# U's and L's when running GNU Radio

**URL:** <https://www.ruby-forum.com/t/us-and-ls-when-running-gnu-radio/233710>\
**Category:** GNU Radio\
**Created:** [January 5, 2015, 5:50pm UTC](https://www.ruby-forum.com/t/us-and-ls-when-running-gnu-radio/233710 "2015-01-05T17:50:50Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [January 5, 2015, 5:50pm UTC](https://www.ruby-forum.com/t/us-and-ls-when-running-gnu-radio/233710/1 "2015-01-05T17:50:50Z")

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Hi,

In the terminal and while using a powerful PC (please see the  
attachment),  
I am occasionally getting capital U’s and L’s when running GNU Radio  
with  
one Tx and 4 Rx channels (Here is a sample of what I am getting:  
ULLLLLLLLLLLLLLLULLLLULLLLLLLLLLLLLLLULLLULLLLLLLLLLLLLLLLLLLLLLLLLLLL).

Please note that I am using timed-commands to start both the Tx and Rx’s  
at  
the same time. When I check the System Monitor while GNU Radio is  
running  
(please see the attachment), the CPU history and RAM usage seem to be  
fine.  
Also, swap memory is disabled as shown in the attachment. What causes  
this  
issue and how to handle it? Could it be due to some instability in the  
writing speed to the hard drive?

BTW, I am having the same issue for my older PC (a PC with less specs).  
The  
System Monitor tells me that my CPU usage is around 60% and my RAM usage  
is  
around 45%, however, I am getting U’s and L’s when I try to use four Rx  
channels. Please note that the system doesn’t throw lots of U’s and L’s  
at  
once, it is a kind of frequent where a bunch of them are thrown out  
every  
many seconds or so.

Thanks in advance for your help.

Best regards,  
Khalid

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<div class="post-metadata">

**Author:** ![khalide](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/3bc359/32.png) [@khalide](https://www.ruby-forum.com/u/khalide)\
**Post date:** [January 7, 2015, 2:48pm UTC](https://www.ruby-forum.com/t/us-and-ls-when-running-gnu-radio/233710/2 "2015-01-07T14:48:48Z")

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Hi,

> In the terminal and while using a powerful PC (please see the attachment), I  
> am occasionally getting capital U’s and L’s when running GNU Radio with one  
> Tx and 4 Rx channels (Here is a sample of what I am getting:  
> ULLLLLLLLLLLLLLLULLLLULLLLLLLLLLLLLLLULLLULLLLLLLLLLLLLLLLLLLLLLLLLLLL).

I can’t find anything in GNURadio itself that would print that.

The only thing it can print AFAICT is aU aO jU

So it must be an OOT block that display those …

Cheers,

Sylvain

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<div class="post-metadata">

**Author:** ![khalide](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/3bc359/32.png) [@khalide](https://www.ruby-forum.com/u/khalide)\
**Post date:** [January 7, 2015, 3:24pm UTC](https://www.ruby-forum.com/t/us-and-ls-when-running-gnu-radio/233710/3 "2015-01-07T15:24:16Z")

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Hello Sylvain, hello Khalid,

these could be underruns (U) and late (L) packets, as reported by UHD,  
used via gr-uhd; however, seeing so many L’s is a very uncommon pattern;  
Do you use timed\_commands? What are your RX and TX sampling rates? From  
your network throughput, I’d assume your using 4x 3.3MS/s, is that  
correct?

Could you share the whole output of your program’s run, not only the  
ULLLL… ?

Best regards,  
Marcus