ITT400
Introduction to Data Communication and
Networking
Chapter 2
Data and Signals
Mazlan Osman, FSKM, UiTM (Terengganu)
2014
Note
To be transmitted, data must be
transformed to electromagnetic signals.
3.2
3-1 ANALOG AND DIGITAL
•Data can be analog or digital.
•The term analog data refers to information that is
continuous; digital data refers to information that
has discrete states.
3.3
ANALOG AND DIGITAL SIGNALS
• Analog signal can have an infinite number of values in
a range.
• Digital signal can only a limited number of values.
Figure 3.1 Comparison of analog and digital signals
3.4
PERIODIC AND NONPERIODIC SIGNALS
• Both analog and digital signals can be periodic or
nonperiodic.
• A periodic signal completes a pattern within a
measurable time frame, called a period, and repeats that
pattern over subsequent identical periods.
• The completion of one full pattern is called a cycle.
• A nonperiodic signal changes without exhibiting a
pattern or cycle that repeat over time.
3.5
3-2 PERIODIC ANALOG SIGNALS
•Periodic analog signals can be classified as simple
or composite.
•A simple periodic analog signal, a sine wave,
cannot be decomposed into simpler signals.
•A composite periodic analog signal is composed
of multiple sine waves.
3.6
SINE WAVE
•Can be represented by three parameters: the peak
amplitude, the frequency, and the phase.
Figure 3.2 A sine wave
3.7
PERIOD AND FREQUENCY
•Period refers to the amount of time, in seconds, a
signal to complete 1 cycle.
•Frequency refers to the number of periods in 1 s.
•Frequency and period are the inverse of each
other.
3.8
Table 3.1 Units of period and frequency
Figure 3.3 Two signals with the same amplitude and phase,
but different frequencies
3.9
PERIOD AND FREQUENCY
• Example 3.1
The power we use at home has a frequency of 60 Hz.
Find the period?
• Example 3.2
Express a period of 100 ms in microseconds.
• Example 3.3
The period of a signal is 100 ms. What is its frequency in
kilohertz?
3.10
PHASE
• Describes the position of the waveform relative to time 0.
Figure 3.4 Three types of phases
3.12
TIME AND FREQUENCY DOMAINS
•A sine wave is comprehensively defined by its
amplitude, frequency, and phase. Sine wave
shown using time-domain plot.
•Frequency-domain plot is using to show the
relationship between amplitude and frequency.
3.13
TIME AND FREQUENCY DOMAINS
Figure 3.5 The time-domain and frequency-domain plots of a sine wave
3.14
TIME AND FREQUENCY DOMAINS
• Example 3.4
Figure 3.6 The time domain and frequency domain of three sine waves
3.15
COMPOSITE SIGNALS
• Made of many simple sine wave.
• A single-frequency sine wave is not useful in data communications;
we need to send a composite signal, a signal made of many simple
sine waves.
Figure 3.7 A composite periodic signal
3.16
COMPOSITE SIGNALS
• Example 3.5
Figure 3.8 shows a nonperiodic composite signal.
Figure 3.8 The time and frequency domains of a nonperiodic signal
3.17
BANDWIDTH
• The range of frequencies contained in a composite signal.
Figure 3.9 The bandwidth of periodic and nonperiodic composite signals
3.18
BANDWIDTH
• Example 3.6
A periodic signal has a bandwidth of 20 Hz. The highest frequency
is 60 Hz. What is the lowest frequency? Draw the spectrum if the
signal contains all frequencies of the same amplitude.
Solution
Figure 3.10 The bandwidth for Example 3.11
3.19
BANDWIDTH
• Example 3.7
A nonperiodic composite signal has a bandwidth of 200 kHz,
with a middle frequency of 140 kHz and peak amplitude of 20 V.
The two extreme frequencies have an amplitude of 0. Draw the
frequency domain of the signal.
Solution
Figure 3.11 The bandwidth for Example 3.7
3.20
3-3 DIGITAL SIGNALS
• In addition to being represented by an analog signal, information can
also be represented by a digital signal.
Figure 3.12 Two digital signals: one with two signal levels and the other with four signal levels
3.21
SIGNAL LEVEL AND DATA LEVEL
• Example 3.8
A digital signal has eight levels. How many bits (data level) are
needed per signal level? We calculate the number of bits from the
formula
Each signal level is represented by 3 bits.
• Example 3.9 A digital signal has nine levels. How many bits
(data level) are needed per signal level?
3.22
BIT RATE
• The number of bits sent in 1s, expressed in bits per
second (bps).
• Example 3.10
Assume we need to download text documents at the rate of
100 pages per second. Each page usually consists of 24 lines.
What is the required bit rate of the channel?
(assumption: each line has 80 characters, and one character
requires 8 bit)
3.23
TRANSMISSION OF DIGITAL SIGNALS
• Digital signal can transmit either by baseband transmission or
broadband transmission (using modulation).
Baseband Transmission
• Sending a digital signal over a channel without changing the
digital signal to an analog signal.
• Requires a low-pass channel, a channel with a bandwidth that
starts from zero.
Figure 3.13 Baseband transmission
3.24
TRANSMISSION OF DIGITAL SIGNALS
Figure 3.14 Bandwidths of two low-pass channels
3.25
TRANSMISSION OF DIGITAL SIGNALS
• Broadband Transmission
• Changing the digital signal to an analog signal for
transmission.
