Ș.l. dr. ing. Lucian-Florentin Bărbulescu

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1 Ș.l. dr. ing. Lucian-Florentin Bărbulescu 1

2 Data: entities that convey meaning within a computer system Signals: are the electric or electromagnetic impulses used to encode and transmit data Characteristics Both exists in either analog or digital form 2

3 Represented as continuous waveforms Can be at an infinite number of points between some given minimum and maximum 3

4 The most important shortcoming: Noise unwanted electrical or electromagnetic energy that degrades the quality of signals and data found in every type of data and transmission system effects range from a slight hiss in the background to a complete loss of data or signal Is also analog - extremely difficult to separate noise from an analog waveform that represents data 4

5 Composed of a discrete or fixed number of values Digital Data - binary 1s and 0s Digital Signals more complex Most simple form is square wave 5

6 Digital signals are more tolerant to noise But not completely immune 6

7 All signals have three characteristics: Amplitude Frequency Phase Amplitude: the height of the wave above (or below) a given reference point 7

8 Frequency: the number of times a signal makes a complete cycle within a given time frame Measured in Hertz (Hz) Period: The time interval of one cycle (1 / frequency) 8

9 Signals are usually composed of more frequencies Spectrum: The range of frequencies that a signal spans from minimum to maximum Eg.: The spectrum of a simple telephone line must be between 300Hz and 3400Hz Bandwidth: the absolute value of the difference between the lowest and highest frequencies Eg.: The bandwidth of a simple telephone line is 3100Hz ( ) Effective bandwidth: the real-life bandwidth Less than the theoretical bandwidth Value influenced by interferences and noise 9

10 Phase: the position of the waveform relative to a given moment of time 10

11 Four possible data-to-signal conversions Analog data-to-analog signal Digital data-to-digital signal Digital data-to-(a discrete) analog signal Analog data-to-digital signal 11

12 An analog waveform is converted in another analog waveform The operation performed is modulation the process of sending data over a signal by varying either its amplitude(am), frequency(fm), or phase(pm) 12

13 AM example 13

14 Digital data is converted to a signal which have a finite set of possible values The operation is called digital encoding Several encoding schemes: NRZ-L NRZ-I Manchester Differential Manchester Bipolar-AMI (alternate mark inversion) 4B/5B 14

15 Non-Return to Zero Non-Return to Zero Level (NRZ-L) 1 -> 0 V 0 -> Positive V Non-Return to Zero Inverted (NRZ-I) 1 -> Voltage change 0 -> Voltage keep 15

16 Non-Return to Zero Advantages: easy to implement, baud rate equal to bit rate Baud rate: the number of times a signal changes value per second. Bit rate: the number of data bits sent in one second. Disadvantages: no signal change for long streams of 0 or 1 (only for NRZ-L) problem with receiver synchronization 16

17 Manchester Manchester 1 -> low to high transition 0 -> high to low transition Differential Manchester 1 -> One transition: at the middle 0 -> Two transitions: one at the beginning, one at the middle 17

18 Manchester Advantage: guaranteed transitions for each bit Disadvantage: Large baud rate With Manchester baud rate = 2 * bit rate Eg.: for 5 zeros the bit rate is 5 and the baud rate is 10 18

19 Bipolar-AMI (alternate mark inversion) 0 -> 0 V 1 -> Either positive or negative voltage, depending on previous 1 Advantages: zero voltage sum - useful in certain types of electronic systems Disadvantages: no signal change for long streams of 0 19

20 4B/5B 4 bits are encoded in 5 bits and sent using NRZ-I The 5 bits never contain more than two consecutive zeros 20

21 4B/5B Advantages: signal transition after at most 3 bits Disadvantages: 20% more data 21

22 Digital data is converted to an analog wave A modulator is used The analog signal takes on a discrete number of signal levels Three simple techniques (plus other complex) Amplitude Shift Keying Frequency Shift Keying Phase Shift Keying 22

23 Amplitude Shift Keying 1 and 0 are represented by different levels of the signal More than two levels can be used 23

24 Amplitude Shift Keying Advantages: the most simple form of modulation Disadvantages: susceptible to sudden noise impulses not very efficient very few levels can be used not used for high data rates 24

25 Frequency Shift Keying 1 and 0 are represented by different frequencies of the signal 25

26 Frequency Shift Keying Advantages: resistant to sudden noise impulses Disadvantages: subject to intermodulation distortion (the frequencies of two or more signals mix together and create new frequencies) not used for high data rates 26

27 Phase Shift Keying 1 and 0 are represented by different phases of the signal 27

28 Phase Shift Keying More phases can be used (quadrature phase shift) 28

29 Complex techniques 12 different phaseshift angles with two different amplitudes 29

30 Analog wave is converted to a signal which have a discrete number of values The equipment used is called a codec Different encoding techniques: Pulse Code Modulation (PCM) Delta Modulation 30

31 Pulse Code Modulation The analog value is converted at specific time moments (sampling points) to the closest level Approximations are made (quantization errors) 31

32 Pulse Code Modulation Correct reconstruction depends on the sampling interval and quantization errors. 32

33 Pulse Code Modulation Better results are obtained with larger sampling rate and more output levels. 33

34 Delta Modulation a codec tracks the incoming analog data by assessing up or down steps not efficient if the analog waveform rises or drops too quickly 34

Chapter 2: Fundamentals of Data and Signals

Chapter 2: Fundamentals of Data and Signals TRUE/FALSE 1. The terms data and signal mean the same thing. F PTS: 1 REF: 30 2. By convention, the minimum and maximum values of analog data and signals are