Communication Systems and Computer Networks PDF

Communication Systems and Computer Networks (1404703) Prepared by: Prof. Shawkat K. Guirguis Professor of Computer Science & Informatics Part 9 2024-12-0...

Communication Systems and Computer Networks (1404703) Prepared by: Prof. Shawkat K. Guirguis Professor of Computer Science & Informatics Part 9 2024-12-03 (c) Prof.Shawkat K. Guirguis 1  2. Wireless Transmission Keywords: The Electromagnetic Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter Waves Lightwave Transmission 2024-12-03 (c) Prof. Shawkat K. Guirguis 2  The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. 2024-12-03 (c) Prof. Shawkat K. Guirguis 3  Radio Transmission (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere. 2024-12-03 (c) Prof. Shawkat K. Guirguis 4  Radio Transmission Easy to generate, travel long distances and penetrate buildings Transmitter and receiver need not be aligned At low frequencies, they path through obstacles but power drops sharply They are subject to interference from motors and other electrical equipment 2024-12-03 (c) Prof. Shawkat K. Guirguis 5  Microwave transmission Above 100MHz waves travel in nearly straight lines and can be narrowly focused Use of parabolic antenna gives higher signal to noise ratios It does not penetrate buildings May be refracted by the atmosphere Delayed waves may arrive out of phase and cancel the signal (multipath fading) At 4Ghz water absorbs wave!! Easier to manage than fibers (only towers and repeaters are needed), no right for road required as in fibers 2024-12-03 (c) Prof. Shawkat K. Guirguis 6  Politics of the Electromagnetic Spectrum The ISM bands 2024-12-03 (c) Prof. Shawkat K. Guirguis 7  Infrared and Millimeter waves Unguided waves are used for short-range applications, e.g. remote controls of TV and VCR, mice.. etc. Disadvantage, they don’t pass through objects Advantage, no interference between rooms when using IR 2024-12-03 (c) Prof. Shawkat K. Guirguis 8  Lightwave Transmission Mostly use LASER beam Requires a very careful alignment It cannot penetrate rain or thick fog Effect of heat convection current may deflect the beam (see next figure) 2024-12-03 (c) Prof. Shawkat K. Guirguis 9  Lightwave Transmission Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here. 2024-12-03 (c) Prof. Shawkat K. Guirguis 10  3. Communication Satellites Geostationary Satellites Polar Orbiting Satellites: -Medium-Earth Orbit -Low-Earth Orbit Comparing Satellites versus Fiber 2024-12-03 (c) Prof. Shawkat K. Guirguis 11  Polar orbit 2024-12-03 (c) Prof. Shawkat K. Guirguis 12 2024-12-03 (c) Prof. Shawkat K. Guirguis 13  Communication Satellites First trials included bouncing signals of metallized weather Balloons, and the moon itself Satellite is typically a big microwave repeater in the sky It has several TRANSPONDERS that listen to different portions of the spectrum, amplifies signals and then send on another frequency to avoid interference with incoming signals The altitude of satellite determines its coverage area and period Some altitudes are prohibited due to a physical phenomena called Van Allen belts (highly charged particles trapped by the earth’s magnetic field) Only three regions are allowed to place satellites (see next figure) 2024-12-03 (c) Prof. Shawkat K. Guirguis 14  Communication Satellites Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage. 2024-12-03 (c) Prof. Shawkat K. Guirguis 15  Geostationary Satellites At 35,800 km it has a circular equatorial orbit It appears motionless in the sky, i.e. need not be tracked At least 2 degrees are required between each pair to avoid interference, so there may only be 180 of them Many transponders with many different frequencies can be used to compensate for this Orbit slot allocation is done through ITU Often they need (station keeping rockets) to compensate for varying effects of gravity Expected life time is about 10 years (because of rocket motor fuel consumption) Downlink transmission may interfere with existing microwave users and therefore ITU allocated certain frequency band to satellite users (see next table) 2024-12-03 (c) Prof. Shawkat K. Guirguis 16  Communication Satellites The principal satellite bands. 2024-12-03 (c) Prof. Shawkat K. Guirguis 17  Satellite Transmission remarks Latency of 270ms is sometimes unacceptable Being a broadcast media it is simple to send to thousands of stations (e.g. popular web pages) Cost of making phone calls is not affected by distance 2024-12-03 (c) Prof. Shawkat K. Guirguis 18  Medium-Earth Orbit Satellites Take about 6 hours to circle the earth Tracking is required They have a smaller footprint Require less powerful transmitters to earth. Not currently suitable for communication There are 24 GPS (Global Positioning System) satellites at about 18,000 km 2024-12-03 (c) Prof. Shawkat K. Guirguis 19  Low-Earth Orbit Satellites A large number is needed to provide a complete coverage Ground stations do not need much power Round trip delay is a few milliseconds Three examples are: 1. Iridium (for voice communication) 2. Globalstar (for voice communication) 3. Teledesic (for internet) 2024-12-03 (c) Prof. Shawkat K. Guirguis 20  Iridium Project submitted by Motorola 77 satellites project (reduced to 66) Idea is that as soon as a satellite went out of view another would appear. Positioned at 750Km at circular polar orbits One satellite every 32 degrees latitude and form 6 necklaces (see figure) All other companies wanted to participate It had a problem with the introduction of mobiles (almost was bankrupted) Restarted in 2001 and supports hand-held devices It provides voice, data, paging, fax and navigation services in land, sea and air Relaying is done in space (see figure) 2024-12-03 (c) Prof. Shawkat K. Guirguis 21  Low-Earth Orbit Satellites Iridium (a) The Iridium satellites form six necklaces around the earth. (b) 1628 moving cells cover the earth. 2024-12-03 (c) Prof. Shawkat K. Guirguis 22  Startlink Starlink is a satellite internet constellation operated by American aerospace company SpaceX, started in 2019, providing coverage to over 60 countries. It also aims for global mobile phone service after 2023. As of November 2023, it consists of over 5,000 mass- produced small satellites in low Earth orbit (LEO), Nearly 12,000 satellites are planned to be deployed with a possible later extension to 42,000. How does Starlink Satellite Internet Work? ☄ - YouTube 2024-12-03 (c) Prof. Shawkat K. Guirguis 23  Relaying (a) Relaying in space. (b) Relaying on the ground. 2024-12-03 (c) Prof. Shawkat K. Guirguis 24  Comparing Satellites vs Fiber A single fiber has, in principle, more potential bandwidth than all the satellites launched so far, but this bandwidth in not available to every one Many people would prefer to use mobiles in their communication Some stations are essentially broadcasting ones Terrain problems might prevent having cables and fiber optics Launching satellites is sometimes cheaper than having reasonable fiber optic coverage When rapid deployment is essential as in military applications, satellites are the choice 2024-12-03 (c) Prof. Shawkat K. Guirguis 25