but I am
> still seeing occasional U's and D's. but not nearly as many as before. 
> I'll play with it at this reduced rate to see if that help some, but I
> am surprised that I had to reduce it so much for an X310.
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