Transcript IST 220 Ch 2

Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 2
Fundamentals of Data and Signals
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Data Communications and Computer Networks
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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
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Introduction
Computer 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 Signals
Data 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
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Data and Signals
Examples 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 Signals
Examples 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 Digital
Analog is a continuous waveform, with examples
such as music and video.
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Analog versus Digital
Digital is a discrete or non-continuous waveform
with examples such as computer 1s and 0s.
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Analog versus Digital
It 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 Digital
Noise in a digital signal. You can still discern a high
voltage from a low voltage.
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Data Communications and Computer Networks
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Analog versus Digital
Noise in a digital signal. Too much noise - you
cannot discern a high voltage from a low voltage.
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All Signals Three Components
Amount -
Amplitude
Time -
Frequency
Phase
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Amplitude
The amplitude of a signal is the height of the wave
above or below a given reference point.
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Frequency
The 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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Phase
The 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 Strength
All 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 Strength
Decibel 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 Strength
Signal 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 Strength
So 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
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X = 50
Data Communications and Computer Networks
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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 Signals
There 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
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• Differential Manchester
Data Communications and Computer Networks
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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 Encoding
Yet 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
Signals
Modulation: 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 Modulation
One amplitude encodes a 0 while another amplitude
encodes a 1.
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Amplitude Modulation
Some systems use multiple amplitudes.
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Frequency Modulation
One frequency encodes a 0 while another frequency
encodes a 1.
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Phase Modulation
One 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
Signals
To 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 Modulation
The analog waveform is sampled at specific intervals
and the “snapshots” are converted to binary values.
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Pulse Code Modulation
When the binary values are later converted to an analog
signal, a waveform similar to the original results.
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Pulse Code Modulation
The more snapshots taken in the same amount of time,
the better the resolution.
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Pulse Code Modulation
Since 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 Modulation
An 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
Signals
Many 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 Technology
A 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 Spectrum
This 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
transmitted
The 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 Action
Let 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 Rates
How 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
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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
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