Transcript Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks
Chapter 1
Network Architecture Models
Logical and physical connections
Data Communications and Computer Networks
Chapter 1
The Internet Model in Action
Note the flow of data from user to web browser and back
At each layer, information is either added or removed,
depending on whether the data is leaving or arriving at a
workstation
The adding of information over pre-existing information is
termed encapsulation
Data Communications and Computer Networks
Chapter 2
Introduction
• Computer networks transmit signals
• Signals are the electomagnetic encoding of data
• Data and signals can be analog or digital
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
Data Communications and Computer Networks
Chapter 2
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
Data Communications and Computer Networks
Chapter 2
Analog versus Digital
Analog is a continuous waveform, with examples
such as music and video.
Data Communications and Computer Networks
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Analog versus Digital
Digital is a discrete or non-continuous waveform
with fixed voltage levels that represent data 1s and
0s.
Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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Analog versus Digital
Noise in a digital signal. You can still discern a high
voltage from a low voltage. Regenerators are
devices that automatically amplify and clean noise
out of digital signals.
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.
Here data will be lost in transmission.
Data Communications and Computer Networks
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Signals Have Three Components
• Amplitude
• Frequency
• Phase
Communicating Data
• Binary data (1s and 0s) are communicated
by changing one or more of these
components (amplitude, frequency, or
phase) in predetermined ways at
predetermined time intervals.
Data Communications and Computer Networks
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Amplitude
The amplitude of a signal is the height of the wave
above or below a given reference point.
Data Communications and Computer Networks
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Frequency
The frequency is the number of times a signal makes a
complete cycle within a given time frame.
Spectrum - The range of frequencies that a signal
spans from minimum to maximum.
Bandwidth - The absolute value of the difference
between the lowest and highest frequencies of a signal.
Attenuation - Loss of signal strength.
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
Chapter 2
Data Communications and Computer Networks
Chapter 2
Loss of Signal Strength
• All signals experience power loss (attenuation) as
they travel over a communications medium.
• Signals must be regenerated or amplified at regular
intervals to prevent total signal loss.
• Attenuation is denoted as a decibel (dB) loss.
Data Communications and Computer Networks
Chapter 2
Converting Digital Data into Digital Signals
There are numerous techniques available to convert digital data into
digital signals.
Let’s examine four techniques:
• NRZ-L
• NRZ-I
• Manchester
• Differential Manchester
Data Communications and Computer Networks
Chapter 2
Self-Clocking Codes
• Big difference between NRZ and Manchester
codes:
– For long strings of 0-bits, NRZ codes generate signal
that does not change over long time period
– Manchester codes always produce signal change during
every bit transmission.
– Manchester codes are called self-clocking codes,
because they provide a guaranteed voltage change (a
“clock signal”) in the middle of every bit received.
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).
Self-Clocking Codes
• Why do we care about self-clocking codes?
– Transmitter / receiver clocks are not perfectly
synchronized to tick at same rate (too expensive).
– NRZ-L or NRZ-I cannot be used at high data rates or
long distances unless a separate clock signal is sent on
another wire.
– Manchester codes can be used at high data rates or long
distances, because receiver continuously gets feedback
on sender clock rate.
Self-Clocking Codes
• Any disadvantage to Manchester codes?
– Manchester codes must transmit two signal changes per
bit (2 baud per bit).
– NRZ codes transmit only one signal change per bit (1
baud per bit).
– Any transmission medium (copper wire, fiber optics,
etc.) has a maximum baud capacity that it can support.
NRZ can send more bits per second.
Data Communications and Computer Networks
Chapter 2
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.
This system is self-clocking, yet provides 4 bits for
every 5 signal changes (more efficient than
Manchester)
Data Communications and Computer Networks
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Data Communications and Computer Networks
Chapter 2
Converting Digital Data into Analog Signals
Three basic techniques:
• Amplitude modulation
• Frequency modulation
• Phase modulation
Data Communications and Computer Networks
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Amplitude Modulation
One amplitude encodes a 0 while another amplitude
encodes a 1.
Data Communications and Computer Networks
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Amplitude Modulation
Some systems use multiple amplitudes.
Data Communications and Computer Networks
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Frequency Modulation
One frequency encodes a 0, while another frequency
encodes a 1.
Data Communications and Computer Networks
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Phase Modulation
One phase change encodes a 0, while another phase
change encodes a 1.
Data Communications and Computer Networks
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Quadrature Phase Modulation
Four different phase angles are used:
• 45 degrees
• 135 degrees
• 225 degrees
• 315 degrees
Data Communications and Computer Networks
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Data Communications and Computer Networks
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Quadrature Amplitude Modulation
In this technology, 12 different phases are combined
with two different amplitudes.
Since only 4 phase angles have 2 different amplitudes,
there are a total of 16 combinations.
With 16 signal combinations, each baud equals 4 bits
of information. (2 ^ 4 = 16)
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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
• Delta modulation
Data Communications and Computer Networks
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Pulse Code Modulation
The analog waveform is sampled at specific intervals
and the “snapshots” are converted to binary values.
Data Communications and Computer Networks
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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 is
created, as long as enough samples are taken
Data Communications and Computer Networks
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Pulse Code Modulation
The more snapshots taken in the same amount of time,
the better the resolution.
Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
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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.
In this situation a data signal is modulated by a carrier signal to
produce a composite signal that can be broadcast in a particular
range of frequencies near the carrier frequency. For example: music
(analog data signal) is modulated by a 91.5 MHz sine wave, which
produces a composite signal with components between 91.4 MHz
and 91.6 MHz. This is transmitted. You retrieve the original signal
by demodulating (tuning into) frequency 91.5 MHz on your radio to
get back the music
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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
Data Communications and Computer Networks
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Frequency Hopping Spread Spectrum
Data Communications and Computer Networks
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Data Code
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 plus a third code set
that has interesting characteristics:
• ASCII
• EBCDIC
• Baudot Code
Data Communications and Computer Networks
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Data Communications and Computer Networks
Chapter 2
Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Data Communications and Computer Networks
Chapter 2
Data and Signal Conversions in Action
Data Communications and Computer Networks
Chapter 2
Data and Signal Conversions in Action
Data Communications and Computer Networks
Chapter 2
Data and Signal Conversions in Action