Chapter 2 Fundamentals of Data and Signals
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Transcript of 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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Data Communications and Computer Networks Chapter 2
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 Communications and Computer Networks Chapter 2
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 Communications and Computer Networks Chapter 2
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 Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
Analog versus DigitalAnalog is a continuous waveform, with examples such as music and video.
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Data Communications and Computer Networks Chapter 2
Analog versus DigitalDigital is a discrete or non-continuous waveform with examples such as computer 1s and 0s.
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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
Analog versus DigitalNoise in a digital signal. You can still discern a high voltage from a low voltage.
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Data Communications and Computer Networks Chapter 2
Analog versus DigitalNoise in a digital signal. Too much noise - you cannot discern a high voltage from a low voltage.
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Data Communications and Computer Networks Chapter 2
All Signals Three ComponentsAmount - Amplitude
Time - Frequency
Phase
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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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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Data Communications and Computer Networks Chapter 2
Amplitude ModulationOne amplitude encodes a 0 while another amplitude encodes a 1.
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Data Communications and Computer Networks Chapter 2
Amplitude ModulationSome systems use multiple amplitudes.
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Data Communications and Computer Networks Chapter 2
Frequency ModulationOne frequency encodes a 0 while another frequency encodes a 1.
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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
Pulse Code ModulationThe analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values.
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Data Communications and Computer Networks Chapter 2
Pulse Code ModulationWhen the binary values are later converted to an analog signal, a waveform similar to the original results.
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Data Communications and Computer Networks Chapter 2
Pulse Code ModulationThe more snapshots taken in the same amount of time, the better the resolution.
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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
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Data Communications and Computer Networks Chapter 2
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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Data Communications and Computer Networks Chapter 2
Frequency Hopping Spread Spectrum
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Data Communications and Computer Networks Chapter 2
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 Communications and Computer Networks Chapter 2
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Data Communications and Computer Networks Chapter 2
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 Communications and Computer Networks Chapter 2
Data and Signal Conversions in Action
Note the order of the signals in time
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Data Communications and Computer Networks Chapter 2
Data and Signal Conversions in Action
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Data Communications and Computer Networks Chapter 2
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