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.

What Stuff Costs, Part 1: OPEX

Most African cellular carriers are partly owned by corporations like Millicom and Vodaphone that are traded on stock exchanges in Europe and America.  They publish regular financial reports.  From those reports we can tell that the typical 2007 African cellular subscriber paid $10-$12/month to talk on the phone for just over half an hour.  That sounds like a rip-off until you do a little more math and realize that it actually cost the carrier about $6/month to provide the service, not counting the cost of internetworking.  What the heck?

Let's say, for simplicity, that all of the traffic is compressed into 6 hours each day, so that you see a load of about 0.003 Erlang per subscriber during this peak traffic time.  A minimum 3-sector GSM BTS site provides about 10.5 Erlangs at 2% blocking and thus serves 3,500 subscribers at your typical daily peak load.  If your cost of operation is $6/sub/mo, that corresponds to a cost of about $252k/year per BTS site to run your network, with most of that cost in the BTS site itself: about $200k/year. (!)  When we first estimated this, we though we'd misplaced a decimal point somewhere.  Then we did we read this article in Balancing Act that put the cost of operating an off-grid BTS site in Africa at around $210k/year.  Then we talked to some telecom people from Africa who said the cost was well over $150k/yr but they didn't know by how much.  So it probably really is around $200k/yr.  Why?

It's all about power.  Suppose you have a BTS that draws 5 kW.  And since it's in the tropics you have to cool it, which brings your power budget up to 7 kW.  To supply that, you need a generator.  And since a generator is a target for theft, you need security lighting and cameras, which drive up your power budget and add at least 1 Mb/s to your backhaul requirement, which requires yet more power.  Before long, the site is drawing over 1o kW continuously and you are burning at least 25 gallons of diesel fuel every day.  Now you need a crew with a truck to drive around fixing generators and fences and filling fuel tanks, which is complicated by the fact that most of these sites aren't even near roads.  It starts looking like war logistics, where Sun Tzu tells us that every sack of rice at the front cost 10 more just to get there.  By the time you have everything in place you're spending nearly $20k/mo to keep this beast running.

This matters a lot to the long term development of these countries, because most of the people who live out in the countryside cannot afford $6/mo for anything, meaning that they will never get telephone service, not even on a non-profit basis.  To achieve universal service, someone will need to try something completely different.

So here's the good news: if you can keep site power consumption down to just a few hundred Watts, this all changes dramatically.  Instead of a generator, you can run the whole site on solar panels or microturbines in many parts of the world.  No more diesel fuel.  No more crews in trucks.  Every two years, you replace the batteries in the power system.  That's all.  That's why the design target for OpenBTS is 75 Watts per transceiver, a target that we are very near already just using off the shelf equipment.

Other other cost components in the subscriber rate are internetworking and capital amortization. Most connections between African carriers happen in Europe. That means that if you call from your MTN cell phone to a wired phone down the street that call may well get routed through France at French long distance rates. And the capital cost of rolling out a rural GSM network is at least $100/subscriber.  But those are topics for other posts.