Transcript Chapter # 3 Data and Signals

Physical layer and Media
CHAPTER # 3
DATA AND SIGNALS
Introduction
2


One of the major functions of physical layer is to
move data in the form of electromagnetic signals
across a transmission medium.
Thus, the data must be transformed to
electromagnetic signals to be transmitted.
1. Analog and Digital Data
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

Data can be analog or digital.
The term analog refers to information that is
continuous
 e.g.

The term digital data refers to information that has
discrete states.
 e.g.
1.
2.
analog clock hh:mm:ss
digital clock hh:mm
Analog data take on continuous values.
Digital data take on discrete values.
2. Periodic and Nonperiodic Signals
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
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Both analog and digital signals can take one of two
forms: periodic or nonperiodic
A periodic signal
 Completes a pattern within a measureable time
frame.
 Repeats that pattern over subsequent identical period
A nonperiodic signal

Changes without exhibiting a pattern or cycle that repeats
over time.
Periodic and Nonperiodic Signals
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
In data communications, we commonly use:
 Periodic
analog signals ( because they need
less bandwidth).
 and
nonperiodic digital signals ( because they
can represent variation in data)
A. Periodic Analog Signals
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Periodic analog signals can be classified as simple or
composite.
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
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.
1) Sine Waves
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
The sine wave is the most fundamental form of a periodic
analog signal.
A sine wave is represented by three parameters: Peak
amplitude, Frequency, and Phase.
1.
Peak amplitude: it is the absolute value of the highest
intensity. It is normally measured in volts.
Sine Waves
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2.
Frequency: it refers to the number of periods in 1 s. It is formally
expressed in Hertz (Hz).
 Period is the amount of time, in seconds, a signal needs to complete one
cycle (the completion of one full pattern).
Therefore , frequency and period are the inverse of each other.
 Note : Frequency is the rate of change with respect to time.
 Change in a short span of time means high frequency.
 Change over a long span of time means low frequency.
 If a signal does not change at all, its frequency is zero

Units of period and frequency
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TABLE 3.1
Sine Waves
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3.

Phase:
 It describes the position of the waveform relative to time
0.
 It is measured in degree or radian
To look to the phase is in term of shift or offsit:
1.
2.
3.
A sine wave with a phase 0° is not shifted.
A sine wave with a phase 90° is shifted to the left by
¼ cycle.
A sine wave with a phase 180° is shifted to the left
by ½ cycle.
Three sine waves with the same amplitude and
frequency, but different phases
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Wavelength
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
Wavelength binds the period or frequency of the simple sine
wave to the propagation speed of the medium.
Wavelength depends on both the frequency and the medium.
 Wavlength = propgation speed X period = progation
speed/ frequency
Time and Frequency Domain
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
A complete sine wave in the time domain can be represented by one
single spike in the frequency domain.
2) Composite Signals
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
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.
 e.g.
if we use single sine wave to convey a
conversation over the telephone. It would just
hear a buzz.
2) Composite Signals
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
According to Fourier analysis, any composite signal
is a combination of simple sine waves with different
frequencies, amplitudes, and phases.

