Chapter 2 Fundamentals of Data and Signals

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Chapter 2

Fundamentals of Data and Signals

Data Communications andComputer Networks: A Business User’s Approach

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Data Communications and Computer Networks Chapter 2

What we cover:

Data vs signals

Signals – digital and analog; aspects

Conversion: A to D; D to A

Data encoding

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IntroductionComputer networks transmit signals

Signals are the electric or electromagnetic encoding of data

Data and signals can be analog or digital

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Data vs SignalsData is what we want to transmit

Data is usually stored

Signals are what we use to transmit the data

Signals are transient

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Data and SignalsExamples of data include:

• computer files

• movie on a DVD

• music on a compact disc

• collection of samples from a blood gas analysis device

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Data and SignalsExamples of signals include:

• telephone conversation over a telephone line

• live television news interview from Europe

• web page download over your telephone line via the Internet

• others?

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Analog versus DigitalAnalog is a continuous waveform, with examples such as music and video.

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Analog versus DigitalDigital is a discrete or non-continuous waveform with examples such as computer 1s and 0s.

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Analog versus DigitalIt is harder to separate noise from an analog signal than it is to separate noise from a digital signal.

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Analog versus DigitalNoise in a digital signal. You can still discern a high voltage from a low voltage.

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Analog versus DigitalNoise in a digital signal. Too much noise - you cannot discern a high voltage from a low voltage.

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All Signals Three ComponentsAmount - Amplitude

Time - Frequency

Phase

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AmplitudeThe amplitude of a signal is the height of the wave above or below a given reference point.

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FrequencyThe frequency is the number of times a signal makes a complete cycle within a given time frame.

Measured in Hz (hertz = cycles/second)

Period is the length of one cycle = 1/frequency

Spectrum - The range of frequencies that a signal spans from minimum to maximum.

Human speech: 300 Hz to 3100 Hz

Bandwidth - The absolute value of the difference between the lowest and highest frequencies of a signal.

Human speech: 2800 Hz

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PhaseThe phase of a signal is the position of the waveform relative to a given moment of time or relative to time zero.

A change in phase can be any number of angles between 0 and 360 degrees.

Phase changes often occur on common angles, such as 45, 90, 135, etc.

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Loss or Gain of Signal StrengthAll signals experience loss (attenuation).

Attenuation and gain is denoted as a decibel (dB) loss or gain.

Decibel losses (and gains) are additive.

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Loss of Signal StrengthDecibel is a relative loss or gain of signal

dB = 10 log10 (output power/input power)

dB = 10 log10 (Po/Pi) = 10 log10 Po – 10 log10 Pi

dB = dBo - dBi

Attenuation is denoted as a decibel (dB) loss.

Decibel losses (and gains) are additive.

dB is RELATIVE; cannot be calculated for a single power level

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Loss or gain of Signal StrengthSignal runs from point A to point C through point B

Both lines and nodes can have a lose or gain.

Rarely do lines have gains

-10 dB + 20 dB – 15 dB = -5 dB

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Signal StrengthSo if a signal loses 3 dB, is that a lot?

A 3 dB loss indicates the signal lost half of its power.

dB = 10 log10 (P2 / P1)

-3 dB = 10 log10 (X / 100)

-0.3 = log10 (X / 100)

10-0.3 = X / 100

0.50 = X / 100

X = 50

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Converting data into signals - types

Transmitting

Digital data to digital signals

Digital data with analog signals

Analog data with digital signals

Analog data with analog signals

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Converting Digital Data (0,1)’s into Digital SignalsThere are numerous techniques available to convert digital data into digital signals.

Many systems are designed only to carry analog signals

Let’s examine four techniques:

• NRZ-L (non return to zero level)

• NRZ-I (non return to zero – inverted)

• Manchester

• Differential Manchester

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Converting Digital Data • NRZ-L & NRZ-I have synchronization problems

•No signal change at the beginning of each bit

•Requires to systems to have clocks in synch

• Manchester & Differential Manchester

•Each bit has signal change

•Encoding schemes are self-clocking

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Bits per second (bps) = number of bits transmitted across a medium in a given second

Baud rate: number of times a signal changes value per second

bps and baud are not always the same

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Note how with a Differential Manchester code, every bit has at least one signal change. Some bits have two signal changes per bit (baud rate is twice the bps).

