USRP TX/RX. Lost bytes and "U" in flowgraph execution

[pilot327]

Hello everybody

I have the flowgraph below PIC 1 (01_flowgraph)
USRP transmits file (txt, picture and video) and receives this file at the same time.
Transmitted video is received without some bytes and i see letters "U". PIC 1 and PIC 2. at gnuradio terminal

1. What do this letters "U" mean?
1* I think there are anothers error letters. Where can i know about it?
2. I don't know on what it (letters) depends, but when i have 3 or more "U", video*doesn't play (but despite the lack of few bytes and 3 "U", received picture is displayed normal). Why?
3. Why I lost my bytes? PIC 2 (lost_bytes)

i use gnuradio 3.7.10 and usrp1.

Please answer the questions or say where i can find answers.
if you want to answer please do it considering i new in gnuradio and usrp.

 

[boatbod]

One or more gnuradio "U" characters on the terminal generally means underrun. i.e. your sink is not taking data fast enough and the queue is overflowing.