Re: [Discuss-gnuradio] Async Messages for Tx Timestamps

[Sean Nowlan]

On 04/08/2013 04:45 PM, Jordan Otomo wrote:
> Hi,
>
> I have a packet-based application where it would be useful to know exactly when 
> each packet has been successfully radiated by the USRP (N200).  It seems that 
> UHD and gr-uhd already provide similar functionality for receiving asynchronous 
> messages when a burst has been successfully transmitted (EVENT_CODE_BURST_ACK). 
>  However, in my application, I'm transmitting continuously and don't use 
> end_of_burst tags. Would it be possible for the USRP to post messages to the 
> event queue when it has transmitted a sample with a generic tag attached?  If 
> not, will I experience an interruption of my signal or any latency if I insert 
> end_of_burst tags into my continuous stream of samples?  Your advice is greatly 
> appreciated.
I'm a bit confused by what you mean when you say you're transmitting 
continuously in a packet-based application. Are the packets back-to-back 
in your sample stream? Or are you stuffing a bunch of zero samples in 
between packets? That isn't the cleanest way to do it. It would be 
better to use start-of-burst ("tx_sob") and end-of-burst ("tx_eob") tags 
if you're using gnuradio.

Gnuradio end-of-burst tags will signal UHD firmware to bring down the TX 
chain. The tag gets converted into a flag in a metadata struct. This 
struct is sent with an 1-sample buffer of samples when send(...) is 
called in gr_uhd_usrp_sink. (See line  376 and thereabouts here: 
http://gnuradio.org/cgit/gnuradio.git/tree/gr-uhd/lib/gr_uhd_usrp_sink.cc).


> Thanks!
> Jordan
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Hope this helps!

--sean