packet sizes are different for TX and RX modes

[AgentAstronaut]

A contributing factor to the difficulty of your application is a lack of a shared time base between the USRPs you are using. I don't think you mentioned the USRP model you are using, but if it is GPS enabled, then you may be able to get them on the same time base. Once you do that, you will have to try to get the devices to reset their clocks to be as close to each other as possible. I don't think you will be able to do any of this with a great degree of accuracy because of the in-deterministic nature of PC timing. But maybe you can look at the wireshark timestamps and get an estimate of the difference in time between the two USRPs. At that point, you can just send the USRP timestamp from the tx to the rx and record the difference, accounting for the time difference between the timestamps. This only works if both USRPs are on the same host.

Another idea we had was that you could send a packet to the RX side, then immediately send the same packet back. You can get the round trip time and divide by two.

I think we understand that you are trying to get the round trip time. We want to know what your application is. What are you using the RTT for?

Also, what is the model of your USRPs?

[Jeff100]

Hi Agent Astronaut,

Thank you for the suggestions.

I am using ni-USRP 2932 now. I used USRP2 in gnuradio as well.

Although I have ni-USRP 2942 with GPS that I can use, but basically USRP clock and synchronization is not my concern.

I tried these two setups:

setup1: host1-----USRP1(TX) ..... USRP2(RX)-----host2, where each USRP connected to its own host.

setup2: .....USRP1(TX)------host-----USRP2(RX)..... , in this setup as you mentioned both USRP connected to same host and timing issue is less.

...... is air interface

----- is Ethernet link

As you mentioned, I am looking to wireshark time-stamps, but I can not map/correlate packets between TX and RX sides, this is my main problem.

As you suggest "send the USRP time-stamp", how I can do that, is there any vi that can refer to that?

Best regards,

Hossein Jafari

[AgentAstronaut]

I would take a look at the USRP palette under Instrument IO » Instr Drivers » NI-USRP. There you can look at the synchronization and utility VI palettes and get idea of what is available. Actually, I was thinking that you may be able to use the wireshark packets to check the time stamps of the commands you are sending to the USRP instead of the data packets. I haven't tried this, but it is an idea. I'm not certain if it is possible.

What are you doing with the RTT information? I don't think there is much more help I can provide with out the knowledge of your over all application. Otherwise we will just be talking in circles, and I want to avoid that.