Commercial MSs do not see the network - OpenBSC

12 Feb 2020

Hi All,

I have been dealing with a weird issue in the past couple of days. I needed to quickly
check something about some phones, therefore I created an Osmocom CNI based 2G network. I
have done this multiple times in the past even with OsmocomBB transceiver, however in the
past some years I was not active in 2G, was more focused on 4G.
I was very happy to see, that all familiar things I initially liked about osmocom are
still in place/improved a lot (meaningful config files, simple VTY interface, a LOT more
and better documentation). So the network is up and running fine, the setup consists of a
laptop and a USRP B210 without GPSDO.

Then I tried to attach some phones to it, and hit an issue:
* Samsung S6, S7 - does not detect the network, even if I put in a SIM card that has
previously been in a phone that was already attached
* iPhone X - does not detect the network

I first fiddled with the power settings, basically trying to turn it down a little bit
(initially tx attenuation was at 0, so I turned it 10-20), because range is not needed,
but I know distortion can happen when driving SDR HW to its max capacity.
No change in the above.

Then I turned quickly to gr-gsm and rtl-sdr to see if I'm transmitting properly, the
network was detected and C0 decoded properly.
Next I tried with a Motorola C115 (yeah, I still have some lying around :)), and it
attached to the network without any issue. Finally something that works as expected,
right?
Last but not least I tried with a Nokia 4.2 phone, which also attached to the network
immediately without any issue.

So the bottom line is: what could cause this behavior? Is it possible that the USRP
B210's accuracy is just not enough for newer phones? If yes how come the Nokia still
attached, it is quite a recent phone with Qcomm chipset. I was also thinking if it is a
Samsung vs Qualcomm thing, but then the iPhone should have at least detected the network.
I tried with different SIM cards as well (not that it would make any difference...but I
didn't have any better idea), and also with different bands and ARFCNs (900 and 1800).
Antenna used is a VERT900.
Configuration: unmodified standard config that came with the binary packages - except for
MCC, MNC, rx gain, tx attenuation and max_power_red.
I also tried to specify valid and also 'invalid' (unknown) MCC-MNC combinations to
see if maybe newer Android/iOS only lists networks that advertise a known combination
(again this would be very weird, but I just had no better ideas in the end).

Anybody have any recent experience with COTS MSs and OsmoBTS(-TRX) based network?

Any input is much appreciated, thanks.

Cheers,
Domi

p.s. It is simply awesome to experience the power of opensource when dealing with 2G.
Every step of the way above I was using software created and shared by you guys. It is a
great feeling, had to share it :-).