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

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

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Represented as continuous waveforms

Can be at an infinite number of points between some given minimum and maximum

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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

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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”

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Digital signals are more tolerant to noise

But not completely immune

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All signals have three characteristics: ◦ Amplitude

◦ Frequency

◦ Phase

Amplitude: the height of the wave above (or below) a given reference point

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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)

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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 (3400 – 300)

◦ Effective bandwidth: the real-life bandwidth

Less than the theoretical bandwidth

Value influenced by interferences and noise

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

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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

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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)

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AM example

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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

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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

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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

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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

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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

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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

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4B/5B ◦ 4 bits are encoded in 5 bits and sent using NRZ-I

◦ The 5 bits never contain more than two consecutive zeros

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4B/5B ◦ Advantages: signal transition after at most 3 bits

◦ Disadvantages: 20% more data

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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

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Amplitude Shift Keying ◦ 1 and 0 are represented by different levels of the signal

◦ More than two levels can be used

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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

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Frequency Shift Keying

◦ 1 and 0 are represented by different frequencies of the signal

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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

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Phase Shift Keying

◦ 1 and 0 are represented by different phases of the signal

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Phase Shift Keying

◦ More phases can be used (quadrature phase shift)

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Complex techniques ◦ 12 different phase-

shift angles with two different amplitudes

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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

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Pulse Code Modulation ◦ The analog value is converted at specific time moments

(sampling points) to the closest level

◦ Approximations are made (quantization errors)

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Pulse Code Modulation ◦ Correct reconstruction depends on the sampling interval

and quantization errors.

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Pulse Code Modulation ◦ Better results are obtained with larger sampling rate and

more output levels.

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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

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