Hi all, I am reading from here:
https://wiki.gnuradio.org/index.php/USRP_Sink how to do burt transmission with the usrp_sink. My questions have to do with the second option: ---- Using tagged streams (See Tagged Stream Blocks <https://wiki.gnuradio.org/index.php/Tagged_Stream_Blocks>). To use this capability, you must specify which string the usrp_sink block should be looking for to denote the length of the next PDU to be transmitted. Set the "tsb_tag_name" parameter in the usrp_sink to whatever string your radio application uses to denote your PDU length. A commonly used string for this purpose is simply "tx_pkt_len". If using Tagged Streams for timed bursts, you must include your "tx_pkt_len" tag and a "tx_time" tag on the first sample of a tx burst. If your first "tx_pkt_len" tag has an offset of 0, and your packet length is 1000 items, your next "tx_pkt_len" and "tx_time" tags must appear with an offset of 1000. TX bursts should not overlap, and there should not be gaps in samples between bursts. ---- Q1: why do we need two tags to make this happen? I would think that "tx_pkt_len" tag would be sufficient: when a tag like this is found by the usrp sink then it waits for that many samples to come in and then it transmits them in burst mode. Then it waits for the next such tag and so on. What is the meaning/use of the second tag "tx_time"? Q2: Does the "tx_time" tag have to be exactly that name (as opposed to the "tx_pkt_len" tag which can be user defined?) Q3: The text above is talking about PDU's but my understanding is that a PDU is a special type of a PMT, while the USRP sink input has to be a tagged stream, not a PMT/PDU. Should the input be a PDU (to a message port?) or should it be a tagged stream? thanks Achilleas
