Cellular Telephones

1. Cellular Telephones How Do They Work and Are They Safe? Meagan Morrell May 3, 2000

2. What Is a Cell Phone? • A cell phone is nothing but a radio. • They work like a two-way walkie-talkie that acts like a phone. • Difference being, a walkie-talkie you can either talk or listen; with a cell phone you can talk and listen at the same time.

3. Analog vs. Digital Cell Phones • The difference between analog and digital phones is how the audio signals are transmitted between the phone and base station. • In either system, the audio at the microphone always starts out as a voltage level that varies continuously over time. • High frequencies (high pitch) cause rapid changes and low frequencies cause slow changes.

4. Analog Systems • In this type of system the audio is modulated directly onto a carrier. This acts like FM radio. • Interference (RF noise or some other anomaly that affects the transmitted signal) gets translated directly into the recovered signal – there is no “check” that the signal makes sense.

5. Digital Systems • In this type of system the audio is converted to digitized samples at about 8000 samples per second. • The digital samples are numbers that represent the time-varying voltage level at specific points in time. These samples (numbers) are transmitted in binary code.

6. Digital Systems Continued • At the other end, the samples are converted back to voltage levels and “smoothed out” so that you receive about the same audio signal that was sent. (Some loss does occur.) • With this system the binary code that is sent can not be easily confused or distorted during transmission, plus extra data is typically included in the transmission to help detect and correct any errors. (A “check” does occur in digital systems.)

7. When a Cell Phone Places a Call • After entering a number and pressing [SND]: • The phone checks all 21 control channels and determines which has the strongest signal. • It chooses this signal to place the call. • The phone now transmits a very short message (about ¼ second) that contains the MIN (Mobile Identification Number), its ESN (Electronic Serial Number), and the number being dialed. • After the cellular service provider verifies validity, the base station sends a Channel Assignment to the phone (also a ¼ second burst). • Conversation can now begin.

8. Cellular Radiotelephone Service • A cellular system operates by dividing a large geographical service area into cells and assigning the same channels to multiple, nonadjacent cells. • This allows channels to be reused, increasing spectrum efficiency (25 MHz for each cellular system). • All cells are connected to a Mobile Telephone Switching Office (MTSO) by landline or microwave links.

9. What does the MTSO do? • The MTSO controls switching between Public Switched Telephone Network (PSTN) and the cell site for all calls, it also processes mobile unit status data. • It also uses the control channel to tell the mobile/portable unit which voice channel has been assigned to the call.

10. Safety Issues • It is still unknown to whether cellular telephones are the cause of any health issues. • There has been an equal amount of data found that can “prove” and “disprove” that there is a correlation between health and the radiation form electromagnetic fields that comes from cellular use. • A thought to leave you with about safety, a microwave and a cellular phone both work on GHz frequencies but a microwave cooks food.

11. Where Radiation Comes From • This is a graphical representation of the radiation emitted by a cell phone. (From http://www.lessemf.com/cellphon.html) • Notice the strong radiation coming from the body of the phone, especially the earpiece.

12. How To Minimize Your Risk • Shorter conversations. • Speak as little as possible inside the car because it amplifies the radiation. • Don’t place mobile phones near babies, even on and not in use cell phones emit microwaves. Children tend to be more sensitive to things than adults and it goes for radiation as well.

13. Minimizing Risks Continued • Always pull out the antenna so it is further away from your head. (The microwaves of the phone heat up human cells.) • You can also buy shields for your phone to reduce radiation directed at you. (Graphical representations follow.)

14. Shielding Performance Radiation Scan of a Normal Cell Phone Before & After Phone/Shield Added

15. Conclusion • What has been covered: • Basic cellular use and how cell phones work. • Differences between Analog and Digital signals. • Safety issues and are cell phones health conscious. • Where radiation actually comes from and how to protect yourself and minimize your possible risks.