27 September 2009

Burning Man 2009: Coverage

On Monday (day 3), we had made our first coverage estimates, sending SMS to ourselves or to short codes that returned status information. We did this a few more times over the course of the week. We had hoped to use RRLP to automate the coverage estimation process, but so few phones were returning useful RRPL results that we had to do it by hand, with a bicycle, a test phone and a note pad. This is a rough estimate of our -90 dBm coverage within BRC:

"A", "B" and "C" are our three sectors, arranged to maximize coverage over the city and insure coverage at the front gate, the airport and the greeters' station. The "x" at 4:30 marks our camp and tower. The "0" at 3:00 marks Fusion Valley, the first link in our backhaul chain. These coverage estimates match those predicted by the Hata suburban model, the same propagation characteristic observed in BRC in our 2008 test.

Despite the failure of RRLP to produce data, we did get some automatic coverage information from measurements of the timing errors of RACH bursts arriving at each BTS. In a GSM system, the timing error of an arriving RACH burst relative to the signal framing yields a rough estimate of the round-trip propagation time from the BTS to the handset and back. We logged every one of the 2,018,321 validated RACH bursts we received and finally got around to sorting through some of that data late last week.

Analysis of RACH burst timing indicated that most of the arriving RACH bursts came from a distance of about 1 km (mode of 1 km and mean of 1.3 km) and nearly all of them (>99%) came from a distance of 3 km or less. That makes sense given our coverage, our tower location, the geometry of the city and the fact that BRC accounts for >90% of the area's population during the festival.

Outside BRC, though, we predicted that coverage would follow the Hata open rural model, meaning that once a handset got out of the shadow of all of the RVs, shipping containers and metal-framed domes, our range could be 15 km or more. There is some evidence of that in our logs. A small fraction of RACH (~0.5%) bursts arrived on sector A from clustered distances of 11 km and 15 km. Gerlach, NV was 15 km away in the direction of sector A, has a population of about 500 and could account for the cluster of access attempts at that distance. We still don't have a good explanation for the cluster of access attempts at 11 km.


  1. What kind of roaming rate do you think AT&T (or $_TELCO anyway) would have applied to someone from Gerlach who accidentally made a call out of your 00 883 x block?

  2. None at all. AT&T has no idea what a phone does while it is on our network. Such a call would have been free to the caller. We would have paid the VoIP fees ourselves, at about $0.012/min for a domestic call.

  3. David, my name is Rizwan Kassim [rizwank@geeky-media.com].

    I was very much looking forward to your presentation at Astricon, but it looks like I'm going to be having to leave the convention a few hours earlier. Will you be posting your slides or a recording anywhere? I missed Burning Man this year, but was very interested to know how the experiment went from your POV.

  4. Rizwan -

    I'm sorry you won't be able to stay to the bitter end. I suppose someone had to be last on the schedule.

    I suspect Astricon will be posting presentations. If not, I'll do it myself. Most of the material will eventually end up here anyway, but it would be bad form to post it all here before the conference presentation.

    As for my own POV, I see the test as great learning experience but not a raging success. Someone in the camp summed it up: "It's a project, not a service." I'm sure if we had another shot in 2-3 months, we would have a solid service, but Burning Man only comes once a year. Maybe before that we can find a real-world testbed somewhere.

  5. Unfortuntely I didn't know you were presenting this at Astricon, so I missed it while there. However I'd like to offer service and support for future endeavors (we're an ITSP). I'm hoping to make it out there next year, but either way I'd be happy to provide no-cost SIP connectivity and Asterisk assistance or servers. My e-mail is my name at my domain.

  6. Is there the possibility that gerlach residents attempting to make 911 calls could have ended up on your network and the result could have been less than useful? Even *trying* this in Australia would see you wind up in prison or dead or both, and that would be the excuse they used.

  7. Gavin -

    That's a very important question that got a lot of thought before we turned on the system and I'm going to answer promptly least anyone out there think we were being reckless. The short answer is this: Given our configuration, that should not have happened, and it if had happened we would have known about it and forwarded the call to appropriate authorities.

    (0) We advertised "test network" parameters. If a handset could see a real AT&T or T-Mobile network it would have preferred the real network and ignored ours.

    (1) Our cell access mask indicated to the handset that we don't support emergency calls, so if someone dialed 911, the handset would have immediately gone looking for another network.

    (2) Upon registration, we put a text message into the handset saying, among other things, "NO EMERGENCY CALLS".

    (3) Emergency call setup is a special procedure the uses different code than a normal phone call. We put alarms all over that code and never saw any of those alarms.

    (4) We configured the system to route emergency calls (if any) to a reliable, dedicated wired phone. It never rang. If it had, with a call from Gerlach, we would have relayed the call to the local sheriff's office, since they know the area best.

    We did all of that to avoid the situation you describe. In the US, they don't kill you; they sue you instead. It's almost as bad and takes much longer because, as a nation, we're sadistic like that.

    As for prison, we were operating under a real license and make our intentions clear to the FCC when we applied for it, so we should have been well clear of any criminal liability.

  8. Simon Barber15 May, 2010 13:19

    How far are the Trego and Bordello hotsprings from the BM site? They are closed during the event, but DPW do use them. This might explain the 11km cluster.

  9. Simon -

    Good idea. I found Trego with Goolgle Earth. 9.7 km. Where's Bordello?

    -- David

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