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Re: [Discuss-gnuradio] Trying to figure out time of packet arrival in US

From: Sapna Dasarath
Subject: Re: [Discuss-gnuradio] Trying to figure out time of packet arrival in USRP
Date: Tue, 8 Mar 2011 21:21:53 -0500

Thanks for your input Tom. I'll look at UHD and stream tags.


On Tue, Mar 8, 2011 at 9:08 PM, Tom Rondeau <address@hidden> wrote:
On Mon, Mar 7, 2011 at 2:26 PM, Sapna Dasarath <address@hidden> wrote:

I am running usrp_transmit_path.py for transmission on one USRP and usrp_receive_path.py for reception of signals on another USRP.
I want to determine the exact arrival time of a packet on the USRP.

I get the message stream which is received from packetutils.
The code in PacketUtils.py gets a packet from the USRP.
This is populated from copy_from_usrp_buffer  in usrp_source_base.cc.
I am assuming there is some polling mechanism to invoke usrp_basic_rx::read for this.
I want to track the point where this buffer is populated. The time stamp of the packet when it is detected by USRP without any modifications by higher layers. 
Can this be done without any changes in Firmware? Any pointers on how to proceed will be appreciated.


It sounds like you don't want to go this route, but the best thing to do is start using the UHD and stream tags. There is an interface in the UHD source block to send stream tags of the sample times. The UHD tags frames with a time stamp based on the internal clock of the FPGA (which you can tell to reset). The tags are then passed downstream for you to collect later and associate a time value with a particular sample.

The legacy USRP2 interface also allows you to collect the time out of the metadata, but you will have to dig into the libusrp2 library code to get access to it.

If you want accurate time stamps for a USRP1, there really isn't one. You would have to have a differential timer. Look at clock_gettime for getting clock information that you might use.


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