A composite signals can be periodic or non
periodic.
A
periodic composite signal can be decomposed into a
series of simple sine waves with discrete frequencies (
with integer values [1,2,3 ,….] ).
A
nonperiodic composite signal can be decomposed
into a combination of an infinite number of simple sine
waves with continuous frequencies. ( with real value)
Bandwidth
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
The bandwidth of a composite signal is the
difference between the highest and the lowest
frequencies contained in that signal.
 e.g.
if a composite signal contain frequencies
between 1000 and 5000, its bandwidth is 50001000 = 4000
The bandwidth of periodic and nonperiodic composite signals
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B- DIGITAL SIGNALS
In addition to being represented by an analog
signal, information can also be represented by a
digital signal. For example, a 1 can be encoded
as a positive voltage and a 0 as zero voltage.
A digital signal can have more than two levels.
In this case, we can send more than 1 bit for each
level.
3.18
Figure 3.16 Two digital signals: one with two signal levels and the
other with four signal levels
3.19
Bit rate and bit interval
Most digital signals are nonperiodic, frequency and period
are not appropriate. Another terms instead of frequency is bit
rate and instead of period: bit interval(bit duration)
Bit rate: number of bits per second bps
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Bit interval=1/bit rate
Bit length =propagation speed x Bit interval
Digital Signal as a composite Analog Signal
Note
A digital signal is a composite analog signal with an infinite
bandwidth.
Fourier analysis can be used to decompose a digital signal
o If the digital signal is periodic (rare in data
communications), the decomposed signal has a frequency
domain representation with an infinite Bandwidth and
discrete frequencies.
o If it is nonperiodic, the decomposed signal still has infinite
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Bandwidth, but the frequencies are continuous.
Transmission of Digital Signals
How can we send a digital signal from point A to point B?
We can transmit a digital signal by using one of two
different approach :
1. Baseband transmission
2. Broadband transmission
3.22
1. Baseband transmission
Means sending a digital signal over a channel
without changing the digital signal to an analog
signal
Base band transmission required a low-pass
channel (channel with a B-W that starts from
zero)
3.23
2. Broadband Transmission (modulation)
Means changing the digital signal to an analog signal for
transmission. Modulation use a band-pass channel (a
channel with a B-W that doesn't start from Zero). This type
of channel is more available than a low-pass channel.
3.24
Digital modulation and multiplexing
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Baseband transmission: the signal occupies
frequencies from zero up to a maximum that depends
on the signaling rate.
•
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Passband transmission: the signal occupies a band
of frequencies around the frequency of the carrier
signal.
•
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It is common for wires.
It is common for wireless and optical channels.
Multiplexing: Channels are often shared by multiple
signals.
1. Baseband transmission
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Line codes: (a) Bits, (b) NRZ, (c) NRZI,
(d) Manchester, (e) Bipolar or AMI.
2- Passband Transmission
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1.
2.
3.
We can take a baseband signal that occupies 0 to B Hz and
shift it up to occupy a passband of S to S +B Hz.
Digital modulation is accomplished with passband
transmission by regulating or modulating a carrier signal that
sits in the passband.
ASK (Amplitude Shift Keying), two different amplitudes are used
to represent 0 and 1.
FSK (Frequency Shift Keying), two or more different tones are
used.
PSK (Phase Shift Keying): the carrier wave is systematically
shifted 0 or 180 degrees at each symbol period.
Passband Transmission
(a) A binary signal. (b) Amplitude shift keying.
(c) Frequency shift keying. (d) Phase shift keying.
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Multiplexing
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1- Frequency Division Multiplexing
FDM: divides the spectrum into frequency bands, with each user
having exclusive possession of some band.
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2- Time Division Multiplexing
• TDM : The users take turns (in a round-robin fashion), each one
periodically getting the entire bandwidth for a little burst of time.
•
Bits from each input stream are taken in a fixed time slot and
output to the aggregate stream.
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3 - Code Division Multiplexing
• CDM : is a form of spread spectrum communication.
(a)
Chip sequences for four stations.
(b)
Signals the sequences represent
3-4 TRANSMISSION IMPAIRMENT
