08 February 2011

Making GSM Future-Compatible

Over the last few weeks, I have been reading through the 3GPP IMS specifications. IMS is the core network for next-generation 4G/LTE mobile data and telephony. Going through the specs is more like bush-whacking than reading; I still can't look at most of the network diagrams without getting dizzy. But I am starting to get a feel for it. In it's essence, IMS is a SIP core network for cellular. Granted, it still looks way more complicated than it needs to be to serve that function, but that's what it is.

Lucky for us, one of the key ideas of OpenBTS is to also use a SIP core network for cellular. So in terms of core networks, we are about five years ahead of the industry, even if the air interface is Um or Uu. We expect the commercial release of OpenBTS to "just plug in" to IMS core networks within a few weeks. IMS compatibility has two big implications for OpenBTS moving forward.

First, it means that there is an application for OpenBTS in incumbent carrier networks that are moving to 4G in the next few years. I've had the opportunity to talk to executives and network engineers from a few carriers who are planning their 4G transitions and have heard the same story over and over. Here it is: "The 4G rollout is expensive, but the performance improvements justify the cost. Except in rural areas, where the subscriber density is too low to justify the expense. But if we keep running GSM/EPGRS or 3G in those areas, then we will have to continue running the old SS7-style core network in addition to the new IMS core network. So we either waste money running two core networks or we waste money installing 4G basestations in the middle of nowhere." The OpenBTS approach offers a solution: Refit your rural sites with an inexpensive OpenBTS-based RAN and then turn off all those BSCs and MSCs.

Second, it eases the minds of carriers looking at greenfield rollouts in the developing world. These carriers need inexpensive networks, but don't want to feel like they are installing obsolete technology. Installing some low-end BTS/BSC/MSC combination just because it's cheap is installing obsolete technology because it will saddle you with an end-of-life core network that you will need to continue to support for years. Sure, you might run your circuit-switched protocols over 802.whatever, a la SIGTRAN, but all that means is that you're not completely stupid; Abis-over-IP is so 1998. On the other hand, installing an IMS-compatible OpenBTS-based network is a first step toward 4G, even if the initial rollout only supports 2G handsets. When the future arrives in your corner of the world, you'll be ready.


  1. Way to go, David ;-) I'm still waiting for your new radio interface! ;-)

  2. Hey David,
    Amazing Job!! Really impressive. Can you help us out with the setup of the USRP and the OpenBTS project that we are trying to implement as a college project in India! I feel this can also create a potential demand for the project at this side of the Globe!! :p
    Please respond!!

  3. Naga -

    As you subscribed to the openbts-discuss mailing list at lists.sourceforge.net? There are several other student projects subscribed there and it is probably the best forum for the kind of support you will need.

  4. Jakob -

    Any week now... Hardware development seems to follow a Zeno's Paradox kind of schedule, but we have a good team on the task and have been making steady progress.

  5. David, i'm keeping my fingers crossed ;-) I'd really like to put that in a few places around places i' frequently at, but there's no GSM reception. It'd make a decent evaluation strategy for the usability of OpenBTS in places like that (more or less like femtocell, but with much more power and control)

  6. Please contact me: leif.eriksson@interestingtimesmagazine.com

  7. I'm looking for open-source, low-cost drive-test kit. Any? br.

  8. There are several other student projects subscribed there and it is probably the best forum for the kind of support you will need.


  9. Brilliant! I love how you can possibly jumpstart mobile development in many countries. It also means immense benefits to companies such as ours. We thrive on the expansion of the mobile space. :-)

  10. Hey David
    what kind of modem do I need to this type of setup? can I use a MultiTech Wireless Modems:Cell:GPRS modem or do I need one of them USRP modems to do this with?

  11. Hi

    Is there a way to connect openbts with existing network?

    For example, There is a telecom operator providing services in most of the cities in a country and it plans to grow it's network. So can they use openbts in the newer cities? so that the calls go via the existing network through bsc and msc instead of Asterisk or Freeswitch. I think it's utilization will increase and operators will say buy to other costly vendors.

    I would say if this can be done then, we can have a more cost effective GSM network.