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