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OpenBTS:A New Approach to
Cellular
David A. BurgessKestrel Signal Processing, Inc.
Fairfield, California
GSM 2G Core NetworkGSM 2G Core Network•From GSM 03.01.
•The key pieces:•MS - Mobile station.•BTS - Base transceiver subsystem (Um L1/L2).
•BSC - Basestation controller (L3 RR)
•MSC - Mobile switching center (L3 CC).
•HLR/VLR - Location registers (L3 MM).
•Must have all 5!ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Understanding “Um”•Um is the GSM subscriber interface.
•Think ISDN, not WiFi.
•~108 kb/s sync serial PHY, TDM’d into logical channels
•shared channels for radio management
•Dm signaling channels (~1.4 kb/s)
•Bm media channels (~13.5 kb/s)
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
OpenBTS• Open-source Um implementation with a
pure SIP/RTP network interface.
• Manages radio resources internally (no BSCs).
• Maps mobility management, call control and SMS to SIP operations (no MSCs, H/VLR).
• Through OpenBTS, GSM handsets appear to VoIP network as SIP endpoints.
• Through OpenBTS, VoIP network appears to handsets as a GSM carrier.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
OpenBTS Node
• Peer-to-peer APs.
• No BSCs, no MSCs, no hair-pinning.
• Network interface is a SIP PBX.
• Network can be as small as a single node, but with a range of 35 km.
OpenBTS Core NetworkOpenBTS Core Network
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
OpenBTS Advantages
•Backhaul GSM through any SIP/IAX carrier.
•Replace GSM core network with commodity HW and open source SW.
•Minimum network is a single cell.
•Use $5 refurb handset as a wireless VoIP terminal.
•Installation/management similar to WISP.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
What OpenBTS Doesn’t Do (Yet)
•Handovers of active calls between cells.
•GPRS data services.
•Encryption.
•Multiple ARFCNs.We expect to find funding to complete these features in 2010. They will just be software
upgrades.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
OpenBTS Source Code
•OpenBTS copyrights assigned to Free Software Foundation in October 2008.
•Kestrel still holds a blanket license.
•Distributed publicly under GPLv3.
•Licensing options and developer copyright assignment similar to Asterisk.
•About 15k lines of C++ in 2.5 release.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
OpenBTS in 2010•Airline application in South America.
•GSM PBX in Netherlands, UK.
•Rural development projects in Brazil and India.
•Rapid deployment system for US DoD.
•Law enforcement and intelligence projects.
•Carrier pilot projects in Polynesia and US.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
• Operating in GSM900 band to avoid contention w/AT&T.
• MCC=001, MNC=01
• ARFCN 29 (940.8 MHz down, 895.8 MHz up)
• Downlink power is about 100 mW.
• Quad-band phone? Try finding us.
• Handsets are unmodified refurbs with a mix of SIMs.
Demo particulars.Demo particulars.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Where to test Um?
• Some things are hard to simulate.
• physical propagation
• heavy loads from many handset models
• Need an open-air range with ready users, but without strong user expectations.
• Need a place within a day’s drive of the SF Bay Area so we can avoid crazy freight costs.
• We have a sense of humor, too: Burning Man.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Fabulous Black Rock City
Aerial view of 2009 Burning Man site.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
First Time Around:BRC 2008
•Experimental license WD9XKN.
•Spectrum consent from Churchill County Communications (GSM850, B-block).
•PSTN gateway from Link2VoIP.
•Static IP and backhaul from Playanet.
•Very tight budget, <$5k.ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
• What it looks like when you build a BTS from eBay and Home Depot.
• No real packaging; just assembled the components on a table.
• Antenna was 10 dBi omni on a 30-ft portable mast.
• Connected about 100 speech calls in last 24 hours.
Camp WD9XKN @ 3:15&G
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Camp WD9XKN @ 3:15&G
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Once More Unto the Breach:BRC 2009
• Experimental license WD9XSP.
• Spectrum consent from Verizon (GSM850, A-block).
• Camp placement (“Papa Legba”, 4:30&H), early access.
• PSTN gateway from Link2VoIP; iNum gateway from Voxbone.
• Static IP and backhaul from Playanet.
