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.