CSC103 How Computers Work

Notes 8

Cellular Telephones

1. History
2. Radio Waves
   • electromagnetic waves
   • transmitter: alternating current, produces waves
   • receiver/antenna: responds to some of the energy in a radio wave
   • Wavelength: 1m-10cm (LAN shorter, RFID longer)
   • blocked by bridges, buildings, hills, trees
3. CDMA: Code Division Multiple Access
   • time division: take turns
   • frequency division: different pitches
   • code division: different languages
   • orthogonal codes: do not interfere with one another when overlapped
   • cells can use same frequency, so no need for frequency planning
4. Base station, Switching Center, Gateway
   • Cell: base station, Base Transceiver Station (BTS)
   • in city, maybe 1/2-mile; rural, 5 miles
   • each cell connected to a Mobile Switching Center (MSC), handles a cluster
   • route calls in their cluster, send instructions to base stations
   • Databases:
     • Home Location Register (HLR), tracks geographics location of all cell phones in its clusters
     • authentication database
     • database to route Short Message Service (SMS) messages to cell phones
   • MSCs route calls to a Gateway Mobile Switching Center (GMSC)
   • routes to normal phone system, or to another cellular network
   • Cell handoff
   • each phone receives several signals from nearby cells
   • MSC continually monitors the power level of your signal
   • knows when you are approaching cell boundary
   • MSC orders another cell to take over
5. Cell phone connection
   • Chip contains System ID (SID) and other identifying numbers
   • Phone listens for overhead signal of certain type & frequency, and SID
   • Tunes to strongest signal
   • If SIDs match, in home network. If not, roaming
   • Base stations control power output of phone! (Many times a second under CDMA)
   • Info to base station, to Mobile Switching Center, to Home Location Register
   • Info updated every few minutes
6. Making a call
   • Send sends out # to base station
   • Base station to MSC, routes to the Gateway MSC, which routes to the phone network
7. Receiving a call
   • Call request to GMSC
   • Routes to MSC
   • MSC checks HLR
   • Sends request to base station containing your cell
   • You cell tells base station OK, send me the call
   • MSC to GMSC, route all set up
   • Your phone rings
8. Two channels: control and communication
   • Control for SID exchange etc.
   • Communication for voice.
9. Inside a phone
   • microprocessor
   • Analog to Digital and Digital to Analog converters
   • microphone to bits, bits to speaker
   • Digital Signal Processing to compress and decompress signals
   • "The FCC's E911 initiative requires cell phone carriers to be able to pinpoint their customers' location within 100 meters, so emergency responders can reach them in a crisis. Some phones, like Motorola's A780, come with Global Positioning System transponders built in, which can find you within a few feet."
   • "The law took effect at the start of 2005. That means cell phone manufacturers have incorporated a GPS receiver in virtually every cell phone."
   • Track a phone number here

10. Short Message System (SMS)
    • "the first person-to-person SMS from phone to phone was sent in Finland in 1993"
    • 2.4 billion active users!
    • Uses control channels
    • Need to be short to fit into existing protocols of control channels: 128 bytes, now 160 bytes
    • messages stored at MSC, forwarded when possible
    • message delivered over the control channel
11. SS7: Signaling System Number 7
    • now an international standard
    • protocol to set up and "tear" down phone connections
    • all this work done in commuication channels that may not be the same route as the ultimate voice channel.
    • separate channels increase efficiency
    • and better security
12. GPS
    • "Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions. "
    • "Each GPS satellite has atomic clocks on board. "
    • "There are at least 24 operational GPS satellites at all times. The satellites, operated by the U.S. Air Force, orbit with a period of 12 hours. Ground stations are used to precisely track each satellite's orbit. "
    • 10-20 meter accuracy
    • Computed by intersecting four spheres
    • "When the system was created, timing errors were inserted into GPS transmissions to limit the accuracy of non-military GPS receivers to about 100 meters. This part of GPS operations, called Selective Availability, was eliminated in May 2000."