• Discussions with BMOrg about social aspects.
• Budget of about $20k.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Papa Legba’s Services•Auto-provisioning via SMS.
•Local SMS and speech calls.
•SMS gateways to iNum and e-mail.
•Speech gateways to iNum and PSTN.
•Expected service radius of about 1.5 miles within BRC, >10 miles in the open desert.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Mobility Strategy
•Every BTS a different “location area”.
•Moving to a new location area causes a handset to perform a “location update”.
•Location update maps to a SIP registration.
•SIP registry for each user points to IP of BTS for most recent location update.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Putting SMS into SIP•SMS is sessionless, page-mode.
•Most IP messaging protocols are session-oriented and inappropriate for cellular.
•OpenBTS SMS interface is RFC-3428.
•smqueue: RFC-3428 store & forward server, uses Asterisk for address resolution.
•Tested integration with Voxbone’s RFC-3428 interface.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Radio Hardware
Power Amp(40 dB, 10 W)
USRP +2 RFX900w/clock
Mini-ITX1.6 GHz Atom
UplinkBPF (90 dB)
Duplexer(70 dB)
LNA(40 dB)Power Buses
(GND, +6, +12)
Pelican Case
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Power System
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Camp “Papa Legba” @ 4:30&G
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Camp “Papa Legba” @ 4:30&G
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Camp “Papa Legba” @ 4:30&G
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Papa Legba IP Network
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Papa Legba GSM Network
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Service Radius Over BRC
Esplanade
Far edgeof BRC
Propagation inside BRC follows Hata suburban model.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Service Radius Over BRC
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Service Radius Beyond BRC
Propagation outside BRC follows Hata open rural model: much longer range.
gate road &Hwy 34
???Gerlach, NV
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Successes•Hardware and packaging
satisfactory.
•Coverage slightly better than predictions.
•Mobility strategy worked.
•Provisioned ~1,300 users via SMS.
•Connected ~1,000 PSTN calls.
•Delivered ~270 user-to-user SMS (and ~3,800 other non-user SMS).
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Back in the Real World•Carrier pilot project scheduled for March
2010.
•Island microstate, 260 sq-km,1,600 residents.
•Cover 90% of population from a single tower.
•Total cost estimate <$50k, including trans-Pacific freight, travel and local network integration.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Building stuff is fun.
Now let’s talk about politics and
economics.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Regulatory Barriers
•Cellular uses licensed spectrum.
•Most countries have licensed out all of their cellular spectrum already.
•New auctions are rare.
•Yet most of the spectrum is fallow, especially in rural areas.
•These things are true all over the world.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Idiotic License Policies•Most US cellular licenses are whole
counties or multiple counties.
•You usually can’t get a rural license without getting cities in the deal.
•Rural customers cost a lot more to serve.
•Once you pay to cover a city, you have no motivation to serve rural customers.
•These things are true all over the world.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Little Motivation to Change
•Spectrum policy might change if there were political pressure.
•Rural areas are typically served by CLECs.
•A minimum GSM network is $100ks.
•Nobody will lobby to get access to spectrum they can’t afford to use.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Changing the Equation?
•OpenBTS can lower cost of entry to $20k for an existing WISP or CLEC.
•The only barrier is a spectrum licensing policy that lets ATT, VZN, etc. lock down huge swaths of fallow rural spectrum.
•If we demonstrate this, will there be more pressure to change the policies?
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
We Will Try•A temporary network in rural Colorado
starting in Spring 2010.
•Existing WISP with tower site, VoIP, trunks. Just add OpenBTS.
•Prove by doing:
•You don’t need $300k CAPEX.
•Profitable network with <200 subs.
•A showpiece for spectrum reform lobbies.
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
Thanks to...•Harvind Samra (the other half of
KSP)
•John Gilmore
•Glenn Edens
•Black Rock City, LLC
•Many others
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010
More Information
•http://openbts.sourceforge.net
•http://gnuradio.org/trac/wiki/OpenBTS
•http://www.kestrelsp.com/OpenBTS.html
•http://openbts.blogspot.com
ANIMAL FARM SALT LAKE CITY - 2010
ANIMAL FARM SALT LAKE CITY - 2010