[Discuss-gnuradio] carrier_sensed() doesn't work in tunnel.py with USRP N210 and XCVR2450

I am doing routing experiment with two USRPs and tunnel.py. Machine A keeps sending, and another machine B forwards the message when it receives from A. Before B forwards message, it calls tb.carrier_sensed() to do exponential back-off. But I have never see the carrier_sensed() works. If it works, it should print 'B' and doesn't send when machine A is sending. In reality, machine B lost a lot of messages from A, which are supposed to be received with carrier_sensed(). 

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Regards,

Weixian Zhou