gsm voice decoding

[Mamoon Ali]

dear sir,
 

1.  my project is GSM call interception.(using rti-sdr in kali rolling 2016)

2.  i have intercepted SMS in a unencrypted network.

3.  i am able to get the caller's no. in a voice call through the call set up packet in wireshark.

4.  i am able to see the assignment command in SDCCH8 channel(LAPDm protocol),in which i am getting the TCH and timeslot no. of the downlink ch.

5.  but i am unable to extract voice from it.

6.  command used is (to intercept voice)

      grgsm_decode -c /root/gr-gsm/apps/project/capture88.cfile -s 1000000 -f 939.5e6 -m TCHF -t 1 -d FR -o /tmp/speech91.au.gsm
 

 7.   but sir its not working. is speech codec and output file parameters correct?

8.   and sir, in encrypted mobilenetwork i am not getting the CHANGE TO CIPHERING MODE message in wireshark. why?

9.   looking forward for your guidance .plzz reply.

 

                                                                                     best regards

[Piotr Krysik]

Hi Mamoon,

My replies are below.

W dniu 07.05.2016 o 16:02, Mamoon Ali pisze:

> dear sir,
>
> 1. my project is GSM call interception.(using rti-sdr in kali rolling 2016)
> 2. i have intercepted SMS in a unencrypted network.
> 3. i am able to get the caller's no. in a voice call through the call
> set up packet in wireshark.
> 4. i am able to see the assignment command in SDCCH8 channel(LAPDm
> protocol),in which i am getting the TCH and timeslot no. of the downlink ch.
> 5. but i am unable to extract voice from it.
>
> 6. command used is (to intercept voice)
>
> grgsm_decode -c /root/gr-gsm/apps/project/capture88.cfile -s
> 1000000 -f 939.5e6 -m TCHF -t 1 -d FR -o /tmp/speech91.au.gsm
>

Usually in timeslot 1 (-t 1 option) on C0 channel there is SDCCH channel
(not TCHF). If you are able to decode SDCCH find Assignment Command and
Channel Description inside of it. You will have timeslot there and
information if channel hopping is used.

> 7. but sir its not working. is speech codec and output file
> parameters correct?
>

You will have to check it yourself. In my opinion the problem is that
you are looking for the TCHF channel in wrong physical channel (timeslot).

> 8. and sir, in encrypted mobilenetwork i am not getting the CHANGE TO
> CIPHERING MODE message in wireshark. why?

In SDCCH channel Cipher Mode Complete message is first one that is
transmitted as encrypted. I'm not sure if you are talking about this
message.

Best Regards,

Piotr Krysik

[Tomcsányi, Domonkos]

Hi Piotr,

He means the Cipher Mode Command, Cipher Mode Complete is on the uplink :).

Cheers,
Domi

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