Seeking Help with Decoding GSM 2G Uplink Using (One USRP B210), (Two Hackrf) or (Two RTL-SDR ) for Classroom cheating Monitoring

[Hassan Ezzainabi]

Dears,

I hope this email finds you well.

First, I would like to thank the authors and contributors of the projects and tools I have been working with, such as gr-gsm and Multi-RTL, for their tremendous work. Your efforts are greatly appreciated and have been invaluable in advancing the field.

I am reaching out to seek assistance with decoding GSM 2G uplink signals to detect simultaneous calls or phone usage within a classroom, passively. I am not looking to decrypt data or crack encryption, unless it is absolutely necessary for the task. The goal is simply to identify active phone usage for classroom monitoring purposes.

I currently have the following hardware available:

• 2 RTL-SDR dongles

• 2 HackRF devices

• 1 USRP B210

I have successfully used gr-gsm to decode the downlink signals, but I am encountering challenges when attempting to work with the uplink. I have also tried to install the Multi-RTL project, but I was unsuccessful in getting it to work. I have a few questions regarding this:

1. Is the Multi-RTL project compatible with the latest version of gr-gsm, or should I use GNU Radio 3.7 and an earlier version of gr-gsm?

2. Would you recommend any specific steps or best practices for utilizing the USRP B210 (which has full duplex capabilities) for decoding GSM 2G uplink signals passively? Or would it be more effective to use two HackRF or two rtl devices for this purpose?

Your guidance and support would be invaluable in helping me develop this solution, which I believe will promote equal opportunities among students and improve skills and performance within the classroom.

Thank you in advance for your help.

Best regards,

[Piotr Krysik]

Hello,

It seems that your project is very ambitious - even conceptually. First
of all most communication currently happens over newer network
generations than 2G (GSM).

But supposing you want to cover all standards - mobile phone can send
something even if subscriber is not communicating anything (like mobile
is doing location update). Distinguishing if encrypted traffic is
carrying some user data or not will not be trivial. At least it wont be
easier than figuring out how to receive uplink and downlink
simultaneously. If mobile phones are not allowed at all during exams the
your task might be easier, but then you don't need gr-gsm for that.

Multi-RTL works only with GNU Radio 3.7. It was doing the job, but it
was always experimental/proof of concept. I.e. if you choose wrong
synchronization signal or the signal will not be available for a moment
it the time synchronization might not work.

If you want to create something new - I would advice to you to check it
on proven hardware that you know will always work predictably instead of
adding another possible headache.

Regarding your hardware - AFAIK none of it can be easily setup work
synchronously on two different carrier frequencies. If you had two USRPs
B210 and some source of multiple 10MHz Ref and PPS signals (like
Octoclock-G) you could use that. Or some SDR with 2 independent RX
chains (like more expensive USRPs - i.e. X3x0).

Best Regards,
Piotr Krysik

W dniu 21.04.2025 o 15:07, Hassan Ezzainabi pisze:

> Dears,
>
> I hope this email finds you well.
>
> First, I would like to thank the authors and contributors of the
> projects and tools I have been working with, such as gr-gsm and Multi-
> RTL, for their tremendous work. Your efforts are greatly appreciated and
> have been invaluable in advancing the field.
>
> I am reaching out to seek assistance with decoding GSM 2G uplink signals
> to detect simultaneous calls or phone usage within a classroom,
> passively. I am not looking to decrypt data or crack encryption, unless
> it is absolutely necessary for the task. The goal is simply to identify
> active phone usage for classroom monitoring purposes.
>
> I currently have the following hardware available:
>

> *
>
> 2 RTL-SDR dongles
>
> *
>
> 2 HackRF devices
>
> *

>
> 1 USRP B210
>
> I have successfully used gr-gsm to decode the downlink signals, but I am
> encountering challenges when attempting to work with the uplink. I have
> also tried to install the Multi-RTL project, but I was unsuccessful in
> getting it to work. I have a few questions regarding this:
>

> 1.

>
> Is the Multi-RTL project compatible with the latest version of gr-
> gsm, or should I use GNU Radio 3.7 and an earlier version of gr-gsm?
>

> 2.

>
> Would you recommend any specific steps or best practices for
> utilizing the USRP B210 (which has full duplex capabilities) for
> decoding GSM 2G uplink signals passively? Or would it be more
> effective to use two HackRF or two rtl devices for this purpose?
>
> Your guidance and support would be invaluable in helping me develop this
> solution, which I believe will promote equal opportunities among
> students and improve skills and performance within the classroom.
>
> Thank you in advance for your help.
>
> Best regards,
>

> --
> You received this message because you are subscribed to the Google
> Groups "gr-gsm" group.
> To unsubscribe from this group and stop receiving emails from it, send
> an email to gr-gsm+un...@googlegroups.com <mailto:gr-
> gsm+uns...@googlegroups.com>.
> To view this discussion visit https://groups.google.com/d/msgid/gr-gsm/
> cb00a404-2fc6-49b6-bd2f-882b4a594b1dn%40googlegroups.com <https://
> groups.google.com/d/msgid/gr-gsm/cb00a404-2fc6-49b6-
> bd2f-882b4a594b1dn%40googlegroups.com?utm_medium=email&utm_source=footer>.


--
Piotr Krysik