# How to pass time tag into usrp sink as a parameter ?\[hurry.\] **URL:** \ **Category:** GNU Radio\ **Created:** [December 16, 2014, 5:23pm UTC](https://www.ruby-forum.com/t/how-to-pass-time-tag-into-usrp-sink-as-a-parameter-hurry/233473 "2014-12-16T17:23:16Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [December 16, 2014, 5:23pm UTC](https://www.ruby-forum.com/t/how-to-pass-time-tag-into-usrp-sink-as-a-parameter-hurry/233473/1 "2014-12-16T17:23:16Z")
Hello, I would like to implement the process as follows at the Rx side: Record the time when the first sample arrived, and then wait for a certain interval and then send ack back to Tx. Meanwhile, finish the cfo calculating and compensating. Here is my whole flow graph design. (Each means a block) ``` pkt_dtc-> cfo_calc -> file_sink cfo_calc -> cfo_cpst file_source -> cfo_cpst -> ``` usrp\_sink where pkt\_dtc, grab\_time\_tag and cfo\_calc was done myself with C++. However, My question are: 1. Is it correct of my whole desgin (I want to implement them in one top block) or are there any easier solutions ? 2. After the block grab\_time\_tag, how should I pass the time information (directly got from the method get\_tags\_in\_range) into the usrp\_sink as a parameter so as to send ack at the intended time? Should I write in Python(but they are not the same level) or C++(but I can’t rewrite a usrp\_sink) ? Any doubts or suggestions are highly appreciated ! ---
**Author:** ![Rui\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/278dde/32.png) [@Rui\_Z](https://www.ruby-forum.com/u/Rui_Z)\ **Post date:** [December 16, 2014, 5:36pm UTC](https://www.ruby-forum.com/t/how-to-pass-time-tag-into-usrp-sink-as-a-parameter-hurry/233473/2 "2014-12-16T17:36:11Z")
Hello Rui, why do you take the detour through a file? That looks unnecessary. the USRP sink understands transmission times you specify using tags [1], so this will be the easiest way to deal with this. You just need to write a really short block (in C++ or python, whatever you prefer) which takes the rx\_time tag that the USRP source added to the sample stream and generates a tag with the desired point in time [2]. There are some examples of the rx\_time and tx\_time tags usage in gr-uhd, so just search for “tx\_time” or “rx\_time” in gnuradio/gr-uhd/examples/ . Greetings, Marcus [1] [http://gnuradio.org/doc/doxygen/page\_stream\_tags.html](http://gnuradio.org/doc/doxygen/page_stream_tags.html) [2] [http://gnuradio.org/doc/doxygen/page\_stream\_tags.html](http://gnuradio.org/doc/doxygen/page_stream_tags.html) ---
**Author:** ![Rui\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/278dde/32.png) [@Rui\_Z](https://www.ruby-forum.com/u/Rui_Z)\ **Post date:** [December 17, 2014, 3:46pm UTC](https://www.ruby-forum.com/t/how-to-pass-time-tag-into-usrp-sink-as-a-parameter-hurry/233473/3 "2014-12-17T15:46:45Z")
Hi, Marcus, I read the doc and examples you recommended, but I still don’t understand fully how to implement my wok. Sorry for my previous omission that the first line of my flow graph design is: usrp\_source -\> ptk\_dtc -\> grab\_time\_tag -\> usrp\_sink 1. Here is the work function of my grab\_time\_tag block(not finished), I don’t know how to code next: const float \*sigin = (const float \*) input\_items[0]; const unsigned char \*detect = (const unsigned char \*) input\_items[1]; pmt::pmt\_t \*out = (pmt::pmt\_t \*) output\_items[0]; 2. I want to send acks at an intended time at usrp\_sink, how to pass this time information into usrp\_sink? I know a C++ method gr::uhd::usrp\_sink::set\_command\_time, but don’t know how to use it. What also confuses me is: if I construct a usrp\_sink object in python, how to pass the time information posted from grab\_time\_tag into it? (They are not at the same level after all). IMHO, Python is not something valuable. It’s just to build block objects and connect them. but if I do the job in C++, does it mean I have to rewrite a new usrp\_sink block(sounds impratical) to receive the desired time value posted from grab\_time\_tag ? or else? BTW, the file\_sink/source was just for testing. I 'm purely a green hand. If something is wrongly understood, please correct me with no hesitation. Hope your kind reply in specific details on the two query above. Many thanks! ??? 2014???12???17??? 00:34, Marcus M??ller ???: ---
**Author:** ![Rui\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/278dde/32.png) [@Rui\_Z](https://www.ruby-forum.com/u/Rui_Z)\ **Post date:** [December 17, 2014, 4:44pm UTC](https://www.ruby-forum.com/t/how-to-pass-time-tag-into-usrp-sink-as-a-parameter-hurry/233473/4 "2014-12-17T16:44:32Z")
On Wed, Dec 17, 2014 at 3:44 PM, Rui Z. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > const unsigned char \*detect = (const unsigned char \*) input\_items[1]; > } > 2. I want to send acks at an intended time at usrp\_sink, how to pass this > time information into usrp\_sink? You could tag your ACK burst with “tx\_time” to send your ACK after a prefered delay. Please look in to Bastian’s packet-pad implementation or pre-cog for reference [1] [https://github.com/bastibl/gr-foo/blob/master/lib/packet\_pad\_impl.cc](https://github.com/bastibl/gr-foo/blob/master/lib/packet_pad_impl.cc) [2] [https://github.com/jmalsbury/pre-cog/blob/master/python/packet\_framer.py](https://github.com/jmalsbury/pre-cog/blob/master/python/packet_framer.py)