I’m going to go ahead and assume you’re using an Ettus device, because
then these ‘S’ indicate a sequence error , not an underrun.
As this means that data reached your PC in the wrong order and most
probably could not be reordered. Your signal could be defective.
Don’t ignore this warning.
Please comment on the USRP you are using.
I’ve cross-posted this to [email protected] , since it is
not a GNU Radio error, but a hardware/operating system problem, possibly
something with your Network Card or USB3 host controller (depends on
what device you are using).
 compare: uhd/host/lib/usrp/common/async_packet_handler.hpp
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