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Unformatted text preview: NOISE: Random disturbances, usually additive Thermal noise Bursttype noises Impulse noise Atmospheric noise THERMAL NOISE Due to random electron motion Proportional to absolute temperature Proportional to bandwidth Spectral density concept: S(f) measures how power of a random noise or signal is distributed in frequency. Thermal noise has a constant spectral density over all frequencies used in communication. Maximum data rate of a channel Crudely, if we send the ideal 2W (2H in text notation) samples/second and can distinguish V = 2k levels, we could send 2Hk = 2H*log2V bits per second. This leaves unanswered how many levels V we could hope to distinguish. This is mainly limited by random noise fluctuations. Shannon developed an equation for a theoretical limit to the maximum data rate, as a function of noise, signal received power, and bandwidth. Assume pure thermal noise. The noise power in a band of H herz is N0H. S is the signal power. S/N = S/N0H is called the signaltonoise power ratio. S/N = S/N0H Shannon proved it is not possible to send data reliably at a rate R > C, but it can be sent reliably at any R < C. In practice, it is difficult to send reliably at R < C if R is very close to C. Often S/N is expressed in decibels (abbreviated dB) (S/N)dB = 10 log10 (S/N) If S/N is very large and bandwidth fixed, C is approximately proportional to (S/N)dB. ATTENUATION is bad because: 1. Reduced S reduces S/N, reduces capacity C. 2. Attenuation is usually greater at the higher frequencies  distortion, pulse spread. 3. Amplification to reduce attenuation contributes to nonlinearities, noise. Amplification after a signal is too weak won't help because noise is equally amplified. In relaying a digital signal, it is better to regenerate it than just amplify. Reason: Regeneration  make binary decisions before signal gets too weak; a clean noisefree signal is sent out. Amplification without decision until final receiver  noises accumulate. GUIDED TRANSMISSION MEDIA (Transported magnetic media  mentioned in text) Twisted pair Coaxial cable Optical fiber Guided media attenuation  measured in dB per kilometer Thus the dB attenuation is additive  measured in dB per kilometer. Attenuation reduces S/N with cable length, which reduces capacity. But there is another factor that reduces capacity further: Higher frequencies generally attenuate more than low frequencies, which reduces the usable bandwidth. Twisted Pair  a pair of wires twisted together to reduce electromagnetic coupling. Common situation: four twisted pairs in a plastic sheath. Often pre wired for 4 telephone connections. Category 3  fewer twists per centimeter Category 5  more twists per centimeter Shielded twisted pair (STP)  metallic sheathing around the pair High rates  even 100 Megabits/second possible for short distances, but high attenuation at high frequencies greatly reduces capacity with length. Coaxial cable Insert Figure 24 Coaxial cable supplies much higher data rates over long distance than twisted pair. Signaling at frequencies up to 500 mhz is feasible up to about a kilometer distance. Applications Television distribution Long distance telephone Short computer system links Local area networks ...
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This note was uploaded on 09/23/2009 for the course CMPEN 362 taught by Professor Johnmetzner during the Spring '09 term at Penn State.
 Spring '09
 JOHNMETZNER

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