Receive WiFi Signal with USRP N210

I try to receive the WiFi signal with 802.11n standard,and the grc as

It showed many “D”(overflow) in the terminal even at low sample rate
(2M). I tried other two notebooks and use simple grc( uhd_usrp_source ->
file_sink), but they are same. How to solve it?

There is the imformation about the set:
freq = 2.412G, gain = 30, samp rate = 20M
USRP N210 Dboard: SBX
GNU Radio Version: 3.7.3

I tried to change net.core.rmem_max & net.core.wmem_max, and that isn’t
solved the problem.
(Besides, I need to use gr-ieee802-11 to get the AP address with 20M
samp rate.)


Is the problem caused by the Dboard (SBX)?