Signals travel through transmission media, which are not
perfect. The imperfection causes signal impairment.
• This means that 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.
Three causes of impairment are attenuation, distortion,
and noise.
3.32
1. Attenuation – a loss of energy
when Signal 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.
Decibel: Measure the relative power(attenuation)
dB=10 log10 P2/P1
3.33
2. Distortion
Distortion : means that signal changes its form or shape.
Each signal component has its own propagation speed
through the medium and therefore ,its own delay in
arriving final destination
3.34
3. Noises
•Thermal noise: is the random motion of electrons in
a wire which creates an extra signal not originally
sent by the transmitter
•Induced noise: Comes from sources such as motors
and appliances.
•Crosstalk noise: Is the effect of one wire on the
other.
Impulse Noise: is a spike ( a signal with high energy
in a very short time) that comes from power lines,
lighting and so on.
3.35
Signal-to-Noise Ratio
SNR: ratio between signal power to the noise power
o A high SNR: means the signal is less corrupted by noise
o A low SNR: means the signal is more corrupted by noise.
SNR can be described in db units
SNR db=10 log10 SNR
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3.5 Data Rate Limits
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◻
A very important consideration in data communications is
how fast we can send data, in bits per second, over a
channel. Data rate depends on three factors:
⬜
The bandwidth available
⬜
The level of the signals we use
⬜
The quality of the channel (the level of noise)
Two theoretical formulas were developed to calculate the
data rete :
1. By Nyquist for a noiseless channel
2. By Shannon for a noisy channel
Nyquist Theorem (noiseless channel)
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The relation between bandwidth and data rate in a
noiseless channel (throughput)
⬜ Maximum data rate = 2 x bandwidth x log2 L
where L: No of signal levels used to represent data
Example :
Consider a noiseless channel with a bandwidth of 3 KHz
transmitting a signal with two signal levels.
The maximum bit rate can be calculated as:
◻
Throughput =2*3000 log2 2 = 6000 bps.
Shannon Theorem (Noisy Channel)
The maximum throughput of a noisy channel of
bandwidth B with a signal to noisy ratio of S/N is:
Maximum throughput = B log2(1+S/N) bps.
Shannon Capacity
Capacity = bandwidth x log2(1+SNR)
is the capacity of the channel in bps ( max data
rate)
Example :
Telephone line Bandwidth=3kHz; S/N=30 dB =>
Max throughput = 3000 * log2(1+1000) =~ 30.000 bps = 28.8
kbps
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3-6 PERFORMANCE
One important issue in networking is the
performance of the network—how good is it?
1. Bandwidth
2. Throughput
3. Latency (Delay)
3.40
1. Bandwidth
In networking, we use the term bandwidth in two
contexts.
❏The first, bandwidth in hertz, refers to the range
of frequencies in a composite signal or the range
of frequencies that a channel can pass.
❏The second, bandwidth in bits per second, refers
to the speed of bit transmission in a channel or
link.
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2. Throughput
Is a measure of how fast we can actually send
data through a network.
 The bandwidth is potential measurement of a
link; the throughput is an actual
measurement of how fast we can send data.
 Throughput less than Bandwidth
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3. Latency ( Delay)
Latency 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 (Delay) =
propagation time + transmission time
+queuing time + processing time
Note the concept : dominate factor
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1) Propagation time
• It is the time required for a bit to travel from the
source to the destination
• Propagation speed depend on the medium and on
the frequency of the signal
Example :
light propagate by 3x108m/s in vacuum. It is lower
in air ; it is much lower in cable.
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2. Transmission time
• It is the time required for transmission of a
message .
• It depends on the size of the message and the
bandwidth of the channel.
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3. Queuing time
• It is the time needed for each end device to
hold the message before it can be processed.
• It changes with the load imposed on the
network, if there is heavy traffic on the
network , the queuing time increases.
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TRANSMISSION MEDIA
McGraw-Hill7.47
©The McGraw-Hill Companies, Inc., 2000
Transmission medium and physical layer
• A transmission media defined as anything that carry
information between a source to a destination
• Located below the physical layer and are directly controlled
by the physical layer
7.48
Classes of Transmission Media
7.49
7-1 GUIDED MEDIA
Guided media, which are those that provide a conduit
