transmission errors in long frames and unstable operation

[pgawlowicz]

Hello Seemoo Lab team,

First of all, thank you for your great work! I have managed to use it successfully. However, I have a few questions regarding the whole system running on Rpi3+.

1. Is sending a waveform through Nexmon SDR subject to CCA? I was not able to find this information, can we somehow enforce it?

2. I have generated a valid waveform for 802.11 data frame with a duration of 1ms (around 2kB of payload, MCS 1) and 48k Nexmon samples. When I send the waveform using USRP the COST WiFi chip is able to receive it perfectly. However, when I send it over Rpi3+ using Nexmon SDR the frame always has some errors in the payload in the middle of it (i.e. I use a simple repeated string as payload and it using tcpdump as ASCII).

3. For the shorter frames, e.g. 0.5ms (payload of 1kB and MCS 1), there are no errors and the frames can be received successfully. However, only one out of a few frames is received. I send NEX_SDR_START_TRANSMISSION command every 2 seconds to the Broadcom chip and I operate on clear WiFi channel in the 5Ghz band. Is the waveform always sent when issuing the command? Have you already experienced a similar behavior? The entire setup is great, and I want to use it for my prototype, however, I need to make it more robust and deterministic.

Thank you for your answers and help! Best Regards, Piotr

[matthiasseemoo]

Hi Piotr,

1. I am not sure, but I think that it does not care about CCA. However, transmissions often fail, if they start during a reception.
2. You should try to capture the frame using the USRP and decode it by hand, to see where the error comes from and how it happens. Maybe 48k is just too much to be stored in shared memory.
3. This is a problem with our current implementation. Sometimes, for whatever reason, only the carrier gets transmitted but no signal is send out. You can observe it well in a waterfall diagram of a spectrum analyzer.

Matthias

On Mon, Nov 19, 2018 at 11:10 AM Piotr [email protected] wrote:

Hello Seemoo Lab team,

First of all, thank you for your great work! I have managed to use it successfully. However, I have a few questions regarding the whole system running on Rpi3+.

Is sending a waveform through Nexmon SDR subject to CCA? I was not able to find this information, can we somehow enforce it? 2.

I have generated a valid waveform for 802.11 data frame with a duration of 1ms (around 2kB of payload, MCS 1) and 48k Nexmon samples. When I send the waveform using USRP the COST WiFi chip is able to receive it perfectly. However, when I send it over Rpi3+ using Nexmon SDR the frame always has some errors in the payload in the middle of it (i.e. I use a simple repeated string as payload and it using tcpdump as ASCII). 3.

For the shorter frames, e.g. 0.5ms (payload of 1kB and MCS 1), there are no errors and the frames can be received successfully. However, only one out of a few frames is received. I send NEX_SDR_START_TRANSMISSION command every 2 seconds to the Broadcom chip and I operate on clear WiFi channel in the 5Ghz band. Is the waveform always sent when issuing the command? Have you already experienced a similar behavior? The entire setup is great, and I want to use it for my prototype, however, I need to make it more robust and deterministic.

Thank you for your answers and help! Best Regards, Piotr

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