• Requires bandpass channel – a channel with a
bandwidth that does not start from zero.
Figure 3.15 Bandwidth of a bandpass channel
3.26
3-4 TRANSMISSION IMPAIRMENT
• Transmission impairment means the signal at the
beginning of the medium is not the same as the signal at
the end of the medium. What is sent is not what is
received.
Figure 3.16 Causes of impairment
3.27
ATTENUATION
• It occurs when a signal, simple or composite, travels
through a medium, it losses some of its energy in
overcoming the resistance of the medium.
• To compensate for this loss, amplifiers are used to amplify
the signal.
Figure 3.17 Attenuation
3.28
DISTORTION
• It occurs in a composite signal when the signal changes
its form or shape at the receiver.
• The shape of the composite signal is therefore not the
same.
Figure 3.18 Distortion
3.29
NOISE
• Noise is unwanted electrical or electromagnetic energy that
degrades the quality of data and signals.
Figure 3.19 Noise
3.30
NOISE
• Several types of noise may corrupt the signal, such as:
i. Thermal noise - Random motion of electronic in
such wire.
ii. Induced noise – Comes from electonic sources such
as motors and appliances.
iii. Crosstalk – Effects of one wire on other wire.
iv. Impulse noise – Spike (a signal with high energy
in a very short time) that comes from power lines,
lighting, and so on.
3.31
SIGNAL-TO-NOISE RATIO (SNR)
• Defined as
SNR = average signal power
average noise
power
• SNR is actually ratio of what is wanted (signal) to what is not
wanted (noise).
• SNR is often described in decibel units, SNRdB, defined as
SNRdB = 10log10SNR
3.32
SIGNAL-TO-NOISE RATIO (SNR)
• Example 3.11
The power of a signal is 10 mW and the power of the noise is 1
μW. What are the values of SNR and SNRdB ?
Solution
The values of SNR and SNRdB can be calculated as follows:
3.33
3-5 DATA RATE LIMITS
• Data rate depends on three factors:
1. The bandwidth available
2. The level of the signals we use
3. The quality of the channel (the level of noise)
• Two theoretical formulas were develop to calculate the
data rate:
i. Nyquist for noiseless channel
ii. Shannon for a noisy channel
3.34
NYQUIST AND SHANNON FORMULA
• Nyquist formula defines the bit rate for noiseless
channel:
BitRate = 2 * bandwidth * log2L
• Shannon formula defines the bit rate for noisy
channel:
Capacity = bandwidth * log2(1 + SNR)
3.35
NYQUIST AND SHANNON FORMULA
• Example 3.11
Consider a noiseless channel with a bandwidth of 3000 Hz
transmitting a signal with two signal levels. The maximum
bit rate can be calculated as
• Example 3.12
Consider the same noiseless channel transmitting a signal
with four signal levels (for each level, we send 2 bits). The
maximum bit rate can be calculated as
3.36
NYQUIST AND SHANNON FORMULA
• Example 3.13
We need to send 265 kbps over a noiseless channel with a bandwidth
of 20 kHz. How many signal levels do we need?
Solution
We can use the Nyquist formula as shown:
Since this result is not a power of 2, we need to either increase the
number of levels or reduce the bit rate. If we have 128 levels, the
bit rate is 280 kbps. If we have 64 levels, the bit rate is 240 kbps.
3.37
NYQUIST AND SHANNON FORMULA
• Example 3.14
We can calculate the theoretical highest bit rate of a regular
telephone line. A telephone line normally has a bandwidth of 3000.
The signal-to-noise ratio is usually 3162. Find the capacity?
• Example 3.15
The signal-to-noise ratio is often given in decibels. Assume that
SNRdB = 36 and the channel bandwidth is 2 MHz. What is the
capacity?
3.38
USING BOTH LIMITS
• In practice, both methods need to find the limits and signal levels.
• Example 3.41
We have a channel with a 1MHz bandwidth. The SNR for this
channel is 63. What are the appropriate bit rate and signal level?
Solution
First, we use the Shannon formula to find the upper limit. Second, we
use the Nyquist theorem to get the signal level.
3.39
3-6 PERFORMANCE
•How to measure the performance of the
network—how good is it?
•The performance can be measured by:
•Bandwidth
•Throughput
•Latency
•Propagation Time
•Transmission Time
•Queuing Time
3.40
PERFORMANCE
• Bandwidth
• In networking, we use the term bandwidth in two contexts.
1. The first, bandwidth in hertz, refers to the range of
frequencies that a channel can pass.
2. The second, bandwidth in bits per second, refers to the speed
of bit transmission in a channel or link.
• Throughput
• Measure of how fast data can send through a network.
• A link may have a bandwidth of B bps, but can only send T bps
through this link with T always less than B.
• Bandwidth is a potential measurement of a link; the throughput
is actual measurement of how fast data can be send.
3.41
PERFORMANCE
• Latency (Delay)
• Defines how long it takes for an entire message to completely
arrive at the destination from the time the first bit is sent out from
the source.
Latency = propagation time + transmission time + queuing
time + processing delay
• Propagation Time
• Measures the time required for a bit to travel from the source to the
destination.
• Propagation Time =
3.42
Distance
Propagation Speed
PERFORMANCE
• Transmission Time
• Time required for transmission of a message.
Transmission time =
Message size
Bandwidth
• Queuing Time
• The time needed for each intermediate or end device to hold the
message before it can be processed.
3.43