Physical Layer Theoretical basis for data communications Fourier analysis distortion by different attenuation rates of different frequency components by different transmission speeds of different frequency components by limited bandwidth

Physical Layer (cont’d) Theoretical basis for data communications (cont’d) baud rate vs. bit rate Nyquist’s result finite bandwidth H Hz noiseless number of discrete signal levels V maximum data rate = 2H log2V

Physical Layer (cont’d) Theoretical basis for data communications (cont’d) Shannon’s result finite bandwidth H Hz with signal-to-noise ratio S/N maximum data rate = H log2(1+S/N) If H=3000 (3K) Hz and S/N=1000 (30dB), then the corresponding maximum data rate is around 30Kbps.

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

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

Physical Layer (cont’d) Transmission media (cont’d) coaxial cable (cont’d) the most versatile higher cost higher noise immunity higher capacity

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

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

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

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

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

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

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

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

Physical Layer (cont’d) Wireless transmission (cont’d) 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

Physical Layer (cont’d) Wireless transmission (cont’d) 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

Physical Layer (cont’d) Wireless transmission (cont’d) 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

Physical Layer (cont’d) Wireless transmission (cont’d) 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

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

Physical Layer (cont’d) Wireless transmission (cont’d) 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