Re: [Discuss-gnuradio] [USRP-users] some questions about transmit time-tags

[Nick Foster]

On Tue, Jul 31, 2012 at 4:03 PM, Francois Quitin <address@hidden> wrote:

Hi all,

 

We are currently trying to send time-synchronized packets from several transmitters, and we are having some difficulties with the transmit time-tags. Especially, we have trouble understanding what happens with the flow of samples when transmit time-tags are added to certain symbols:

 

-          When a stream of samples arrives at the FPGA, and sample i has a time-tag corresponding to a future time, what happens? Does the FPGA stop transmitting samples until that future time (and does the FPGA generates an underflow? )

Yes, it will stop, but no underflow will occur unless the USRP is streaming continuously (in other words, no tx_eob tag was issued).

 

-          When a stream of samples arrives at the FPGA, and sample i has a time-tag correspond to a past time, the FPGA generates an “L”, but is the stream of samples transmitted anyway (ASAP) or are the samples discarded?

The samples are discarded.

 

-          When a stream of samples arrives at the FPGA, and sample i has a time-tag corresponding to a future time, but corresponding to e.g. the time of sample (i-10). What happens in that case? Does the FPGA transmit all samples and generates a “L” when it gets to sample i, or does the FPGA transmit sample i at the scheduled time and discard the samples (i-10:i-9) ?

I'm not sure what you mean here. Timestamps are intended to be used to send bursts. If you wish to send a burst of data, tag the first sample with "tx_sob" to indicate start of burst as well as "tx_time" to set the time you wish the sample to be transmitted, and tag the last with "tx_eob" to indicate end of burst. See gr_uhd_usrp_sink.cc/tag_work() for the implementation.

Hope this helps,

Nick

 

 

We are working with USRP-N200 with WBX daughterboards, UHD_003.004.001, and we are coding our custom C-blocks that we integrate in a GRC-flowgraph afterwards.

 

Thanks a lot for your help!

 

Francois

 

François Quitin, Ph.D.

 

BAEF Post-doctoral research fellow

Electrical & Computer Engineering Dept.

University of California

Santa Barbara, CA 93106-9560

Phone: +1-805-883-8599

Email: address@hidden

Home: www.ece.ucsb.edu/~fquitin

 

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