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The approach taken with OpenBSC is markedly different from that adopted by OpenBTS, and rather than seeking to implement as little of GSM as possible it aspires to a more faithful architecture that is plug-compatible. It uses a minimal base station controller (BSC) – the GSM network component that sits upstream of a BTS – and additional network components that run on a commodity Linux PC and can be combined with off-the-shelf BTS hardware to provide network service.

Zoom A Siemens BS-11 Base Transceiver Station
Source: Bjoern Heller, CC-BY-SA-3.0 or GFDL

The motivation behind the project came in 2006 when its leader, Harald Welte, acquired surplus BTS equipment via eBay. Kept busy at the time in his capacity as Lead System Architect for Openmoko, development did not commence until 2008 when Dieter Spaar picked up the same Siemens BS-11 hardware and made rapid progress. Within a matter of months, Welte and Spaar had software capable of controlling the BS-11 via the "A-bis" protocol that is used between a BTS and BSC. The first public demonstration then took place at the Chaos Communications Congress event held in December 2008, and with the first voice call being made in early 2009.

The OpenBSC software can be configured as a BSC only and in this mode of operation it is situated between a BTS and a mobile switching centre (MSC). Alternatively, it can be configured to function as a BSC plus MSC, home location register (HLR), authentication centre (AuC) and equipment identity register (EIR). In this latter mode – dubbed "network in the box" – and when coupled with BTS hardware, it offers a turnkey solution that could be considered roughly analogous to an OpenBTS setup. The similarities don't end there: OpenBSC also uses an Sqlite3 database for maintaining a registry of subscribers, whilst providing tools that include a command line interface for managing this and for system configuration.

Zoom Key elements of a GSM Network
Source: Wikimedia user: Tsaitgaist, GPL v3

OpenBSC was originally developed as a platform for research and experimentation, and indeed it has gone on to find such use at numerous universities, whilst enabling Welte and others to conduct probing research that has exposed serious shortcomings in GSM security. But this is by no means where the story ends, and OpenBSC has gone far beyond original intentions, having been put to use in real world applications that include rapidly-deployable networks for the emergency services and disaster relief, and the provision of maritime mobile phone networks for passengers and crew.

"Privacy in residential applications is a desirable marketing option."

(ETSI EN 300 175-7 Ch. A6)

Quote from an ETSI specification used by Harald Welte as an email signature

In addition to the Siemens BS-11, support is also provided for the Wi-Fi router-sized nanoBTS from ip.access, with development under way for at least four other models of BTS hardware. One of these is the upcoming sysmoBTS from sysmocon, the company set up by Welte to provide mobile communications products and services. There is also even the future possibility of using GSM handsets to provide the BTS network component.

Zoom OpenBSC configured with GPRS support
Source: OpenBSC Project, CC BY-SA 3.0

The project has not drawn the line at voice and SMS and provides experimental support for GPRS data services via the OsmoSGSN software. It is also worth mentioning that OpenBSC is just one of a growing number of projects that are part of the Osmocom family, which includes open source implementations of DECT and TETRA – the digital radio systems that are used by cordless telephones and by public services such as police, fire and ambulance.

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