Synchronize multiple USRP-N210s

Hi,
I wanna synchronize two USRPs to sample signal at the time.And the
synchronization accuracy should be of microseconds without external
clock and cable.This leads me to design a synchronization protocol but
not on PC because of the jitter of ethernet.It seems openBTS add a
timestamp block in FPGA.Can anybody give me some detail or other
suggestions.
Thanks.