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From: | Marcus Müller |
Subject: | Re: [Discuss-gnuradio] Late Packet Arrivals |
Date: | Mon, 23 Nov 2015 10:46:59 +0100 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
Hi Alexander, ha! good catch: I have one block (in python) that sends (via asyncronous message) a command that has the following fields:tx_time internally uses these command. I suspect that processor loading may be an issue because when I run a simpler (but less capable) version of this flowgraph, I do not get the late packet issue.That sounds very reasonable. How does your system calculate the tx_time value? To explain: The PC and the USRP have two completely different notions of time; hence, if the PC wants the USRP to do something at a specific point in time, e.g. in two seconds, then it either has to ask the USRP what time it is now, or explicitly set the device time of the USRP. Of course, if you know the time of the first sample, and you stream continously, then then the device time at the Nth samples is the first sample time + N / f_sample. However, the first sample time somehow has to be known. Since you didn't find anything that sets the time, I'd assume the program figures out what time the USRP has; this can either be done asynchronously via get_time_now, get_time_last_pps, or extracted from the packets coming from the USRP (that only makes sense if you're simultaneously RX'ing with the USRP). Based on that time, the host then calculates tx_time; probably something like rx_time = USRP_time + some_delay. If your flow graph gets too complex, the delay might need to increase. Best regards, Marcus On 22.11.2015 20:44, Alexander Levedahl
wrote:
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