USRP1 + FLEX900 -> receive "transmitted" message

Dear all,I am using bbn 802.11 simple mac code, and I encountered a
problem.
The packets which to be sent are read by the code of
"os.read(tun_fd,10*1024).
Just in the second round of transmission, usrp will receive the packet
which
is sent in the last round of transmission.
For testing, I removed the antenna and it was still the same.
Could anyone here give any idea??

Thank you all, sincerely.