Cellular Phones

1
Cellular Phones

• SIT Conference 2005
• Heartland Community College

2
Items of Interest

• How Do Cell Phones Work
• Why Is It Called Cellular
• Cell Phone Identity Systems
• Digital Verse Analog Cell Phones

• What Happens When Your Cell Phone Places A Call
• How Does Roaming Work
• How A Telephone System Works

3
How Cell Phones Work

• A cell phone is a radio like a walkie-talkie
• Walkie Talkie is simplex
• Only 40 channels to choose from
• 5 Watts can go about 4 miles
• Cell phone is Duplex
• There are around 1664 channels
• Has few milliWatts only limited range

4
Block Diagram of Cell Phone
From Texas Instruments Web Site http//focus.ti.co
m/vf/docs/blockdiagram.tsp?familyvfblockDiagramI
d2007
5
Why is it called Cellular?

• An antenna tower handles calls from a short
  distance from it (Base Station)
• More antenna towers means more users can talk on
  cell phones
• Lower power the cell phone emits
• Repeat the frequencies used
• The antennas form small cells of coverage around
  the city

6
Cell Sites Around A City
7
Cell Phone Identity Systems

• Every cell phone has 2 unique identifications
• MIN Mobile Identification Number
• A 10 digit telephone number
• 309-555-1212
• ESN Electronic Serial Number
• 32 bit binary number
• Built into the cell phone

8
A Cell Tower

• Inside of a Phone

9
Digital Verse Analog Systems

• Older cell phones were analog
• Analog AMPS, NMT
• Digital GSM, PCS, CDMA
• The difference is between how your voice is
  transmitted between the phone and base station
• Cassette tape as analog
• CDs as Digital

10
What Happens When Your Cell Phone Places a Call

• Enter Phone
• Scan Control Channels finds closest base
  (determines which control signal is strongest)
• Sends Signal ¼ second
• MIN and ESN
• Number you want to call
• Base sends a Channel Assignment
• What frequency pair conversation will be on
• Connection
• The person you are calling is connected

11
How Does Roaming Work

• Phone is on sends data message to cell site (MIN
  and ESN)
• Computer switches at phone company compare MIN of
  your phone to all of their customers MIN
• If you are a customer it gives you HLR (Home
  Location Register)
• If your are from another system, telephone system
  sends messages that you are on the system and
  creates a VLR (Visitor Location Register)

12
How a Telephone System Works

• http//electronics.howstuffworks.com/telephone-ima
  ge.htm