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chap4

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Information about chap4
Travel-Nature

Published on March 24, 2008

Author: Soffia

Source: authorstream.com

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Chap 4. Transmission Media:  Chap 4. Transmission Media Transmission Media (1):  Transmission Media (1) Physical path between transmitter and receiver Wired and Wireless Communication is in the form of electromagnetic waves Characteristics and quality of data transmission are determined by characteristics of medium and signal In wired media, medium characteristics is more important, whereas in wireless media, signal characteristics is more important Transmission Media (2):  Transmission Media (2) Electromagnetic Spectrum Wired Transmission Media (1):  Wired Transmission Media (1) Twisted Pair The oldest, least expensive, and most commonly used media Pair of insulated wires twisted together to reduce susceptibility to interference : ex) capacitive coupling, crosstalk Skin effect at high frequency Up to 250 kHz analog and few Mbps digital signaling ( for long-distance point-to-point signaling) Need repeater every 2-3 km (digital), and amplifier every 5-6 km (analog) Wired Transmission Media (2):  Twisted Pair (Cont’d) Telephone (subscriber loop: between house and local exchange) High-speed (10 - 100 Mbps) LAN : ex) token ring, fast-Ethernet Wired Transmission Media (2) Wired Transmission Media (3):  Wired Transmission Media (3) Coaxial Cable Most versatile medium => LANs, Cable TV, Long-distance telephones, VCR-to-TV connections Noise immunity is good Very high channel capacity => few 100 MHz / few 100 Mbps Need repeater/amplifier every few kilometer or so (about the same as with twisted pair) Wired Transmission Media (4):  Wired Transmission Media (4) Coaxial Cable (Cont’d) Wired Transmission Media (5):  Wired Transmission Media (5) Optical Fiber Flexible, thin (few to few hundred m), very pure glass/plastic fiber capable of conducting optical rays Extremely high bandwidth : capable of  2 Gbps Very high noise immunity, resistant to electromagnetic interference Does not radiate energy/cause interference Very light Need repeaters only 10’s or 100 km apart Very difficult to tap : Better security but multipoint not easy Wired Transmission Media (6):  Wired Transmission Media (6) Optical Fiber (Cont’d) Need optical-electrical interface (more expensive than electrical interface) Wired Transmission Media (7):  Wired Transmission Media (7) Principle of optical fiber transmission: Based on the principle of total internal reflection If >, medium B (water) has a higher optical density than medium A (air) In case the index of refraction<1 (>), if  is less than a certain critical angle, there is no refracted light i.e., all the light is reflected. This is what makes fiber optics work. Optical Fiber Wired Transmission Media (8):  Wired Transmission Media (8) Principle of optical fiber transmission (Cont’d) - The cladding surrounding the core is also glass but is optically less dense than the core Three types of optical fiber Multimode stepped index Multimode graded index Singlemode (Monomode) Optical Fiber Wired Transmission Media (9):  Wired Transmission Media (9) Three types of fiber transmission - Step index multimode => Variety of angles that reflect. Each angle defines a path or a mode => Limited data rate due to the different path lengths Optical Fiber Wired Transmission Media (10):  Wired Transmission Media (10) Multimode graded index Use the fact that speed of light depends on the medium; light travels faster through less optically dense media The boundary between core and cladding is not sharply defined; Moving out radially from the core, the material becomes gradually less dense Optical Fiber Wired Transmission Media (11):  Wired Transmission Media (11) Three types of fiber transmission (Cont’d) - Single mode => The diameter of the core is reduced to the order of wavelength s.t. only a single angle or mode can pass => Superior performance Optical Fiber Slide15:  Optical Fiber Transmission Mode Wireless Transmission(1):  Wireless Transmission(1) Unguided media Transmission and reception via antenna Directional Focused beam Careful alignment required Omnidirectional Signal spreads in all directions Can be received by many antennae Antenna Gain :  Antenna Gain Antenna gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna: Radiates power equally in all directions) Effective area Related to physical size and shape of antenna Antenna Gain:  Antenna Gain Relationship between antenna gain and effective area G = antenna gain Ae = effective area f = carrier frequency c = speed of light (» 3 x 108 m/s)  = carrier wavelength Propagation Modes:  Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation Ground Wave Propagation:  Ground Wave Propagation Ground Wave Propagation:  Ground Wave Propagation Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example AM radio Sky Wave Propagation:  Sky Wave Propagation Sky Wave Propagation:  Sky Wave Propagation Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops, back and forth between ionosphere and earth’s surface Reflection effect caused by refraction Line-of-Sight Propagation:  Line-of-Sight Propagation Line-of-Sight Propagation:  Line-of-Sight Propagation Transmitting and receiving antennas must be within line of sight Satellite communication – signal above 30 MHz not reflected by ionosphere Ground communication – antennas within effective line of site due to refraction Refraction – bending of microwaves by the atmosphere Velocity of electromagnetic wave is a function of the density of the medium When wave changes medium, speed changes Wave bends at the boundary between mediums Line-of-Sight Equations:  Line-of-Sight Equations Optical line of sight Effective, or radio, line of sight d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction, rule of thumb K = 4/3 Line-of-Sight Equations:  Line-of-Sight Equations Maximum distance between two antennas for LOS propagation: h1 = height of antenna one h2 = height of antenna two LOS Wireless Transmission Impairments:  LOS Wireless Transmission Impairments Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise Attenuation:  Attenuation Strength of signal falls off with distance over transmission medium Attenuation factors for unguided media: Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal Signal must maintain a level sufficiently higher than noise to be received without error Attenuation is greater at higher frequencies, causing distortion Free Space Loss:  Free Space Loss Free space loss, ideal isotropic antenna Pt = signal power at transmitting antenna Pr = signal power at receiving antenna  = carrier wavelength d = propagation distance between antennas c = speed of light where d and  are in the same units (e.g., meters) Free Space Loss:  Free Space Loss Free space loss equation can be recast: Free Space Loss:  Free Space Loss Free space loss accounting for gain of other antennas Gt = gain of transmitting antenna Gr = gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna Free Space Loss:  Free Space Loss Free space loss accounting for gain of other antennas can be recast as Wireless Transmission(2):  2GHz to 40GHz Microwave Highly directional Point to point Satellite 30MHz to 1GHz Omnidirectional Broadcast radio 3 x 1011 to 2 x 1014 Infrared Wireless Transmission(2) Frequencies Wireless Transmission(3):  Wireless Transmission(3) Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates Terrestrial Microwave Satellite Microwave:  Satellite Microwave Satellite is relay station Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Requires geo-stationary orbit Height of 35,784km Optimum transmission in 1 - 10 GHz range; Bandwidth of 100’s MHz Significant propagation delay (270 ms) Application: Television, long distance telephone, Private business networks Wireless Transmission(4) Broadcast Radio:  Broadcast Radio Electromagnetic wave in the range 30 MHz - 1 GHz Omnidirectional Less attenuation than microwave since  is larger Cellular mobile networks, Wireless LAN FM radio, UHF and VHF television Suffers from multipath interference Reflections Wireless Transmission(5) Infrared:  Infrared For short-range communication Remote controls for TVs, VCRs and stereos IRD port Indoor wireless LANs Do not pass through solid walls Better security and no interference (with a similar system in adjacent rooms) No government license is needed Cannot be used outdoors Wireless Transmission(6)

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