Chapter 2 Fundamentals of Data and Signals Flashcards
Data, Signals and Computer networks ?
What can data signals be
Data are entities that convey meaning
Signals are the electric or electromagnetic encoding of data
Computer networks and data/voice communication systems
transmit signals
Data and signals can be analog or digital
What is the layer 1 of the OSI model?
What is Point-to-point transmission of data across nodes:
Layer 1 of the OSI model is all about the physical
transmission of signals over media
Point-to-point transmission of data across nodes:
– Specifies the type of connection and the signals that pass through it
– Signals can be analog or digital, broadband or baseband
– The capacity (throughput) of the network depends on the type of
cabling used
Single properties of amplitudes, frequencies Wavelength and phase
Amplitude:
– The “height” of the wave above (or below) a central point, often
measured in volts (V)
* Frequency:
– The number of waves that pass a given point per second, measured in Hertz (Hz)
* Wavelength:
– The distance from the start to the end of the wave, measured in meters
(m)
* Phase:
– Position of the waveform at a given time, measured in degrees of shift (o
More on frequencies, what is the spectrum and bandwidth
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
More on 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.
Signal strength?
All signals experience loss (attenuation)
* Attenuation is denoted as a decibel (dB) loss
* Decibel losses (and gains) are additive
4B and 5B data encoding?
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
Amplitude shift keying?
One amplitude encodes a 0 while another amplitude
encodes a 1 (amplitude modulation)
Frequency shift keying?
One frequency encodes a 0 while another frequency
encodes a 1 (frequency modulation)
Phase shift keying?
One phase change encodes a 0 while another phase change
encodes a 1 (phase modulation)
How do you send more data?
Use a higher frequency signal (make
sure the medium can handle the higher
frequency
Use a higher number of signal levels
In both cases, noise can be a problem
The most common (because it’s cheaper)
is amplitude, or frequency
Distinguish between data and signals?
Data are entities that convey meaning within a computer or
computer system
* Signals are the electric or electromagnetic impulses used to
encode and transmit data
distinguish between analog vs digital?
Data and signals can be either analog or digital
* Analog is a continuous waveform, with examples such as
(naturally occurring) music and voice
* It is harder to separate noise from an analog signal than it is
to separate noise from a digital signal (see the following two
slides)
Digital is a discrete or non-continuous waveform
* Something about the signal makes it obvious that the signal
can only appear in a fixed number of forms (see next slide)
* Noise in digital signal
– You can still discern a high voltage from a low voltage
– Too much noise – you cannot discern a high voltage from a low
voltage
What are the three components of a signal
Amplitude
– Frequency
– Phase
Amplitude
– The height of the wave above or below a given reference point
– Amplitude is usually measured in volt
Frequency: Spectrum and bandwidth
The number of times a signal makes a complete cycle within a given
time frame; frequency is measured in Hertz (Hz), or cycles per
second (period = 1 / frequency)
– Spectrum – Range of frequencies that a signal spans from minimum
to maximum
– Bandwidth – Absolute value of the difference between the lowest and
highest frequencies of a signal
– For example, consider an average voice
* The average voice has a frequency range of roughly 300 Hz to 3100
Hz
* The spectrum would be 300 – 3100 Hz
* The bandwidth would be 2800 Hz
Phase
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
Phase and Phase Angles
If a signal can experience two different phase angles, then 1 bit can
be transmitted with each signal change (each baud)
– If a signal can experience four different phase angles, then 2 bits can
be transmitted with each signal change (each baud)
– Note: number of bits transmitted with each signal change = log2
(number of different phase angles)
– (You can replace “phase angles” with “amplitude levels” or “frequency
levels”)
Loss of signal strength
All signals experience loss (attenuation)
* Attenuation is denoted as a decibel (dB) loss
* Decibel losses (and gains) are additive
- Formula for decibel (dB):
dB = 10 x log10 (P2
/ P1
)
where P1
is the beginning power level and P2
is the ending power level
So if a signal loses 3 dB, is that a lot?
