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Transcript
Question #2 (p.146-151): impairments w/ guided media (twisted pair, coaxial cable, and optical fiber): Attenuation and attenuation distortion: a signal becomes weaker when transmitted along any medium at a great distance. Delay distortion: occurs in guided media. It is caused by the fact that the velocity of propagation of a signal through a cable is different for different frequencies. For a signal with a given bandwidth, the velocity tends to be highest near the center frequency of the signal and to fall off toward the two edges of the band. Thus, various components of a signal will arrive at the receiver at different times. Noise: when information is transmitted in the form of an electromagnetic signal, the received signal will consist of the transmitted signal, modified by attenuation and the various distortions imposed by the transmission system, plus the addition of unwanted electromagnetic energy that is inserted somewhere between transmission and reception. The latter, undesired signals are referred to as noise. Noise is the major limiting factor in communication system performance. There are four kinds of noise: thermal noise, intermodulation noise, crosstalk, and impulse noise. Thermal noise is due to thermal agitation of electrons in a conductor. It is present in all electronic devices and transmission media and is a function of temperature. Thermal noise is uniformly distributed across the frequency spectrum and hence is often referred to as white noise. Thermal noise cannot be eliminated. Intermodulation noise occurs when different frequencies share the same transmission medium. Intermodulation reduces frequency at a level equal to the sum or difference of the two original frequencies. For example, intermodulation can produce 12,000 Hz frequency from the two separate signals of 4000Hz and 8000 Hz. Crosstalk happens when you’re having a telephone conversation and you hear another telephone conversation. It is the result of electrical coupling between nearby cables or by the overlap of signals transmitted by antennas. Crosstalk has the same or less magnitudes than thermal noise. Impulse noise: is not continuous, irregular pulse with high amplitude generated by lightning, fault and flaws in the communication system. Impulse noise is the primary source of error in digital communication. Impairments w/ wireless transmission: Free-space loss: data, when transmitted wirelessly, may be lost over a long distance because signals disperse with distance. Atmospheric absorption: Water vapor and oxygen contribute most to attenuation. At 22 GHz, water vapor causes greatest attenuation. At 15 GHz or less, the attenuation is less. Multipath: Obstacles reflect signal creating multiple copies Refraction: radio waves are bent when they go through the atmosphere. Thermal noise: as described above Question #3 explain the three types of media used for transmission of voice and data. Give details on the nature of their use, suitability for an application, bandwidth, and security. (p.158 –167). Descriptions Nature of use & suitability Bandwidth Security Price Twisted Pair Consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs Most common for both analog and digital signals. Used for telephones, local area network for personal computers Digital link for ISN Low frequency. 4 Mbps with long distance applications and 10 – 100 Mbps for local area networks Susceptible to interference and noise, easy to tap. Least expensive and readily available, easy to work with, and light weight Coaxial Cable A cable consisting of one conductor insulated and within another conductor of larger diameter Use for cable TVs, LAN, and telephone Higher bandwidth: 400 – 600 MHz Up to 10,800 conversations Optical Fiber Flexible medium capable of conducting an optical ray. An optical fiber has three sections: center, cladding, and, and protective jacket. Used by telephone companies for long distance trunk lines Used by private companies for local telecommunication network Very high bandwidth: up to 2 Gbps Can be tapped easily Immuned to interference, very hard to tap Very expensive over Small size and light long distance weight, expensive, and require high skill installers Question #4: Describe satellite communication. Give technical details. (p.168 – 172) Satellite communication consists of a satellite and a number of transmitter/receivers, known as earth stations or ground stations. The satellite receives a signal on one frequency (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink). A single orbiting satellite will operate on a number of frequency bands, called transponder channels or transponders. There are two common configurations for satellite communication: point to point between two ground-based antennas and between one-ground based transmitter and a number of ground-based receivers. A satellite is required to be stationary with respect to the earth. It means that the satellite rotation period must be equal to the earth rotation period. Applications of satellite communication include television, long distance telephone, and private business networks. Television: a TV network program is transmitted to the satellite and then broadcast down to a number of stations, which then distribute to the program to individual viewers. The new satellite technology, direct broadcast satellite (DBS), transmits satellite signals directly to home users. Transmission Characteristics: Optimum frequency range for satellite transmission is 1 to 10 GHz. Under 1 GHz, there will be noise from natural sources like galactic, solar, and atmospheric noise, and humanmade devices. Above 10 GHz, the signals will be attenuated by atmospheric absorption and precipitation. There are three bands: C band: C band has 4GHz downlink and 6 GHz uplink. C band is the first band to be designated. Ku band: has 12 GHz downlink and 14 GHz uplink. Rain interference is the major problem with Ku band. Ka band: has 19 GHz downlink and 29 GHz uplink. The problem with Ka band is that equipment needed to use the band is still very expensive. Question #5: Describe the three data encoding techniques, highling their practical uses. (p 183 – 184 and p.188 – 191) 1. Amplitude Shift Keying (ASK) In radio transmission, known as amplitude modulation (AM) The amplitude (or height) of the sine wave varies to transmit the ones and zeros Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude. 2. Frequency Shift Keying (FSK) In radio transmission, known as frequency modulation (FM) Frequency of the carrier wave varies in accordance with the signal to be sent Signal transmitted at constant amplitude More resistant to noise than ASK Less attractive because it requires more analog bandwidth than ASK 3. Phase Shift Keying (PSK) Also known as phase modulation (PM) Frequency and amplitude of the carrier signal are kept constant The carrier signal is shifted in phase according to the input data stream Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying) Remember to draw the diagram on p.184. It is very very important