On September 3, 2008, Harvind Samra
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
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
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
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,
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
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
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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