Benchmark Duplex

Hi

When I try to establish the duplex comunication between two USRPs, I
found
that if I put the code A infront of code B, only the transmitter works,
but
if I put code B infront of code A, only receiver works.
Could anyone tells me the reason?
Thanks a lot!

code A:
self.source = uhd_receiver(options.args, symbol_rate,
options.samples_per_symbol,
options.rx_freq, options.rx_gain,
options.spec, options.antenna,
options.verbose)

code B:
self.sink = uhd_transmitter(options.args, symbol_rate,
options.samples_per_symbol,
options.tx_freq, options.tx_gain,
options.spec, options.antenna,
options.verbose)

Best regards!

Dong W.