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Re: [Discuss-gnuradio] High packet loss problem (samples dropped?) and f

From: Dan Halperin
Subject: Re: [Discuss-gnuradio] High packet loss problem (samples dropped?) and fusb parameters
Date: Wed, 25 Jul 2007 12:02:36 -0700
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> One interesting thing I noticed is that when I increased fusb_nblock
> and fusb_block_size, it shows a lot of buffer overrun ('uO'). When I'm
> using the default setting, these uO disappear. Why is this happening?
> However, the packet loss ratio are high in both cases. And I also
> tried enabling real time scheduling, but it doesn't have much
> effects.

uO means "USRP Overrun". That is, the USRP is sending data to the
computer faster than the computer can handle it. This probably means
that you are sending at too high a data rate -- the online decoder is
taking too much CPU. When you start logging with the file_source, then
this CPU (and memory, etc) overhead disappears and all samples are
logged. Then when the decoded samples are replayed, your receiver has
ample CPU time to work.

Fixes to decrease CPU load are:

1) Decrease the data rate (you tried this, and it worked!)
2) Insert a pwr_squelch_cc block with gating on to decode only packets
and not all random noise. Note this requires some parameter tuning to
get it to work properly.
3) Optimize the receiver.

Once you've eliminated the uO's, if there are still packet losses, note
that the default GNU radio receivers are not perfect or even necessarily
that great... there might just be some problems recovering from the
default channel distortions. Finally, you can also play with the gain on
each end to attempt to improve the SNR. Which modulation scheme are you
using with which boards at which frequency?

Also, just to make sure, when you connect two USRPs directly make sure
to use sufficient attenuation (~40dB I think, check the archives for
emails from Matt Ettus for the actual number) with any of the RFX boards
to avoid damage to the receiver!

- -Dan
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