In practice it is difficult to send reliably if r is

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Unformatted text preview: . In practice, it is difficult to send reliably 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 noise-free signal is sent out. Amplification without decision until final receiver - noises accumulate. G UIDED T RANSMISSION MEDIA Twisted pair Coaxial cable Optical fiber Guided media attenuation - dB/kilometer Thus the dB attenuation is additive - measured in dB per kilometer. At...
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