data&signal5 - modulation Digital signal analog signal...

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modulation) : Digital signal analog signal If the available channel is a “ bandpass ”, i.e., the channel lower bound freq., f low , can be any non zero value, we need to convert our digital signal to analog signal ( modulation process). l Signal to Noise Ratio: SNR dB = 10log 10 SNR where SNR = Signal power watt / Noise power watt = 10 (SNR(DB)/10) Noise channel: Nyquist Capacity (max channel bit rate) Max Bit Rate ––– C = 2 BW ch * log 2 L Where L is the number of signal levels Noisy Channel: Shannon Capacity C = BWch log 2 (1 + SNR) Performance of Networks
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1) Throughput: The actual speed of data traveling a link (notice, the BW of a link is its potential speed) Throughput (T) <= BW Example: bad encoding might waste BW in case of Ethernet ME. Therefore T ME < BW 2) Delay (Latency): L = propagation + T x + queuing + processing Propagation delay = Distance (m) / propagation speed (m/s) T x delay = message size (bit) / data-rate (bit/sec) 3) Jitter: Packets of data will encounter different delays. (not very good for real time applications!) Digital Transmission Digital to digital conversion (Line Coding) Converting a sequence of data bits (text, numeric, audio, or video) into a digital signal, at the sender, then recovering the original bit sequence from the signal, at the destination data rate bit rate whereas signal rate baud rate The goal is to increase the data rate (information flow) while decreasing band rate (better utilization of channel BW, cheaper links) Factors to consider in digital signaling:
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1) Baseline wandering: The receiver averages the signal power: “Baseline”, and uses it to decode the received signal bit value. Long strings of 0’s or 1’s causes a drift of the obtained baseline, hence “baseline wandering” that leads to incorrect bit decoding. 2) DC components: Constant level for long period of time creates very low frequency components in the frequency spectrum, that might not pass through some medium (e.g., TP of 200Hz 3000Hz). Hence, we need to remove the DC from the Digital Signal (How?) 3) Self-Synchronization: To match the sender and receiver clocks, hence bit intervals match for correct decoding.
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