* What if a signal starts at 100 watts and ends at 50 watts?
What is dB loss?
dB = 10 x log10 (P2
/ P1
)
dB = 10 x log10 (50 / 100)
dB = 10 x log10 (0.5)
dB = 10 x -0.3
dB = -3.0
* So a 3.0 decibel loss losses half of its power
There are four main combinations of data and signals:
Analog data transmitted using analog signals
– Digital data transmitted using digital signals
– Digital data transmitted using discrete analog signals
– Analog data transmitted using digital signals
Transmitting Analog Data with
Analog Signals
In order to transmit analog data, you can modulate the data
onto a set of analog signals
* Broadcast radio and the older broadcast television are two
very common examples of this
* We modulate the data onto another set of frequencies so
that all the different channels can coexist at different
frequencies
Transmitting Digital Data with Digital Signals:
Digital Encoding Schemes: There are numerous techniques available to convert
digital data into digital signals. Let’s examine five:
There are numerous techniques available to convert
digital data into digital signals. Let’s examine five:
– NRZ-L
– NRZI
– Manchester
– Differential Manchester
– Bipolar AMI
* These are used in LANs and some telephone systems
Nonreturn to Zero Digital Encoding
Schemes
Fundamental difference exists between NRZ-L and NRZI
Nonreturn to zero-level (NRZ-L) transmits 1s as zero
voltages and 0s as positive voltages
* Nonreturn to zero inverted (NRZI) has a voltage change at
the beginning of a 1 and no voltage change at the beginning
of a 0
* Fundamental difference exists between NRZ-L and NRZI
– With NRZ-L, the receiver has to check the voltage level for
each bit to determine whether the bit is a 0 or a 1,
– With NRZI, the receiver has to check whether there is a change
at the beginning of the bit to determine if it is a 0 or a 1
Manchester Digital Encoding Schemes
Note how with a Differential Manchester code, every bit has
at least one significant change. Some bits have two signal
changes per bit (baud rate = twice bps)
Bipolar-AMI Encoding Scheme
The bipolar-AMI encoding scheme is unique among all the
encoding schemes because it uses three voltage levels
– When a device transmits a binary 0, a zero voltage is transmitted
– When the device transmits a binary 1, either a positive voltage or a
negative voltage is transmitted
– Which of these is transmitted depends on the binary 1 value that was
last transmitted
4B/5B Digital Encoding Scheme
Yet another encoding technique; this one 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 NRZI encoded
signal
Transmitting Digital Data with
Discrete Analog Signals
* Three basic techniques:
Transmitting Digital Data with
Discrete Analog Signals
* Three basic techniques:
– Amplitude shift keying
– Frequency shift keying
– Phase shift keying
* One can then combine two or more of these basic
techniques to form more complex modulation techniques
(such as quadrature amplitude modulation)
Amplitude Shift Keying
Amplitude Shift Keying
* One amplitude encodes a 0 while another amplitude
encodes a 1 (a form of amplitude modulation)
Frequency Shift Keying
One frequency encodes a 0 while another frequency
encodes a 1 (a form of frequency modulation)
Phase Shift Keying
Phase Shift Keying
* One phase change encodes a 0 while another phase change
encodes a 1 (a form of phase modulation)
Phase Shift Keying (continued)
* Quadrature Phase Shift Keying
– Four different phase angles used
45 degrees
* 135 degrees
* 225 degrees
* 315 degrees
Quadrature amplitude modulation
As an example of QAM, 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
(log2
(16) = 4, or inversely, 2 ^ 4 = 16)
. Transmitting Analog Data with
Digital Signals, and two different techniques
To convert analog data into a digital signal, there are two
techniques:
– Pulse code modulation (the more common)
– Delta modulation
Pulse Code Modulation (continued)
Pulse Code Modulation (continued)
* Since telephone systems digitize human voice, and since the
human voice has a fairly narrow bandwidth, telephone
systems can digitize voice into either 128 or 256 levels
* These 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)
Pulse Code Modulation
Pulse Code Modulation
* The analog waveform is sampled at specific intervals and the
“snapshots” are converted to binary values
When the binary values are later converted to an analog
signal, a waveform similar to the original results
The more snapshots taken in the same amount of time, or
the more quantization levels, the better the resolution
Delta Modulation
Delta Modulation
* An analog waveform is tracked, using a binary 1 to represent
a rise in voltage, and a 0 to represent a drop
Pulse Code Modulation (continued)
Pulse Code Modulation (continued)
* How fast do you have to sample an input source to get a
fairly accurate representation?