Presence of transition at beginning of bit time means a zero

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4B/5B Digital EncodingYet another encoding technique that converts four bits of data into five-bit quantities.

The five-bit quantities are unique in that no five-bit code has more than 2 consecutive zeroes.

The five-bit code is then transmitted using an NRZ-I encoded signal.

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Converting Digital Data into Analog SignalsModulation: change from one version to another

Three basic techniques:

• Amplitude modulation

• Frequency modulation

• Phase modulation

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Modem

• Device that converts digital data to analog signal and back again.

• MOdulator/DEModulator

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Amplitude ModulationOne amplitude encodes a 0 while another amplitude encodes a 1.

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Amplitude ModulationSome systems use multiple amplitudes.

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Frequency ModulationOne frequency encodes a 0 while another frequency encodes a 1.

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Phase ModulationOne phase change encodes a 0 while another phase change encodes a 1. Here, only phase change for a 1.

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Converting Analog Data into Digital SignalsTo convert analog data into a digital signal, there are two basic techniques:

• Pulse code modulation (used by telephone systems)

• Delta modulation

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Pulse Code ModulationThe analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values.

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Pulse Code ModulationWhen the binary values are later converted to an analog signal, a waveform similar to the original results.

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Pulse Code ModulationThe more snapshots taken in the same amount of time, the better the resolution.

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Pulse Code ModulationSince telephone systems digitize human voice, and since the human voice has a fairly narrow bandwidth, telephone systems can digitize voice into either 128 levels or 256 levels.

These levels are called quantization levels.

If 128 levels, then each sample is 7 bits (2 ^ 7 = 128).

If 256 levels, then each sample is 8 bits (2 ^ 8 = 256).

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Delta ModulationAn analog waveform is tracked, using a binary 1 to represent a rise in voltage, and a 0 to represent a drop.

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Converting Analog Data into Analog SignalsMany times it is necessary to modulate analog data onto a different set of analog frequencies.

Broadcast radio and television are two very common examples of this.

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Spread Spectrum TechnologyA secure encoding technique that uses multiple frequencies or codes to transmit data.

Two basic spread spectrum technologies:

• Frequency hopping spread spectrum

• Direct sequence spread spectrum

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Frequency Hopping Spread Spectrum

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Direct Sequence Spread SpectrumThis technology replaces each binary 0 and binary 1 with a unique pattern, or sequence, of 1s and 0s.

For example, one transmitter may transmit the sequence 10010100 for each binary 1, and 11001010 for each binary 0.

Another transmitter may transmit the sequence 11110000 for each binary 1, and 10101010 for each binary 0.

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Data Code – common digital data transmittedThe set of all textual characters or symbols and their corresponding binary patterns is called a data code.

There are two basic data code sets:

• ASCII (7 bit, 1 parity bit)

• EBCDIC (8 bit) [IBM computers]

Number of possible codeable terms 2*N

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Data and Signal Conversions in ActionLet us transmit the message “Sam, what time is the meeting with accounting? Hannah.”

This message first leaves Hannah’s workstation and travels across a local area network.

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Data and Signal Conversions in Action

Note the order of the signals in time

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Data and Signal Conversions in Action

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Higher Data Transfer RatesHow do you send data faster?

1. Use a higher frequency signal (make sure the medium can handle the higher frequency)

2. Use a higher number of signal levels

In both cases, noise can be a problem.

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What we covered• Data and Signal• Signal components – amplitude, frequency, phase• Transmission factors – noise and attenuation• Four combinations data and signal• Digital encoding schemes

– Bit rate vs baud

• Digital data on analog signals requires modulation– AM, FM, PM

• Analog data to digital signals– PCM, delta M

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What we covered• Analog data over analog signal – modulate to

another frequency• Spread spectrum – frequency hopping• Data codes – transmission of data codes

– ASCII (American National Standard code for Information Interchange) the most common

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Problems Set

Documents

Chapter 2 Fundamentals of Data and Signals