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L39review

# L39review - Review for final exam Layered architecture...

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Review for final exam Layered architecture APPLICATION LAYER TRANSPORT LAYER NETWORK LAYER DATA LINK LAYER PHYSICAL LAYER

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PHYSICAL LAYER Limitations to Data Communication rates. Fourier Transform (finite energy time signals) . Important properties: Linear system: Y(f) = X(f)H(f) Scale change: Modulation property:
The Sampling Theorem : 1. Sampling a bandlimited (W) waveform - (Sampling rate at least 2W samples/sec.) Reconstruction from samples: 2. Sending pulses through a bandlimited communication channel: response H(f) . If H(f) = 0 for * f *\$ W, up to a maximum 2W pulses per second (bauds) can be sent through the channel.

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CHANNEL IMPAIRMENTS, LIMITATIONS Distortion, noise, attenuation, interference, limited bandwidth LINEAR DISTORTION: H(f) = A(f)e J è (f) Combat with equalizer filter E(f), so E(f)H(f) has ideal filter characte ristic. NONLINEAR DISTORTION: nonlinearity creates new difference frequencies, (intermodulation distortion). NOISE: Random disturbances, usually additive Thermal noise Burst-type noises
THERMAL NOISE Proportional to absolute temperature and 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 0 density, N /2. Maximum data rate of a channel - Shannon formula - assume thermal noise. 0 The noise power in a band of H herz is N H . S is the signal power.

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ATTENUATION is bad because: 1. Weakened signal reduces information capacity. 2. Attenuation greater at higher frequencies. In relaying a digital signal, it is better to regenerate it than to just amplify it.
GUIDED TRANSMISSION MEDIA Twisted pair, Coaxial cable, Optical fiber Guided media attenuation - in dB per kilometer V is voltage, P is power.

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