Hello,
I have been working with the gr-ieee802-11 project for some time but always in software, and now i managed to acquire B210 USRPs to do some testing on air. I am testing the transmitter only, and sniffing from the medium with another pc that has a wi-fi card using TCPDump.
I am experiencing massive overruns on the transmiter when using a pdu of more than 10 bytes. This results in tcpdump not detecting the frames transmitted when the USRP is being overrun. However, when transmitting a frame of 5 bytes, for example, the output from GNU Radio is clean from 'U's and tcpdump detects every frame correctly.
These sizes also vary on the modulation scheme used, if i use 64 QAM 3/4 i am able to transmit more than 10 bytes with no overruns, however, in BPSK 1/2 the limit is 10, which suggest that has to do with the number of symbols transmitted.
I attach an extract of the output of GNU Radio during an execution:
-- Detected Device: B210
-- Operating over USB 2.
....
....
UHD Warning:
The total sum of rates (20.000000 MSps on 1 channels) exceeds the maximum capacity of the connection.
This can cause underruns (U).
.....
.....
UUUUUUUUUUUUUUUUUUUUUUU
I understand that the message states that the capacity of my connection is not sufficient, however, it does not specify how to solve it, or either if it is a configuration problem of the OS / USRP, or a hardware limitation of either the USRP or the PC. I am currently using USB 2.0 to connect the USRP to my computer, which is an Ubuntu 14.04.
Is there a setting from the OS that should be tweaked? How can i fix this problem?
Thanks in advance.