1

Physical Layer

Nelson Fonseca

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

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

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

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

Optical Fibers: connecting continents

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Optical fibers

Disadvantas: need of experts Expensive interfaces Unidirection

31

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