[Discuss-gnuradio] Question about stream tag

[Pan, Luyuan]

Hello,
    I want to use my USRP2 to implement a TDMA system, and I want to send 5 seconds, receive 5 seconds, and do the recycle. I used the stream tag demo in gr-uhd/taguhd. The question is:
1. As sean have said in http://lists.gnu.org/archive/html/discuss-gnuradio/2012-04/msg00506.html we should use "tx_time", "tx_sob" (start-of-burst), and "tx_eob" (end-of-burst) to tag the data flow, but yend B in http://www.ruby-forum.com/topic/3832986   said the data not enclosed by sob-eob tag pair was dropped. Is this problem really exist? And any solution?
2. Is there any way to switch the Tx/Rx? And how to tell the USRP to receive for a desired time slot such as 5 seconds then stop? The only parameter I can see in gr_uhd_usrp_source is 'rx_time'(tells when to start receive), I do not know how to stop the streaming after 5 seconds.
Thank you for your help. Any suggestion is appreciated.

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Best regards,
Pan, Luyuan