# Packet Flowgraph and underflows **URL:** \ **Category:** GNU Radio\ **Created:** [August 6, 2015, 12:07pm UTC](https://www.ruby-forum.com/t/packet-flowgraph-and-underflows/242901 "2015-08-06T12:07:00Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Rafa\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/958977/32.png) [@Rafa\_F](https://www.ruby-forum.com/u/Rafa_F)\ **Post date:** [August 6, 2015, 12:07pm UTC](https://www.ruby-forum.com/t/packet-flowgraph-and-underflows/242901/1 "2015-08-06T12:07:00Z")
Hi, I’ve been looking for a solution to this and can’t seem to find any examples. If I have a flowgraph that transmits packets for instance from a TCP stream - if this TCP stream only receives data occasionally it will create underflows (showing up as U characters on STDOUT). What is the proper way of handling such a scenario? I thought of using something like the Mute block to transmit a 0 data stream during periods of no data. However that seems excessively complicated to implement and I think I am missing some pointer here … Thanks, Wolfgang ---
**Author:** ![Wolfgang\_Nagele](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/e495f1/32.png) [@Wolfgang\_Nagele](https://www.ruby-forum.com/u/Wolfgang_Nagele)\ **Post date:** [August 6, 2015, 12:51pm UTC](https://www.ruby-forum.com/t/packet-flowgraph-and-underflows/242901/2 "2015-08-06T12:51:24Z")
Hi Wolfgang, no, you do not need to use something like a mute block. The functionality you are looking for is already build into UHD (I’m assuming you are using a USRP as your transmitting device) The UHD sink in GNU Radio is able to react to some specific stream tags [1] one of which is the length tag. You can assign a name for this tag in the UHD source block (‘Length tag name’ field). Now, if you have assigned a value to this ‘Length tag name’ field the UHD sink is somehow reacting as a tagged stream block [2]. If samples arrive at the UHD sink, the USRP will only transmit that amount of samples specified in the length tag starting from that length tag. Therefore, you should not see any ‘U’ messages no more as the USRP automatically stops transmitting and does not expect any more samples follow. However, keep in mind that with tagged streams every first sample of a ‘packet’ needs to have the length tag attached. E.g consider the following stream of samples where the first sample has a length tag assigned to it with value 10: 1 2 3 4 5 5 7 8 9 10 11 | | length tag next length tag needs to be exactly here value = 10 samples Hope that helps. Cheers, Julian [1] [https://gnuradio.org/doc/doxygen/page\_stream\_tags.html](https://gnuradio.org/doc/doxygen/page_stream_tags.html) [2] [https://gnuradio.org/doc/doxygen/page\_tagged\_stream\_blocks.html](https://gnuradio.org/doc/doxygen/page_tagged_stream_blocks.html) ---
**Author:** ![Wolfgang\_Nagele](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/e495f1/32.png) [@Wolfgang\_Nagele](https://www.ruby-forum.com/u/Wolfgang_Nagele)\ **Post date:** [August 6, 2015, 1:58pm UTC](https://www.ruby-forum.com/t/packet-flowgraph-and-underflows/242901/3 "2015-08-06T13:58:35Z")
Hi Julian, Thanks for the pointer. I’m using a HackRF - doesn’t look like the Osmocom sink supports the same construct. Any way of doing something like this in a more generic approach? Cheers, Wolfgang ---
**Author:** ![Wolfgang\_Nagele](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/e495f1/32.png) [@Wolfgang\_Nagele](https://www.ruby-forum.com/u/Wolfgang_Nagele)\ **Post date:** [August 9, 2015, 3:29pm UTC](https://www.ruby-forum.com/t/packet-flowgraph-and-underflows/242901/4 "2015-08-09T15:29:54Z")
Hi Julian, Thanks for this - after a bit of googling I managed to find that as well. Was a bit hard to get started but after fiddling with the block buffers got it running the way I need it. Also for reference - in the Osmocom sink the HackRF does not have the payload length implementation based on tags yet. In case somebody else should need it. Cheers, Wolfgang ---
**Author:** ![Wolfgang\_Nagele](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/e495f1/32.png) [@Wolfgang\_Nagele](https://www.ruby-forum.com/u/Wolfgang_Nagele)\ **Post date:** [August 9, 2015, 3:25pm UTC](https://www.ruby-forum.com/t/packet-flowgraph-and-underflows/242901/5 "2015-08-09T15:25:48Z")
Hi Wolfgang, sorry for the late reply. I’m not familiar with the HackRF but something that might be useful for you as it makes burst mode transmissions in GNU Radio more generic and hardware independent is Eventstream by Tim O’Shea [1]. It should be exactly what you were looking for. It kind of implements the ‘mute block’ concept you were talking about. I hope that gets you started. Let me know if you have any more questions. [1] [http://oshearesearch.com/2015/03/15/burst-transmission-in-gnu-radio-sample-streams-with-eventstream/](http://oshearesearch.com/2015/03/15/burst-transmission-in-gnu-radio-sample-streams-with-eventstream/) Cheers, Julian