FakeTRX (Virtual Um-interface)

FakeTRX is a virtual Um-interface implementation written in Python, that allows to connect both OsmocomBB and OsmoBTS without the actual RF hardware. The main purpose of this software is to facilitate and simplify development and testing process. In other words, you don't need to physically run your GSM network nor use any kind of special hardware - just run a few scripts and do anything you want / need in your virtual GSM network!

Source code

FakeTRX is a part of OsmocomBB. The source code is available at

You can browse it via gitea:, and download using git:

$ git clone

Contributions are welcome via Gerrit.


What is the difference from VIRT-PHY?

The main difference is that FakeTRX basically works on GSM L1 and deals with bursts, while VIRT-PHY works on GSM L2, using GSMTAP and multicast sockets to exchange MAC-blocks. FakeTRX provides the TRX Interface for both OsmocomBB and OsmoBTS, and forwards GSM bursts between both sides. So, there is no need to do any modifications in the OsmoBTS source code, just use osmo-bts-trx.


Of course, Python is slower than C, for example. But it's more than enough for exchanging UDP messages between OsmocomBB and OsmoBTS, and vice versa. Moreover, it can be easily reimplemented in C/C++, if someone needs better performance.

Python2 or Python3?

The current implementation is known to work with both versions of Python. FakeTRX is not using any version-specific features or modules.

What about RSSI and ToA (Timing of Arrival)?

Since we are talking about the virtual Um-interface, it's possible to emulate different parameters of the physical RF interface:

  • ToA (Timing of Arrival) - measured difference between expected and actual time of burst arrival in units of 1/256 of GSM symbol periods.
  • RSSI (Received Signal Strength Indication) - measured "power" of the signal (per burst) in dBm.
  • Path loss - bit errors, burst clipping, interference, etc.

By default, ToA is 0 and RSSI is -60 dBm. All simulation parameters mentioned above can be changed at runtime using the control commands with prefix 'FAKE_'. For more details, see documentation of the main FakeTRX class.

Can I run multiple BTS and / or multiple MS instances?

Yes (since #3667 is done)!


This guide assumes that you already have the Osmocom GSM network side stack compiled and installed. If not, the simplest way is to use the Network in the Box.

TRX Interface is a part of the upstream OsmocomBB, just make sure that you have compiled the latest version of trxcon application. FakeTRX is a part of TRX toolkit, that is located in 'src/target/trx_toolkit/'. See README for more details.

Tip: feel free to use tmux or screen to avoid a mess with multiple windows

1. Run the network side stack you have. In this example we will use the Network in the Box:

$ osmo-nitb -c ./openbsc.cfg -l ./hlr.sqlite3 -P -C --debug=DRLL:DCC:DMM:DRR:DRSL:DNM

2. Run the

$ cd osmocom-bb/src/target/trx_toolkit/
$ python ./

3. Start OsmoBTS:

$ osmo-bts-trx -c ./osmo-bts-trx.cfg

Congratulations! Now you have a virtual GSM network running. As you can see, the virtual transceiver emulates the clock source, as this is required for OsmoBTS. Also, it handles only a few important commands, such as RXTUNE and TXTUNE, but ignores other irrelevant ones.

4. In order to "bridge" L2&3 applications with FakeTRX, you need to run trxcon:

$ cd osmocom-bb/src/host/trxcon/
$ ./trxcon

5. Finally, run any L2&3 application, e.g. ccch_scan:

$ cd osmocom-bb/src/host/layer23/src/misc/
$ ./ccch_scan -a ARFCN -i

Please note that ARFCN value should match the one your BTS configured to.

At this stage, you should see the broadcast messages coming from the virtual network, like in case of a real one. You can use Wireshark to analyze them.

Running mobile application

As you may already know, mobile application implements GSM mobile phone with SMS, USSD and voice calls. In the virtual network we can benefit from using a virtual SIM card. Just configure one according to your network configuration, see the example below. If you are starting with the default config from the source tree (osmocom-bb/doc/examples/mobile/default.cfg), make sure to change sim reader to sim test in the ms 1 section.

  imsi 901700000000000
  no barred-access
  rplmn 901 70

Make sure you have the virtual network running, then run mobile the same way as in case of a Calypso based phone:

$ cd osmocom-bb/src/host/layer23/src/mobile/
$ ./mobile -i

Now you can use mobile's telnet interface to manage your virtual phone:

$ telnet localhost 4247
$ ...

Multiple transceivers

It's possible to handle multiple MS and/or BTS connections in a single FakeTRX process using --trx option.

Additional MS-/BTS-side transceiver:

$ ./ --trx

Two child transceivers of the main BTS:

$ ./ --trx --trx

Additional transceiver with name:

$ ./ --trx foo@

Additional transceiver with IPv6 address:

$ ./ --trx ipv6@[2001:0db8:85a3:0000:0000:8a2e:0370:7334]:5700/5


Project status


  • Simulation and randomization of both RSSI and ToA
  • Multiple MS / BTS transceivers (see #3667)
  • Burst capture to file (see
  • Injection of bursts and commands
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Updated by neels over 1 year ago · 44 revisions

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