1. 2-1 Physical Layer l Theoretical basis for data communications n Fourier analysis n distortion –by different attenuation rates of different frequency components –by different transmission speeds of different frequency components –by limited bandwidth

2. 2-2 Physical Layer (cont’d) l Theoretical basis for data communications (cont’d) n baud rate vs. bit rate n Nyquist’s result –finite bandwidth H Hz –noiseless –number of discrete signal levels V –maximum data rate = 2H log 2 V

3. 2-3 Physical Layer (cont’d) l Theoretical basis for data communications (cont’d) n Shannon’s result –finite bandwidth H Hz –with signal-to-noise ratio S/N –maximum data rate = H log 2 (1+S/N) –If H=3000 (3K) Hz and S/N=1000 (30dB), then the corresponding maximum data rate is around 30Kbps.

4. 2-4 Physical Layer (cont’d) l Transmission media n twisted pair –low cost –few Mbps capacity –widely used in telephone networks (loops) –UTP (unshielded twisted pair) l category 3 l category 5 –STP (shielded twisted pair)

5. 2-5 Physical Layer (cont’d) l Transmission media (cont’d) n coaxial cable –two transmission modes l baseband: 50 ohms, digital, 10Mbps over 1 Km l broadband: 75 ohms, analog, a few hundred Mbps over 100 Km Fig. 2-4 (p. 86)

6. 2-6 Physical Layer (cont’d) l Transmission media (cont’d) n coaxial cable (cont’d) –the most versatile –higher cost –higher noise immunity –higher capacity

7. 2-7 Physical Layer (cont’d) l Transmission media (cont’d) n fiber optics –extremely high capacity (tens of Tbps) –based upon total internal reflection Fig. 2-5 (p. 88)

8. 2-8 Physical Layer (cont’d) l Transmission media (cont’d) n fiber optics (cont’d) –single mode versus multimode Fig. 2-7 (p. 90)

9. 2-9 Physical Layer (cont’d) l Transmission media (cont’d) n fiber optics (cont’d) –operating wavelength Fig. 2-6 (p. 89)

10. 2-10 Physical Layer (cont’d) l Transmission media (cont’d) n fiber optics (cont’d) –LED/laser -- transmission media -- photodiode Fig. 2-8 (p. 91)

11. 2-11 Physical Layer (cont’d) l Transmission media (cont’d) n fiber optics (cont’d) –extremely high noise immunity –thin and light –difficult to splice and tap (good and bad) –interfaces l passive interface: simple, cheap, reliable, limited no. of nodes l active interface: less reliable, longer links, no limit on the no. of nodes

12. 2-12 Physical Layer (cont’d) l Wireless transmission n the electromagnetic spectrum –wavelength times frequency = speed of light Fig. 2-11 (p. 95)

13. 2-13 Physical Layer (cont’d) l Wireless transmission (cont’d) n radio transmission –air/free space as media –easy to generate –can travel long distance (also cause interference) –high penetration capability –omnidirectional

14. 2-14 Physical Layer (cont’d) l Wireless transmission (cont’d) n radio transmission (cont’d) –frequency-dependent transmission properties Fig. 2-12 (p. 98)

15. 2-15 Physical Layer (cont’d) l Wireless transmission (cont’d) n microwave transmission –air/free space as media –line-of-sight transmission –directional transmission (with parabolic antenna) –repeaters needed periodically (spaced 80Km apart with 100-m high towers) –low penetration capability –multipath fading problem –sensitive to weather condition –no right of way needed

16. 2-16 Physical Layer (cont’d) l Wireless transmission (cont’d) n microwave transmission –air/free space as media –line-of-sight transmission –directional transmission (with parabolic antenna) –repeaters needed periodically (spaced 80Km apart with 100-m high towers) –low penetration capability –multipath fading problem –sensitive to weather condition –no right of way needed

17. 2-17 Physical Layer (cont’d) l Wireless transmission (cont’d) n infrared and millimeter waves –air/free space as media –line-of-sight transmission –for short range communication, e.g. TV remote control, wireless LANs –directional, cheap and easy to build –no government license required –very low penetration capability (also low interference) –indoor use only

18. 2-18 Physical Layer (cont’d) l Wireless transmission (cont’d) n lightwave transmission –air/free space as media –line-of-sight transmission –laser is usually used –directional, high bandwidth, cheap and easy to build –no government license required –sensitive to weather condition –aiming problem

19. 2-19 Physical Layer (cont’d) l Wireless transmission (cont’d) n lightwave transmission (cont’d) –deflection problem Fig. 2-13 (p. 101)

20. 2-20 Physical Layer (cont’d) l Wireless transmission (cont’d) n satellites –air/free space as media –big microwave repeater/frequency converter in the air –broadcasting mode –500 MHz bandwidth –significant propagation delay –expensive –large geographic coverage –geosynchronous satellites