05-SignalEncoding - ACOE312 Signal Encoding Techniques 1 1...

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Unformatted text preview: ACOE312 Signal Encoding Techniques 1 1 Data Communications & Computer Networks Lecture 5 Signal Encoding Techniques Fall 2007 2 Agenda Digital Data, Digital Signals Digital Data, Analog Signals Home Exercises ACOE312 Signal Encoding Techniques 2 3 Encoding Techniques There are a number of transmission options available today, depending on the encoding technique There are four possible combinations of encoding techniques Digital data, digital signal Digital data, analog signal Analog data, digital signal Analog data, analog signal We shall examine only the first two techniques 4 Digital Data Digital Signals ACOE312 Signal Encoding Techniques 3 5 1. Digital Data, Digital Signals Digital signal Discrete, discontinuous voltage pulses Each pulse is a signal element Binary data encoded into signal elements 6 Terms (1) Unipolar All signal elements have same sign, i.e. all positive or all negative Polar One logic state represented by positive voltage the other by negative voltage Data rate Rate of data transmission in bits per second Duration or length of a bit Time taken for transmitter to emit the bit eg. For a data rate R, the bit duration is 1/R ACOE312 Signal Encoding Techniques 4 7 Terms (2) Modulation rate Rate at which the signal level changes Modulation rate is measured in baud = signal elements per second Mark and Space Mark is Binary 1, Space is Binary 0 8 Interpreting Signals Receiver needs to know Timing of bits - when they start and end Signal levels What factors determine how successful the receiver will be interpreting the incoming signal? Signal to noise ratio Data rate Bandwidth Encoding Scheme ACOE312 Signal Encoding Techniques 5 9 Encoding Schemes considerations (1) Signal Spectrum Lack of high frequencies reduces required bandwidth Lack of dc component also desirable since it allows ac coupling via transformer, providing electrical isolation Concentrate tx power in the middle of tx bandwidth Clocking Synchronizing transmitter and receiver External clock Sync mechanism based on tx signal with suitable encoding 10 Encoding Schemes considerations (2) Error detection Can be built in to signal encoding Signal interference and noise immunity Some codes are better than others Cost and complexity Higher signal rate (& thus data rate) lead to higher costs Some codes require signal rate greater than data rate ACOE312 Signal Encoding Techniques 6 11 Encoding Schemes Return to Zero (RZ) Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar - AMI Pseudoternary Manchester Differential Manchester 12 Return to zero (RZ) Signal amplitude varies between a positive voltage, i.e. unipolar Binary 1: a constant positive voltage Binary 0: Absence of voltage (i.e. 0 Volts or Ground) Example: 1 0 1 1 0 0 0 1 +V 0 Volts ACOE312 Signal Encoding Techniques...
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05-SignalEncoding - ACOE312 Signal Encoding Techniques 1 1...

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