1 physical layer nelson fonseca
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Physical Layer
Nelson Fonseca
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Principle in Action: Nyquist Theorem vs. Shannon Theorem
Nyquist Theorem: Nyquist sampling theorem
fs ≧ 2 x fmax
Maximum data rate for noiseless channel 2 B log2 L (B: bandwidth, L: # states to represent a
symbol) 2 x 3k x log2 2 = 6 kbps
Shannon Theorem: Maximum data rate for noisy channel
B log2 (2(1+S/N)) (B: bandwidth, S: signal, N: noise) 3k x log2 (2 x (1+1000)) = 32.9 kbps
14Chapter 2: Physical Layer
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Twisted Pair Long distances between repeaters Bandwidth depends on the diameter
and length of the cabe Crosstalk and atenuation telephony and data communicatiom Introduces delay (skew) in vídeo
atraso
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Twisted Pair Category 3, 5 e 6 (UTP, Unshield
Twisted Pair) UTP 25 pairs
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Twisted Pair Cat3 (16 MHz) 10BASE TX e
100BASET Cat5 (100 MHz) 100BASE TX e
1000BASET Cat 6 (250 MHz) 1000Base T (1 Gbps) Cat 6e 10000Base T (10 gbps) Cat7 and Cat7a (1Gbps and 10 Gbps) Maximum distance – 100 meters
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Coaxical Cable Base band Coaxical cabel Oliver
Heaviside
50 ohms Digital transmission Maximum 2 Gbits in 1 Km
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Coaxical cabel Broad Band
75 ohms Analog transmission Cable TV, channels 6 MHz - 3 Mbps Unideractional repeaters: single and
duo cable systems
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Coaxial cable
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Power Line Uses
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Optical Fibers Optical Refraction Multimode and
unimode 100Gbps – no
need of amplifier for 100 Km
Three componentes: light source, fiber and optical detector
Solitons: pulse inverse seno hyperpolic format pulsos – long distances without distortion
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Optical Fibers
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Optical Fibers Diameter: multimode (50 micra), unimode
(10 micra). Conectors lose 10% to 20% of light ,
encaixadores (10% de perda), fusão. Source of light: LEDs and semicondictor
lasers, Reception: photodiode 100 Gbps.
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Fiber Cables (a) Side view of a single
fiber. (b) End view of a sheath
with three fibers.
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Fiber Cables (2) A comparison of semiconductor
diodes and LEDs as light sources.
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Optical Fibers Passive Interface:
Conectors, LEDs and photodiode connected with the fiber – no electronic conversion
Does not impair transmission in case of na elemento stops working
Loss of light in connections
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Optical fibers Repeaters:
Converts optical to electronic signal and then to optical again to renerate the power
In case of a fualt of a device, transmission
Long distances
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Optical Fibers: connecting continents
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Optical fibers
Disadvantas: need of experts Expensive interfaces Unidirection
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Electromagnetic Spectrum The electromagnetic spectrum and
its uses for communication.
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Electromagnectic Spectrum
Electromagnetic spectrum Speed of light 3 x 108 m/s (2/3 in
cables) f = c spread spectrum – change of
frequence Hedy Lamarr
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Radio
Omnidirectional: transmitter and receiver do not need to be aligned
Low frequence waves penetrate walls.
High frequencies signals tend to propagate in straight
AM uses MF band, 1000 Km raio.
HF, VHF –refracted at ionosphere
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Microwave
Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly focused Receiver and transmitter aligned. Towers located higher than 100 m, need of
repeaters every 80 Km. Multipath fading, refracted in lower ionosphere Widely used in telephony and TV distribution
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Microwaves High frequencies (10 GHz) absorved
by rain Low costs industrial,cientific and medical band –
no need for permission -902 a 928 MHz 5.725 a 5.850 GHz – wireless phones, gates