Chapter 2 - Fundamentals of Data and Signals

A)      Introduction

a.       Possible data-to-signal conversion combinations

                                                               i.      Analog data-to-analog signal – amplitude and frequency modulation techniques

                                                             ii.      Digital data-to-square-wave digital signal – encoding techniques

                                                           iii.      Digital data-to-(a discrete) analog signal – modulation techniques

                                                           iv.      Analog data-to-digital signal – digitization techniques

1.       Digitization – converting analog data to digital signals

B)      Data and Signals

a.       Data – entities that convey meaning within a computer or system

                                                               i.      Example

1.       Computer file of names and addresses

b.       Signal – electric or electromagnetic impulses used to encode and transmit data

                                                               i.      Example

1.       Transmission of a telephone conversation over a telephone line

c.       Analog vs. Digital

                                                               i.      Analog data and analog signals are represented as continuous waveforms that can be at an infinite number of points between some given minimum and maximum. Usually presented as voltages

1.       Noise – unwanted electrical or electromagnetic energy that degrades the quality of signals and data

                                                             ii.      Digital data and digital signals are composed of a discrete or fixed number of values, rather than a continuous or infinite number of values

                                                           iii.      A look at how noise distorts digital signals

d.       Fundamentals of signals

                                                               i.      Amplitude – the height of the wave above (or below) a given reference point

1.       Denotes the voltage level of the signal (measured in volts), can also denote the current level of the signal (measured in amps), or the power level of the signal (measured in watts

                                                             ii.      Frequency – number of times a signal makes a complete cycle within a given time frame

                                                           iii.      Period – length, or time interval, of one cycle for frequency

                                                           iv.      Spectrum – range of frequencies that a signal spans from minimum to maximum

                                                             v.      Bandwidth – the absolute of the difference between the lowest and highest frequencies

                                                           vi.      Because extraneous noise degrades original signals, an electronic device usually has an effective bandwidth less than its bandwidth

                                                          vii.      Phase – the position of the waveform relative to a given moment of time, or relative to time zero

                                                        viii.      Attenuation – loss of power or loss of signal strength

1.       When traveling through any type of medium, a signal always experiences some loss of its power due to friction

                                                            ix.      Decibel (dB) – relative measure of signal loss or gain and is used to measure the logarithmic loss or gain of a signal

1.       Equation to measure signal loss or gain: 

a.       P2 = ending power level

b.       P1 = beginning power level

                                                             x.      Amplification – signal gains in decibel

C)      Converting Data into Signals

a.       Transmitting analog data with analog signals

                                                               i.      Modulation – process of sending data over a signal by varying its amplitude, frequency, or phase

b.       Transmitting digital data with square-wave digital signals: digital encoding schemes

                                                               i.      Nonreturn to zero digital encoding schemes

1.       Nonreturn to zero-level (NRZ-L) – transmits 1s as zero voltages and 0s as positive voltages

2.       Nonreturn to zero inverted (NRZI) – voltage change at the beginning of a 1 and no voltage change at the beginning of a zero

                                                             ii.      Manchester Digital Encoding Schemes

1.       Properties of Manchester encoding scheme

a.       To transmit a 1, the signal changes from low to high in the middle of the interval, and to transmit a 0, the signal changes from high to low in the middle of the interval

2.       Differential Manchester

a.       If there is a transition at the beginning of the interval, then a 0 is being transmitted, if there is no transition at the beginning of the interval, then a 1 is being transmitted

3.       Self-clocking – the occurrence of a regular transition is similar to second ticking on a clock

4.       Baud rate (baud) – the number of times a signal changes value per second

5.       Data rate is measured in bits per second (bps)

                                                           iii.      Bipolar-AMI encoding scheme – uses three voltage levels

1.       When transmitting a 0, a zero voltage is transmitted

2.       When transmitting a 1, either a negative or positive voltage is transmitted

                                                           iv.      4B/5B Digital encoding scheme – takes 4 bits of data, coverts the 4 bits into a unique 5-bit sequences, and encodes the 5 bits using NRZI

D)      Transmitting digital data with discrete analog signals

a.       Amplitude shift keying – amplitude is represented by two different amplitudes for a signal

                                                               i.      Can use more amplitudes in order to achieve two-bit representation

b.       Frequency shift keying – uses two different frequency ranges to represent data values of 0 and 1

                                                               i.      Intermodulation distortion – a phenomenon that occurs when the frequencies of two or more signals mix together and create new frequencies

c.       Phase shift keying – represents 0s and 1s by different changes in the phase of a waveform

                                                               i.      Quadrature phase shift keying – incorporates four different phase angles, each of which represents 2 bits

1.       a 45-degree phase shift represents 11

2.       a 135-degree phase shift represents 10

3.       a 225-degree phase shift represents 01

4.       a 315-degree phase shift represents 00

                                                             ii.      quadrature amplitude modulation – uses each signal to represent 4 bits

E)      Transmitting analog data with digital signals

a.       Pulse code modulation (PCM) – a codec, coverts the analog data to a digital signal by tracking the analog waveform and taking “snapshots” of the analog data at fixed intervals

                                                               i.      The binary value is then transmitted by means of a digital encoding format

                                                             ii.      Pulse amplitude modulation (PAM) – tracking an analog waveform and converting it to pulses that represent the wave’s height above a threshold

                                                           iii.      Sampling rate – frequency at which snapshots are taken

                              

b.       Delta modulation – a codec tracks the incoming analog data by assessing up or down “steps”. During each time period, the codec determines whether the waveform has risen one delta step or dropped one delta step. If the waveform rises on delta step, a 1 is transmitted, if the waveform drops, a 0 is transmitted

                                                               i.      Slope overload noise – when the analog waveform rises or drops too quickly, the

codec is not able to keep up with the change

F)       Data Codes – set of all textual characters or symbols and their corresponding binary patterns

a.       EBCDIC – 8 bit code allowed 256 (2⁸ = 256) possible combinations of textual symbols

b.       ASCII (American Standard Code for Information Interchange) – 7-bit version that allows for 128 (2⁷ = 128) possible combinations

                          

c.       Unicode – encoding technique that provides a unique coding value for every character in every language, no matter the platform