OpenBTS

OpenBTS

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OpenBTS is a software-based GSM access point, allowing standard GSM-compatible mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

s to make telephone calls without using existing telecommunication providers' networks. OpenBTS is notable for being the first free software
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

 implementation of the industry-standard GSM protocol stack
Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....

.
It is written in C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...

 and released as free software under the terms of version 3 of the GNU Affero General Public License
Affero General Public License
The Affero General Public License, often abbreviated as Affero GPL and AGPL , refers to two distinct, though historically related, free software licenses:...

 (AGPL).

Open GSM infrastructure


OpenBTS replaces the traditional GSM operator network switching subsystem
Network Switching Subsystem
Network switching subsystem is the component of a GSM system that carries out call switching and mobility management functions for mobile phones roaming on the network of base stations...

 infrastructure, from the Base Transceiver Station
Base Transceiver Station
A base transceiver station or cell site is a piece of equipment that facilitates wireless communication between user equipment and a network. UEs are devices like mobile phones , WLL phones, computers with wireless internet connectivity, WiFi and WiMAX gadgets etc...

 (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
Asterisk PBX
Asterisk is a software implementation of a telephone private branch exchange ; it was created in 1999 by Mark Spencer of Digium. Like any PBX, it allows attached telephones to make calls to one another, and to connect to other telephone services including the public switched telephone network and...

 via SIP
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

 and Voice-over-IP (VoIP).

The reference air interface
Air interface
In mobile or wireless communication, the air interface is the radio-based communication link between the mobile station and the active base station...

 (Um
Um Interface
The Um interface is the air interface for the GSM mobile telephone standard. It is the interface between the mobile station and the Base transceiver station . It is called Um because it is the mobile analog to the U interface of ISDN. Um is defined in the GSM 04.xx and 05.xx series of...

) uses a software-defined radio
Software-defined radio
A software-defined radio system, or SDR, is a radio communication system where components that have been typically implemented in hardware are instead implemented by means of software on a personal computer or embedded computing devices...

 (SDR) on top of the Universal Software Radio Peripheral
Universal Software Radio Peripheral
The Universal Software Radio Peripheral products are a family of computer-hosted hardware offered by Ettus Research LLC and its parent company, National Instruments, for making software radios. The USRP product is intended to be a comparatively inexpensive hardware device for software radio...

 (USRP) USB board.

History


The project was started by Harvind Samra and David A. Burgess. 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—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 that an ARMEL
Armel
Armel may refer to:*Armel, Virginia, unincorporated community in Frederick County, Virginia, United States*Armel , a United States singer*Saint Armel, early 6th century Breton holy man* Armel, a member of the Wu-Tang Clan affiliates...

 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
DEFCON
A defense readiness condition is an alert posture used by the United States Armed Forces. The DEFCON system was developed by the Joint Chiefs of Staff and unified and specified combatant commands. It prescribes five graduated levels of readiness for the U.S...

) 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
Burning Man
Burning Man is a week-long annual event held in the Black Rock Desert in northern Nevada, in the United States. The event starts on the Monday before the American Labor Day holiday, and ends on the holiday itself. It takes its name from the ritual burning of a large wooden effigy on Saturday evening...

 festival in August 2008, a week-long live field test was run under special temporary authorization (STA) license WD9XKN. 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.

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

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
Niue
Niue , is an island country in the South Pacific Ocean. It is commonly known as the "Rock of Polynesia", and inhabitants of the island call it "the Rock" for short. Niue is northeast of New Zealand in a triangle between Tonga to the southwest, the Samoas to the northwest, and the Cook Islands to...

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.

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