David A. Burgess, Kestrel Signal Processing, Inc.
What You are About to See
• Hybrid GSM/SIP wireless access point.
• AP looks like a GSM network to GSM handsets.
• AP looks like Asterisk to IP network.
• Handsets treated as SIP endpoints.
• Bigger picture? Other aspects?
• Find us at the conference:
• David A. Burgess
• Harvind S. Samra
“Phase 2” GSM Core Network
(GSM 03.02 version 7.1.0 Release 1998) 18 ETSI TS 100 522 V7.1.0 (2000-02)
• 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!
OpenBTS Core Network
• Peer-to-peer APs.
• No BSCs, no MSCs,
• Network interface is
packet-switced network Database
• Network can be as
small as a single
node, but with a
range of 35 km.
air interface, Um
Advantages of Peering
• Core network is commodity IP switching equipment.
• Minimum viable network is a single AP.
• Low cost of entry for a small operator.
• “Bare dirt” infrastructure.
• Low power; solar/wind.
• Any IP backhaul; low bit rates with GSM codecs.
• Very robust.
• Every AP or collection of APs is service area.
• “Islands” continue to function locally.
• “Islands” continue to function globally if they can route enough
packets to the public internet.
• Big vision: “Base of the Pyramid”.
• Rural WLL at the lowest possible cost.
• Our target is US$1/mo with US$5 handset.
• Near-term markets to fund the work.
• Isolated facilities (oil rigs, cruise ships, airliners, etc.)
• Disaster relief (rapid deployment, scant infrastructure)
• Test equipment (network simulation, handset testing)
• GPL alternative to Vanu
• Can replace hybrid L3 with A-over-IP interface.
• Avoid vendor lock with open source.
OpenBTS AP Architecture
• Local RR.
• Map MM
and CC in
Analog the GSM
RF Gear Air Interface
• 10k lines
GSM bursts Private
of C++ in
• Operating in GSM900 band to avoid contention.
• MCC=901, MNC=55
• ARFCN 29 (940.8 MHz down, 895.8 MHz up)
• No BTS tx antenna, just leakage, <30 mW.
• Did your quad-band phone get a text? Well, that’s your fault.
• Handsets are unmodiﬁed refurbs.
• Mostly old Nokias with AT&T SIMs.
• Some have real DID E.164’s in Solano County, California and
• Thanks to Lee Dryburgh, John Gilmore, GNU Radio.
The Perfect Test Site
• Black Rock
City, the 5th
largest city in
• But 50,000
Camp WD9XKN @ 3:15 & G, BRC
• What it looks Preamps
like when you Duplexer
PAP2 Adapter and
build a BTS Test Equipment
Wired Phone for
on Ice Pack
• Going again in