OFDM Timout

sorry if i keep asking a lot of questions but this time i tried google
1st and didn’t find a solution to my problem

i am trying to transmit audio through ofdm modulator then USRP
then from the Rx of the USRP (same USRP) to OFDM demod to audio sink
i am using basic Tx and basic Rx connected to each other by a SMA cable

when i do that i receive no audio and get a TIMEOUT message
all of the above is done using GRC and i am using the latest version of
gnuradio

Thanx alot for your help