CPSC 3600 - DAY 9 FEBRUARY 8, 2017 ================================================================================ TRANSMISSION MEDIA ------------------ Guided and unguided transmission .Specific vs. no specific path. .unguided ex. radio transmission Background Radiatin and Electrical Noise .Electrical current flows along a complete circuit. Thus, all transmmissions of electrical energy need two wires to form a circuit - a wire to the receiver and a wire back to the sender. .Computer networks use an alternative form of wiring. .Random electromagnetic radiation permeates the environment .When it hits metal, electromagnetic radiation induces a small signal .Metal acts as a shield Twisted Pair Copper Wiring .Unshielded Twisted Pair .Coaxial Cable .Shielded Twisted Pair Twisted pair wiring or unshielded twisted pair wiring is used extensively in communications. One wire tends to act as a shield that absorbs some of the electromagnetic radiation .Extra metal shielding can be used for further protection. .Coaxial Cable .Thick metal shield formed from braided wires that surround a center wire that carries the signal .Shielded twisted pair (STP) .thinner, more flexible metal shield (usually metal foil) and twisted pairs of wires Optical fibers .Thin strand of glass or plastic encaased in a plastic wrapper .for two-way communication two fibers are used, one to send information and one to receive .Optical fiber cannot be bent at a right angle .Energy is lost when light is reflected in a fiber optic cable .The amount of energy received is always laess than the amount sent .The longer a photon has to travel, the more energy is lost. .Three forms .Multimode Step Index .Multimode Graded Index .Single Mode .Differs in performance and cost .Multimode Step Index Fibers .Least expensive form of fiber optics .Boundry between fiber and wrapper is abrupt which causes the light inside to reflect a lot .High levels of radiation cause dispersion .Best used over short distances .Multimode Graded Index .slightly more expensive .density of fiber is increased at edge .This causes a decrease in light dispersion .mediumn grade fiber optic wire, with performance better than the step index, however is not the best and still has its pitfalls .Single Mode Fiber .Most expensive and provides the least dispersion .Done by using a fiber with a small diameter .Used for longer distances and higher bit rates. .Equipment at each end of a single mode fiber focus light, this allows light to travel over very long distances with little dispersion. InfraRed Communication .Wavelength longer than visible light .Used in remote controls .InfraRed beams are easily obstructed by opaque surfaces and moisture .Maximum speed at 4.0 Mbps Point-to-point .A laser is pointed at a light receptor, the laser relays information across a distance. .Must be precisely mounted in order for information to be transferred .Point to Point communication is usually mounted on two buildings for direct communication Sattelites .Low Earth Orbit (LEO) .Has the advantage of low delay, but the disadvantage that from an observer's point of view on the earth, the satellite appears to move across the sky. .Medium Earth Orbit (MEO) - An eliptical (rather than circular) orbit primarily used to provide communication at the north and south poles .Geostationary Earth Orbit (GEO) - has the advantage that the satellite remains at a fixed position with respect to a location on the earth's surface, but the disadvantage of being further away. Low Earth Orbit Satellites and Clusters .short delays .does not match Earth's rotation .requires ground station to have an antenna and tracking .LEO satellites are designed to work together and can communicate with other satellites in a group Measure a medium of transmitting data .Propogation Delay .Channel Capacity .Harry Nyquist proved that an analog waveform can only be restored from a digital copy if it was sampled more than twice its frequency .Claude Shannon adjusted Nyquist's to account for backgound noise.