from one device to another, include twisted-pair cable,
coaxial cable, and fiber-optic cable.
Twisted –pair cables and coaxial cable:
use metallic (copper) conductors that transport signals in
the form of electric current
Optical fiber :
transport signals in the form of the light
7.50
1. Twisted-pair cable
• One of the wire used to carry signal and the other as a
ground. The receiver uses the difference between the two.
• If the two wires are parallel, the effect of interference
noise and crosstalk is big
• Twisting the pair of wire balance the effect of unwanted
signal and reduce it.
7.51
Applications of Twisted pair
Used in
1. Telephone lines to provide voice and data
channels
2. The DSL lines that are used by the telephone
companies to provide high-data-rate connections
3. Local area networks, such as 10-base-T and
100-base-T
7.52
2. Coaxial cable
• Coax cable carries signals of higher frequency ranges
than those in Twisted pair cable because the two media
are constructed quite differently.
• The outer conductor serves both as a shield against noise
and as second conductor, which complete the circuit
7.53
Applications of coaxial cable
1.Analog telephone network where a single cable could
carry 10,000 voice signals. Later it was used in Digital
telephone networks where cable can carry 600Mbps
2.Cable TV network: hybrid network use coaxial cable only
at the network boundaries , near the consumer.
3.Traditional Ethernet LANs.
7.54
3. Fiber Optic Cable
7.55
• Is made of glass or plastic and transmit signals in the
form of light.
• Light travels in a straight line as long as it is moving
through a single uniform substance. If a ray of light
traveling through one substance enters another
substance of different density , the ray change
direction as shown:
Optical fiber
7.56
Fiber Optical : uses reflection to guide light through a
channel. A glass or plastic core is surrounded by a
cladding of less dense glass or plastic
Back to the book for advantages and disadvantages
Applications for Fiber Optic cable
7.57
Used in :
1.Cable TV network: hybrid network use a combination of
optical fiber and coax cable. Optical provides the
backbone while coaxial cable provide the connation to
the user.
2.Local area networks such as ( fast Ethernet)
3.Backbone networks because its wide bandwidth
Propagation modes using fiber optics
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Multimode Fiber: any light ray incident on the
boundary above the critical angle will be reflected
internally, many different rays will be bouncing around
at different angles.
•
Single-mode Fiber: light can propagate only in a
straight line, without bouncing.
Fiber cable composition
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(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.
Core: 50 microns for multi-mode, 8-10 microns for single mode
Cladding: glass with a lower refraction index, to keep the light
in the core
7- 2 UNGUIDED MEDIA - wireless
• Unguided media transport electromagnetic waves
without using a physical conductor.
• Signals are normally broadcast through free space and
thus are available to anyone who has a device capable
of receiving them
• Unguided signals can travel from the source to
destination in several ways:
1. Ground propagation
2. Sky propagation
3. Line – of – sight propagation
7.60
7.61
1. Radio Transmission
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(a) In the VLF, LF, and MF bands, radio waves follow the
curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
1. Radio Transmission
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Frequency ranges: 3 KHz to 1 GHz
Omnidirectional
Susceptible to interference by other antennas using same
frequency or band
Ideal for long-distance broadcasting
May penetrate walls
Apps: AM and FM radio, TV, maritime radio, cordless
phones, paging
2. Microwaves
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•
•
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Frequencies between 1 and 300 GHz
Unidirectional.
Narrow focus requires sending and receiving antennas to
be aligned.
Issues:
•
•
Line-of-sight (curvature of the Earth; obstacles)
Cannot penetrate walls
2. Satellite Microwaves
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
Similar to terrestrial microwave except the signal travels
from a ground station on earth to a satellite and back to
another ground station.



Satellite receives on one frequency, amplifies or repeats signal
and transmits on another frequency
Satellite is relay station
Applications
 Television
 Long distance telephone
 Private business networks
3. Infrared
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•
•
•
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Frequencies between 300 GHz and 400 THz.
Short-range communication in a closed area.
High frequencies cannot penetrate walls.
Requires line-of-sight propagation.
Advantage: prevents interference between systems in
adjacent rooms.
Disadvantage: cannot use for long-range communication
or outside a building due to sun’s rays.