OpenBTS

OpenBTS

Stable release	2.6 / August 1, 2010; 15 months ago (2010-08-01)
Operating system	Unix-like
Available in	C++
Type	GSM protocol stack
License	AGPL (free software)
Website	openbts.sf.net

OpenBTS (Open Base Transceiver Station) is a software-based GSM access point, allowing standard GSM-compatible mobile phones to make telephone calls without using existing telecommunication providers' networks. OpenBTS is notable for being the first free software implementation of the industry-standard GSM protocol stack. It is written in C++ and released as free software under the terms of version 3 of the GNU Affero General Public License (AGPL).

Open GSM infrastructure

OpenBTS replaces the traditional GSM operator network switching subsystem infrastructure, from the Base Transceiver Station (BTS) upwards. Instead of forwarding call traffic through to an operator's mobile switching centre (MSC) the calls are terminated on the same box by forwarding the data onto the Asterisk PBX via SIP and Voice-over-IP (VoIP).

The reference air interface (Um) uses a software-defined radio (SDR) on top of the Universal Software Radio Peripheral (USRP) USB board.

History

The project was started by Harvind Samra^[1] and David A. Burgess.^[2] An aim of the project is to reduce the cost of GSM service provision in rural areas and the developing world to below $1 per month per subscriber. The original developers working on the project have access to licensed test spectrum, but have previously faced legal disputes (now resolved) over earlier related work^[3][4]—meaning that some of the lower-level GSM code is being rewritten.

Platforms

A large number of experimental installations (presumably run without licenses) proved that OpenBTS ran on extremely low overhead platforms including even some CDMA handsets (making a GSM gateway to a CDMA network). Chris Paget reported ^[5] that an ARMEL device (a Droid) could "act as a base station to which handsets can connect; the Droid then connects calls using an on-board Asterisk server and routes them to the PSTN via SIP over Verizon’s 3G network." Paget has also shown (in an unrelated event at Defcon) that GSM calls can be intercepted easily - presumably this can be made much easier using OpenBTS. Paget claims that on an architecture with the clock issues taken care of "it should be as simple as dropping 2 files on the SD card and then running one of them as root."

Field tests

Live tests of OpenBTS have been conducted in the United States in Nevada and northern California. The necessary temporary radio licenses were applied for through Kestrel Signal Processing (KSP)—the original authors' consulting organisation firm—and granted for a short period of time.

Burning Man

During the Burning Man festival in August 2008, a week-long live field test was run under special temporary authorization (STA) license WD9XKN.^[6][7] Although this test had not been intended to be open to Burning Man attendees in general, a number of individuals in the vicinity succeeded in making real out-going calls after a mis-configured Asterisk PBX installation allowed test calls prefixed with an international code through.^[8]

The Burning man test successfully connected about 120 phone calls to 95 different numbers in area codes over North America.^[8]

A second, larger test was run using a 3-sector system at the 2009 Burning Man festival under the STA license WD9XSP, and a 2-sector 3-carrier system was run in 2010.

Niue

During 2010, an OpenBTS system was permanently installed on the island of Niue and became the first installation to be connected and tested with by telecommunication company. Niue is a very small (in terms of population) country with a population of about 1,700, too small to attract mobile telecommunications providers. The cost structure of OpenBTS suited Niue, which urgently required a mobile phone service but did not have the volume of potential customers to justify buying and supporting a conventional GSM basestation system. In March 2011, the single BTS system was removed without the permission of its owner and replaced with a commercial solution from network integrator Challenge Networks based around a commercial system from vendor Lemko. This replacement project was funded entirely with aid from the government of New Zealand. As of September 2011, the calling rates on this new system are high enough to severely limit it use by most Niueans.

See also

Free software portal

• Base station subsystem (BSS)

Related projects

• OpenBSC
• AirProbe

External links

• Official website maintained by OpenBTS original authors.
• Project blog.
• Source code repository at Sourceforge.
• Burning Man test network photographs at Flickr.
• OpenBTS Wiki in Chinese A OpenBTS Wiki in Chinese.

Media

• Phones at Burning Man: Can you hear me now?
• The trouble with OpenBTS
• Extending VoIP to the GSM Air Interface - eComm 2009 talk by David Burgess

References

1. ^ http://www.linkedin.com/in/harvindsamra
2. ^ http://ecommconf.com/2009/speakers/davidburgess/
3. ^ David A. Burgess (2008-09-04). "Homebrew GSM with OpenBTS (comment reply)". Tim Pozar. http://timpozar.blogspot.com/2008/09/homebrew-gsm-with-openbts.html. Retrieved 2008-09-19. "the developing world is our big vision. We've run a lot of numbers and done a lot of research and believe that this kind of technology can be used to drive opex" 
4. ^ http://lists.gnu.org/archive/html/discuss-gnuradio/2008-09/msg00055.html
5. ^ http://www.tombom.co.uk/blog/?p=144
6. ^ "File Number: 0353-EX-ST-2008; Call sign: WD9XKN" (PDF). Experimental Special Temporary Authorization. Federal Communications Commission. 2008-08-20. p. 2. http://openbts.sourceforge.net/FieldTest/WD9XKN.pdf. Retrieved 2009-02-09. "Kestral Signal Processing, Inc. Performing on-the-air testing of an experimental GSM base station using realistic test loads. Locations Gerlach (PERSHING), NV - NL 40–46–09; WL 119–13–12. 869–894 MHz FX 270KGXW 8 W (ERP), 1930–1945 MHz FX 270KGXW 8 W (ERP) ... authorization effective August 24, 2008 .. September 01, 2008" 
7. ^ David A. Burgess (2008-09-04). "The OpenBTS project - an open-source GSM base station (comment reply)". LWN. http://lwn.net/Articles/297038/. "the FCC application for the Burning Man GSM test, I can tell you that what we did was completely legal. Our license was WD9XKN, STA file #0353-EX-ST-2008, good for one week in an area 10 miles NE of Gerlach, NV. We also coordinated spectrum use with a licensed cellular carrier in the area." 
8. ^ ^{a b} "Black Rock City Field Test, Surprise! The BTS is a Little Too Open". The Unofficial Non-Carrier of Burning Man 2008 (Please do not call us Black Rock City Cellular. We are not a telephone company.). Kestrel Signal Processing, Inc.. http://openbts.sourceforge.net/FieldTest/. Retrieved 2010-08-30. "unprovisioned users were making outgoing trunk calls. All they had to do was dial 1 at the beginning of the number to get routed to our VoIP carrier"