2-1

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

2-2

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

2-3

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.

2-4

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)

2-5

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)

2-6

Physical Layer (cont’d)

Transmission media (cont’d) coaxial cable (cont’d)

– the most versatile

– higher cost

– higher noise immunity

– higher capacity

2-7

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)

2-8

Physical Layer (cont’d) Transmission media (cont’d)

fiber optics (cont’d)– single mode versus multimode

Fig. 2-7 (p. 90)

2-9

Physical Layer (cont’d) Transmission media (cont’d)

fiber optics (cont’d)– operating wavelength

Fig. 2-6 (p. 89)

2-10

Physical Layer (cont’d) Transmission media (cont’d)

fiber optics (cont’d)– LED/laser -- transmission media -- photodiode

Fig. 2-8 (p. 91)

2-11

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

2-12

Physical Layer (cont’d) Wireless transmission

the electromagnetic spectrum– wavelength times frequency = speed of light

Fig. 2-11 (p. 95)

2-13

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

2-14

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

radio transmission (cont’d)– frequency-dependent transmission properties

Fig. 2-12 (p. 98)

2-15

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

2-16

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

2-17

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

2-18

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

2-19

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

lightwave transmission (cont’d)– deflection problem

Fig. 2-13 (p. 101)

2-20

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