19 January 2009

A Tale of Two Licenses

OpenBTS includes a partial implementation of the GSM air interface, Um, the radio link between a GSM handset and its serving basestation. (By "partial", I mean it includes the minimum set of features to support speech telephony and, in release 2.0 and later, text messaging.)

Now, before I go any further, let me say this it not legal advice. This is my best understanding of a situation based on discussions with some strong authorities in the FOSS world.

Since late September 2008, after some convincing from John Gilmore, all distributions of OpenBTS have been under GPLv3. We like GPLv3. We'd do everything under GPLv3 if we could, but that's a story for a different blog. With OpenBTS there's a catch, though. Even though GSM is a publicly available specification, it is not"open". Many essential elements of GSM are covered by patents. These patents are held by companies like Ericsson, AT&T and Alcatel and are registered in the ETSI IPR database. The current GPL distributions of OpenBTS are offered for only private experimental use, which is generally exempt from patent licensing. Furthermore, OpenBTS is presently distributed as software, not an actual, usable end product. Anyone using OpenBTS is expected to comply with all applicable laws, including patent laws.

But let's say you use OpenBTS in a complete product that provides GSM service. You got the source code under GPLv3. And you have licenses for GSM patents. And you sell your GSM product to network operators. Section 6 of GPLv3 requires that you make the source code available to your customers and Section 11 requires that you extend your GSM patent licenses to anyone to whom you distribute the source code. Because these GSM patent licenses cost money and are granted under limited terms, these requirements appear to be in conflict. (This is not a hypothetical situation, BTW.)

Thankfully, there's a loophole of sorts. Look closely at Section 6. It does not say you must distribute the source code. It just says that you must make sure that people who have your product know where to get that source code. So the key to delivering a commercial GSM system under GPLv3 is to make sure that there is at least one party to distribute the source code who
  1. does not hold GSM patent licenses,
  2. has up-to-date copies and
  3. did not receive that code under GPLv3 from anyone who does hold GSM patent licenses.
Note the compound in requirement (3). A party can meet (3) by receiving the code from a party with no GSM patent licenses or by receiving the code outside of GPLv3. There are two parties in the world who have complete releases of OpenBTS who didn't get it under GPLv3: the authors (Kestrel Signal Processing, Inc.) and the Free Software Foundation, who were granted copyrights on 24 October 2008. Either party could fill the role of the distributor as long as that party does not hold GSM patent licenses.

So, here are some scenarios for equipment providers wanting to use OpenBTS:

  • Kestrel distributes OpenBTS to an equipment provider under GPLv3 and then Kestrel makes the source code available to that equipment provider's customers in compliance with GPLv3. That works as long as Kestrel doesn't hold GSM patent licenses and the equipment provider does not want to add proprietary features.
  • Kestrel distributes OpenBTS to an equipment provider under GPLv3 and the FSF makes the source code available to that equipment provider's customers in compliance with GPLv3. That works as long as the equipment providers' software is identical to some public release of OpenBTS. It does not work if the equipment provider wants to add proprietary features.
  • Kestrel provides OpenBTS to an equipment provider under a different license that relieves the equipment provider of all GPL obligations. This would allow the equipment provider to add proprietary features to OpenBTS and operate with no ongoing reliance on Kestrel or the FSF. It is probably the option that most equipment providers would choose. This would cost money, though, since OpenBTS uses other GPL libraries that would need to be sub-licensed.
And here are some scenarios if the project founders want to get into the equipment business:

  • Kestrel starts producing a complete turn-key GSM box, not just source code. Now Kestrel needs GSM patent licenses, so Kestrel no longer meets requirement (1) and can no longer distribute OpenBTS under GPLv3. But Kestrel could transfer source code to FSF let them distribute it under GPLv3 to preserve the open source project.
  • Kestrels' owners set up a new, distinct company to produce and sell GSMequipment and hold the GSM patent licenses. Kestrel extends a non-GPL license to that new company. This last option preserves Kestrel's ability to release under GPLv3, but still allows the project founders to pursue the equipment business.
Is that complicated enough for everyone?


  1. Sounds real complicated. From reading through some of your posts ( and web pages ), would it legal for me to build and sell femtocells in the U.S. based on OpenBTS software?

  2. Almost complicated enough. I'm interested in "the rest of the story". Have Kestrel's owners set up the new, distinct company? (BTW, I refer to option three as "going over to the dark-side". The option to go over to the dark-side is available to the FSF as well because of their large portfolio of copyright assignments.)

  3. Mark, to answer your question, we launched Range Networks, Inc. at DEMO last week for the purpose of producing carrier-grade network equipment based on OpenBTS.