Need help with configuration of Timeslots and of multible USRPs - OpenBSC

7 Jul 2014

Hello everybody,

after starting on my new Job at an University I have been tasked to build up
an BTS for our students.

Being a complete noob to both Linux and GSM it took me some time, but I
finally got OpenBSC and Osmo-BTS up and running on Ubuntu Linux 12.04 with
our ETTUS N210.
After doing some experiments with this setup, our students now ask me to do
some changes which I couldn't figure out yet, how to do.

1. The students told me, that OpenBSC doesn't behave like all the BTS from
commercial Providers they analysed:
The say that on commercial nets at the beginning of a phone call there will
be send an "Immediate assignment" sending the MS always to a SDCCH on TS 1
or TS2 where the encryption will be startet. After establishing the
encryption another "Immediate assignment" will occour and send the MS to a
Traffic Channel (which also contains the FACCH / SACCH) for the actual
Communication.
OpenBSC also sends the "Immediate assignment" on TS0 but sends the MS
directly to Timeslot with a Traffic Channel where all the signalling, like
encryption, will be done in the SACCH.
Is there any way to configure OsmoBSC to behave like the commercial BTS the
students described?

2. I have been asked if I could enable "Frequency Hopping" 
Right now I already fail to bring up a second TRX with OpenBSC. 
I start 2 instances of "osmo-trx" with 2 differents IP-Addresses but the
same port, after that I start "osmobts-trx" with the "-t 2"
parameter.
OpenBSC seems to see both USRPs but the LED'S an the USRPs indicate that
only one is completely powered up and transmitting (a quick check with an
Motorola with RSSI also shows only one ARFC).
So how do I have to start  "osmo-trx" to enable transmitting on both URSPs
as TRX0 and TRX1 in one BTS?
After that, is it possible to configure "Frequency hopping" with this setup?
(Because when I changed the value "hopping enabled" of the timeslots in
open-bts.cfg to "1" I get a message that this type of BTS does not support
hopping)

I hope that anyone can clarify those two points to me.

Thanks in advance

Norbie