# Syncronising transmitting and receiving streams to and from a USRP **URL:** \ **Category:** GNU Radio\ **Created:** [May 19, 2015, 5:44pm UTC](https://www.ruby-forum.com/t/syncronising-transmitting-and-receiving-streams-to-and-from-a-usrp/242001 "2015-05-19T17:44:39Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\ **Post date:** [May 19, 2015, 5:44pm UTC](https://www.ruby-forum.com/t/syncronising-transmitting-and-receiving-streams-to-and-from-a-usrp/242001/1 "2015-05-19T17:44:39Z")
Hi All I’m using N210 USRP with XCVR2450 (Rev2) hardware. I’m writing an out of tree module for GRC in C++ for a consensus synchronisation system. The block takes in a certain number of samples from the USRP, then estimated when it needs to transmit its signal. I set the transmit time for the outputted data using the tx\_time tag. I then need to start recapturing the data after the transmission has ended. I was wondering what is the best way to do this? My current plan is to use nitems\_read to count how many samples have been in-putted into the block. So I would have the fist set of data (call this BlockA) that I process to estimate the required Tx time, I would then have a certain period of guard samples (lets call this BlockB) that would occupy the time between the end of BlockA and the start of the transmission. (I have tried to illustrate the frame structure below for two repetitions, with sample number increasing along the page). * * * ## | BlockA(1) | BlockB(1) | Transmit(1) | BlockA(2) | BlockB(2) | Transmit(2) | … I need to know when to expect the first sample of the next BlockA. If the USRP sent a stream tag when it switched from a transmitter to a receiver this would be easy, is this possible?? I thought the USRP are meant to produce a tag on each stream disruption, but I have not been able to see any!? If not, I need to use nitems\_read. I can estimate the length of BlockB by seeing how many samples would exist between the end of BlockA and the requested transmit time. But what would happen if I set the transmit time to not be an integer multiple of the sample period (1/rx\_rate) from the start (I use the initial tag to set a time zero). i.e. Say the samples are arriving at 1us, 2us, 3us… and I set a transmit time of 8.5us. Will the length of BlockB be rounded down to the number of complete samples between the end of BlockA and the start of transmission? I’m worried that if I get this wrong I will start to mix up samples from BlockA and BlockB when the block is run for a long time. Any help with this would be much appreciated. (This question was originally posted in removed\_email\_address@domain.invalid but I was advised to post it here as well) Kind regards Will * * * NOTE: The information in this email and any attachments may be confidential and/or legally privileged. This message may be read, copied and used only by the intended recipient. If you are not the intended recipient, please destroy this message, delete any copies held on your system and notify the sender immediately. Toshiba Research Europe Limited, registered in England and Wales (2519556). Registered Office 208 Cambridge Science Park, Milton Road, Cambridge CB4 0GZ, England. Web: www.toshiba.eu/research/trl ---
**Author:** ![Will\_Thompson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/65b543/32.png) [@Will\_Thompson](https://www.ruby-forum.com/u/Will_Thompson)\ **Post date:** [May 20, 2015, 6:56pm UTC](https://www.ruby-forum.com/t/syncronising-transmitting-and-receiving-streams-to-and-from-a-usrp/242001/2 "2015-05-20T18:56:30Z")
A couple of pointers: - This is a bit like radar, where you want to time rx and tx in sync very tightly. Check out the gr-radar toolbox as an example how to do this. - You can evaluate and write tags to control the transmission time of bursts. Check out the gr-uhd manual for tag names. …I wish I had more advice right now; and I’m still thinking about when the USRP source produces tags. If you’re running it as a full duplex block (i.e. rx is ‘on’ permanently), I’m not sure the tx/rx switching would incur tags, because the GR block wouldn’t know about that. M