Wireless Transmission Media PDF

University of Science & Technology Faculty of Computer Science & Information Technology Department of information for communication and technology Mobile communications & wireless technology_ it 709 4year/sem 7 Tawffeeg Mohammed Tawfeeg Wireless...

University of Science & Technology Faculty of Computer Science & Information Technology Department of information for communication and technology Mobile communications & wireless technology_ it 709 4year/sem 7 Tawffeeg Mohammed Tawfeeg Wireless Transmission Media 2 Outline Overview of transmission Media Wireless Transmission Media The Electromagnetic Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter Waves Lightwave Transmission 3  Transmission Media Transmission medium:: the physical path between transmitter and receiver. 1. Guided media :: waves are guided along a physical path (e.g, twisted pair, coaxial cable and optical fiber) 2. Unguided media :: means for transmitting but not guiding electromagnetic waves (e.g., the atmosphere and outer space). 4 Guided Transmission Data Magnetic Tapes Coaxial Cable Twisted Pair Fiber Optics 5  Wireless Transmission The Electromagnetic Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter Waves Lightwave Transmission 6  Electromagnetic Waves How they are Kind of wave Sometimes behave formed as Transverse with Waves made by alternating electric Waves or as vibrating electric and magnetic fields Particles (photons) charges that can travel through space where there is no matter 7 The electromagnetic spectrum(1) 8 The electromagnetic spectrum(2) The radio, microwave, infrared, and visible light portions of the spectrum can all be used for transmitting information by modulating the amplitude, frequency, or phase of the waves. Ultraviolet light, X-rays, and gamma rays would be even better, due to their higher frequencies, but they are hard to produce and modulate, do not propagate well through buildings, and are dangerous to living things. 9 The electromagnetic spectrum(3) The bands listed at the bottom of the Figure are the official ITU (International Telecommunication Union) names and are based on the wavelengths, so the LF band goes from 1 km to 10 km (approximately 30 kHz to 300 kHz). The terms LF, MF, and HF refer to Low, Medium, and High Frequency, respectively. 10 The electromagnetic spectrum(4) Clearly, when the names were assigned nobody expected to go above 10 MHz, so the higher bands were later named the Very, Ultra, Super, Extremely, and Tremendously High Frequency bands. Beyond that there are no names, but Incredibly, Astonishingly, and Prodigiously High Frequency (IHF, AHF, and PHF). 11 The electromagnetic spectrum(5) The amount of information that a signal such as an electromagnetic wave can carry depends on the received power and is proportional to its bandwidth. It should now be obvious why networking people like fiber optics so much. 12 The electromagnetic spectrum 13  Radio Transmission Radio waves are easy to generate, can travel long distance, and penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. Radio waves are also omnidirectional, meaning that they travel in all directions from the source, so that the transmitter and receiver do not have to be carefully aligned physically. 14  Indoor and Outdoor Campus Distribution In Building Services Cellular Back Haul CELLULAR SERVICE 15  VLF, LF, and MF Bands In the VLF, LF, and MF bands, radio waves follow the ground. These waves can be detected for perhaps 1000 km at the lower frequencies, less at the higher ones. AM radio broadcasting uses the MF band. 16 VLF, LF, and MF Bands Cont. Radio waves in these bands pass through buildings easily. which is why portable radios work indoors. The main problem with using these bands for data communication is their low bandwidth. 17  HF and VHF bands In the HF and VHF bands, the ground waves tend to be absorbed by the earth. However, the waves that reach the ionosphere, a layer of charged particles circling the earth at a height of 100 to 500 km, are reflection by it and sent back to earth. 18  Microwave Transmission Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam by means of a parabolic antenna (like the familiar satellite TV dish) gives a much higher signal-to-noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. 19 Microwave Transmission Cont. In addition, this directionality allows multiple transmitters lined up in a row to communicate with multiple receivers in a row without interference, provided some minimum spacing rules are observed. Before fiber optics, for decades these microwaves formed the heart of the long- distance telephone transmission system. 20  Microwaves Repeaters Microwaves travel in a straight line, so if the towers are too far apart, the earth will get in the way. Thus, repeaters are needed periodically. The higher the towers are, the farther apart they can be. 21 Microwaves Repeaters Cont. The distance between repeaters goes up very roughly with the square root of the tower height. For 100-meter-high towers, repeaters can be 80 km apart 22  Microwaves Microwaves - electromagnetic waves with a frequency between 1GHz (wavelength 30cm) and 12GHz (wavelength 1mm) microwaves frequency are further categorized into frequency bands: L (1-2 GHz), S (2-4 GHz), C (4- 8 GHz), X (8-12 GHz) receivers need an unobstructed view of the sender to successfully receive microwaves microwaves are ideal when large areas need to be covered and there are no obstacles in the 23 path  Microwaves usages  carrier waves in satellite communications  cellular communication  bluetooth  wimax  wireless local area network  GPS (Global Positioning System) 24 Microwave communication concepts microwaves are generated by magnetrons through vibration of electrons LoS (Line of Sight) – is a visible straight line between the sender and the receiver LoS propagation – propagation of microwaves in a straight line free from any obstructions Fresnel zone – eliptical area around the LoS between a sender and receiver; microwaves spread into this area once are generated by an antenna; this area should be free of any obstacles: 25 Microwave propagation modes microwaves, one generated, propagate in a straight line in all directions there are 3 modes of propagation possible, and the mode is decided based on distance and terrain: 26 Microwave signal attenuation 27 Microwaves Multipath Fading Unlike radio waves at lower frequencies, microwaves do not pass through buildings well. In addition, even though the beam may be well focused at the transmitter, there is still some divergence in space. Some waves may be refracted off low-lying atmospheric layers and may take slightly longer to arrive than the direct waves. 28 Microwaves Multipath Fading Cont. The delayed waves may arrive out of phase with the direct wave and thus cancel the signal. This effect is called multipath fading and is often a serious problem. It is weather and frequency dependent. Some operators keep 10% of their channels idle as spares to switch on when multipath fading temporarily wipes out some frequency band. 29 Microwaves absorption by water Bands up to 10 GHz are now in routine use, but at about 4 GHz a new problem sets in: absorption by water. 30 Microwaves absorption by water Cont. These waves are only a few centimetres long and are absorbed by rain. As with multipath fading, the only solution is to shut off links that are being rained on and route around them. 31  Light Transmission Unguided optical signalling or free-space optics has been in use for centuries. A more modern application is to connect the LANs in two buildings via lasers mounted on their rooftops. 32  Light Transmission Optical signaling using lasers is inherently unidirectional, so each end needs its own laser and its own photodetector. This scheme offers very high bandwidth at very low cost and is relatively secure because it is difficult to tap a narrow laser beam. It is also relatively easy to install and, unlike microwave transmission, does not require any license. 33  Light Transmission Cont. Heat from the sun during the daytime caused convection currents to rise up from the roof of the building. This turbulent air diverted the beam and made it dance around the detector, much like a shimmering road on a hot day. 34 Communication Satellites Satellite is relay station Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Types based on orbital altitude: – Geostationary Orbit Satellites (GEO) – Medium-Earth Orbit Satellites (MEO) – Low-Earth Orbit Satellites (LEO) Applications : Television, Long distance telephone, Private business networks 35 Satellite Point to Point Link 36  Satellites Types Communication satellites and some of their properties, including altitude above the earth, round trip delay time and number of satellites needed for global coverage. 37 Thanks for attention These slides are adapted from Computer Networking: A Top Down Approach Jim Kurose, Keith Ross Addison-Wesley March 2012 38