The OpenBTS project creates a stand-alone cell phone network [LWN.net]

By Forrest Cook
September 10, 2008

On September 3, 2008, Harvind Samra announced the new OpenBTS project:

The Open BTS Project is an effort to construct an open-source Unix application that uses the Universal Software Radio Peripheral (USRP) to present a GSM air interface ("Um") to standard GSM handset and uses the Asterisk software PBX to connect calls. The combination of the ubiquitous GSM air interface with VoIP backhaul could form the basis of a new type of cellular network that could be deployed and operated at substantially lower cost than existing technologies in greenfields in the developing world.

OpenBTS is currently a work in progress, released components (and the associated pile of telecom acronyms) include a Gaussian minimum-shift keying (GMSK) radio modem and interface code for the USRP hardware, GSM forward error correction (FEC) coders and decoders, GSM L3 message serializers/deserializers, a hybrid GSM/SIP control layer, and a partial short message service (SMS) stack implementation. There are plans for expanding the functionality of the various components of the code.

The fairly short project FAQ notes a potential legal issue with a proposed workaround solution: "Although the project founders have built a more complete GSM BTS (base transceiver station), some of that code may be the subject of a legal dispute. While the authors deny any wrongdoing is this matter, it would still not be prudent to release all of the code in these circumstances... Hopefully, the incomplete parts can be replaced quickly."

The OpenBTS developers ran a recent alpha-level system field test at the 2008 Burning Man art/technology festival in the Nevada desert. They applied for and received a temporary FCC license, memorialized by this poster, in order to keep everything legal with the licensing authorities. Around $7000 worth of radio equipment was assembled. To top it off, everything was powered by a small wind generator and a 12V battery.

A WiFi backhaul connection was made to a nearby satellite ground station to provide VoIP connectivity to the external world. Some interesting technical problems were encountered, including being flooded by connections from active cell phones that were looking for connection points when the system was first activated. Another issue discovered was a "security hole" involving unlimited external long distance dialing. After sorting through the various issues, the system was declared operational. Many in-system and external voice and text connections were made, the alpha test was declared a success.

The live field test resulted in exposing a lot of real-world problems that led to numerous code improvements. There's no doubt that sitting in a tent in a hot and windy desert is a fairly difficult environment to develop code in, but progress was made nonetheless. The OpenBTS project illustrates the kind of technical advances that can be made by a small, but dedicated group of people using open-source software and open hardware.

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