* Nyquist says 2 times the highest frequency
* Thus, if you want to digitize voice (4000 Hz), you need to
sample at 8000 samples per second
The Relationship Between Frequency
and Bits Per Second (continued)
The Relationship Between Frequency
and Bits Per Second (continued)
* Maximum Data Transfer Rates
– How do you calculate a maximum data rate?
– Use Shannon’s equation
* S(f) = f x log2
(1 + S/N)
– Where f = signal frequency (bandwidth), S is the signal power in
watts, and N is the noise power in watts
– For example, what is the data rate of a 3400 Hz signal with 0.2
watts of power and 0.0002 watts of noise?
* S(f) = 3400 x log2
(1 + 0.2/0.0002)
= 3400 x log2
(1001)
= 3400 x 9.97
= 33898 bps
The Relationship Between Frequency
and Bits Per Second
The Relationship Between Frequency
and Bits Per Second
* Higher Data Transfer Rates
– How do you send data faster?
* Use a higher frequency signal (make sure the medium can handle the
higher frequency
* Use a higher number of signal levels
– In both cases, noise can be a problem
Data and Signal Conversions In Action:
Data and Signal Conversions In Action:
Two Examples
* Let us transmit the message “Sam, what time is the
meeting with accounting? Hannah.”
* This message leaves Hannah’s workstation and travels
across a local area network
Data codes and three common data codes
The set of all textual characters or symbols and their
corresponding binary patterns is called a data code
* There are three common data code sets:
– EBCDIC
– ASCII
– Unicode
Unicode
Unicode
* Each character is 16 bits
* A large number of languages / character sets
* For example:
– T equals 0000 0000 0101 0100
– r equals 0000 0000 0111 0010
– a equals 0000 0000 0110 0001
After reading this chapter,
you should be able to:
• Distinguish between data and signals, and cite the
advantages of digital data and signals over analog data and
signals
• Identify the three basic components of a signal
• Discuss the bandwidth of a signal and how it relates to data
transfer speed
• Identify signal strength and attenuation, and how they are
related
2
After reading this chapter,
you should be able to (continued):
• Outline the basic characteristics of transmitting analog data
with analog signals, digital data with digital signals, digital
data with analog signals, and analog data with digital signals
• List and draw diagrams of the basic digital encoding
techniques, and explain the advantages and disadvantages
of each
• Identify the different shift keying (modulation) techniques,
and describe their advantages, disadvantages, and uses
3
After reading this chapter,
you should be able to (continued):
• Identify the two most common digitization techniques, and
describe their advantages and disadvantages
• Identify the different data codes and how they are used in
communication systems
After reading this chapter,
you should be able to:
• Distinguish between data and signals, and cite the
advantages of digital data and signals over analog data and
signals
• Identify the three basic components of a signal
• Discuss the bandwidth of a signal and how it relates to data
transfer speed
• Identify signal strength and attenuation, and how they are
related
2
After reading this chapter,
you should be able to (continued):
• Outline the basic characteristics of transmitting analog data
with analog signals, digital data with digital signals, digital
data with analog signals, and analog data with digital signals
• List and draw diagrams of the basic digital encoding
techniques, and explain the advantages and disadvantages
of each
• Identify the different shift keying (modulation) techniques,
and describe their advantages, disadvantages, and uses
3
After reading this chapter,
you should be able to (continued):
• Identify the two most common digitization techniques, and
describe their advantages and disadvantages
• Identify the different data codes and how they are used in
communication systems
