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Chapter Two

Fundamentals of Data and Signals

Data Communications and Computer Networks: A Business User's Approach

Eighth Edition

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Introduction

Data are entities that convey meaning (computer files, music on CD, results from a blood gas analysis machine)
Signals are the electric or electromagnetic encoding of data (telephone conversation, web page download)
Computer networks and data/voice communication systems transmit signals
Data and signals can be analog or digital

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Introduction (continued)

Table 2-1 Four combinations of data and signals

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Analog vs. Digital (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Analog vs. Digital (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Analog vs. Digital (continued)

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Analog vs. Digital (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Analog vs. Digital (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Analog vs. Digital (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Fundamentals of Signals

All signals have three components:
Amplitude
Frequency
Phase

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Fundamentals of Signals – Amplitude

Amplitude
The height of the wave above or below a given reference point
Amplitude is usually measured in volts

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Fundamentals of Signals – Amplitude

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Fundamentals of Signals – Frequency

Frequency
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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Fundamentals of Signals – Frequency

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Fundamentals of Signals – Phase

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.

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Fundamentals of Signals – Phase

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Fundamentals of Signals

Phase
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”)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Loss of Signal Strength

All signals experience loss (attenuation)
Attenuation is denoted as a decibel (dB) loss
Decibel losses (and gains) are additive

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Loss of Signal Strength (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Loss of Signal Strength

Formula for decibel (dB):

dB = 10 x log10 (P2 / P1)

where P1 is the beginning power level and P2 is the ending power level

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Loss of Signal Strength (continued)

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Converting Data into Signals

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
Let’s look at each these

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

1. 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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

1. Transmitting Analog Data with
Analog Signals (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

2. 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:
NRZ-L
NRZI
Manchester
Differential Manchester
Bipolar AMI
These are used in LANs and some telephone systems

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

2. Transmitting Digital Data with Digital Signals: Digital Encoding Schemes (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Nonreturn to Zero Digital Encoding Schemes

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Manchester Digital Encoding Schemes (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

4B/5B Digital Encoding Scheme (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

3. 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)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Amplitude Shift Keying

One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Amplitude Shift Keying (continued)

Note: here we have four different amplitudes, so we can encode 2 bits

in each signal change (bits per signal change = log2 (amplitude levels)).

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Frequency Shift Keying

One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Phase Shift Keying

One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Phase Shift Keying (continued)

Quadrature Phase Shift Keying
Four different phase angles used
45 degrees
135 degrees
225 degrees
315 degrees

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Phase Shift Keying (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Phase Shift Keying (continued)

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)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Phase Shift Keying (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

4. Transmitting Analog Data with
Digital Signals

To convert analog data into a digital signal, there are two techniques:
Pulse code modulation (the more common)
Delta modulation

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Pulse Code Modulation

The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Pulse Code Modulation (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Pulse Code Modulation (continued)

When the binary values are later converted to an analog signal, a waveform similar to the original results

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Pulse Code Modulation (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Pulse Code Modulation (continued)

The more snapshots taken in the same amount of time, or the more quantization levels, the better the resolution

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Pulse Code Modulation (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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: A Business User's Approach, Eighth Edition

Delta Modulation (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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 Communications and Computer Networks: A Business User's Approach, Eighth Edition

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

EBCDIC

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

ASCII

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

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 Communications and Computer Networks: A Business User's Approach, Eighth Edition

Data and Signal Conversions In Action:
Two Examples (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Data and Signal Conversions In Action:
Two Examples (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Data and Signal Conversions In Action:
Two Examples (continued)

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Summary

Data and signals are two basic building blocks of computer networks
All data transmitted is either digital or analog
Data is transmitted with a signal that can be either digital or analog
All signals consist of three basic components: amplitude, frequency, and phase
Two important factors affecting the transfer of a signal over a medium are noise and attenuation
Four basic combinations of data and signals are possible: analog data converted to an analog signal, digital data converted to a digital signal, digital data converted to a discrete analog signal, and analog data converted to a digital signal

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Summary (continued)

To transmit analog data over an analog signal, the analog waveform of the data is combined with another analog waveform in a process known as modulation
Digital data carried by digital signals is represented by digital encoding formats
For digital data to be transmitted using analog signals, digital data must first undergo a process called shift keying or modulation
Three basic techniques of shift keying are amplitude shift keying, frequency shift keying, and phase shift keying

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition

Summary (continued)

Two common techniques for converting analog data so that it may be carried over digital signals are pulse code modulation and delta modulation
Data codes are necessary to transmit the letters, numbers, symbols, and control characters found in text data
Three important data codes are ASCII, EBCDIC, and Unicode

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition