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UNIT 3
DATA TRANSMISSION
AND NETWORKING
MEDIA 1
Outcomes 1
By the end of this subtopic, student should be able to :
Explain the basic concept of data transmission: 1. Analog and
digital signaling
2. Data Modulation
3. Simplex, half-duplex, and full-duplex transmission
4. Multiplexing
5. Point to point Transmission
6. Broadcast Transmission
7. Throughput
8. Bandwidth
9. Baseband and Broadband
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1. SIGNAL
SIGNAL
Digital Analog
ANALOG SIGNAL
Analog
Continuous signal
Examples of analog data is the human voice When somebody speaks,
a continuous wave is created in the air
This can captured by a microphone an converted to and analog
signal.
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DIGITAL SIGNAL
Digital
Discrete signal.
Examples of digital data; is data stored in memory of a computer
in the form of 0s and 1s.
Digital signal is more reliable than any other signal.
Waveform of analog & digital signal
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Differences between analog &digital
signal
ANALOG SIGNAL DIGITAL SIGNAL
Continuous signal Discrete signal
Examples of analog data is the human voice
(when somebody speaks, a continuous wave is
created in the air).
Examples of digital data is the data stored in
memory of a computer in the form of 0s and
1s or on-off.
Cannot perform high-quality data transmission
(very difficult to remove noise and wave
distortions during the transmission).
Noise and distortions have little effect, making
high-quality data transmission
No security/encryption implemented in
analog cordless products (analog cordless
phone).
Able to encrypt all 1s and 0s during
transmission so your conversation is safe from
eavesdroppers (digital cordless phone).
2. DATA MODULATION
Data modulation is a technology used to modify analog signals to
make them suitable for carrying data over a communication path.
In modulation, a simple wave, called a carrier wave, is combined
with another analog signal to produce a unique signal that gets
transmitted from one node to another. The carrier wave has preset
properties (including frequency, amplitude, and phase).
Its purpose is to help convey information; in other words, its
only a messenger.
Another signal, known as the information or data wave, is added
to the carrier wave. When the information wave is added, it
modifies one property of the carrier wave (for example, the
frequency, amplitude, or phase). The result is a new, blended
signal that contains properties of both the carrier wave and added
data. When the signal reaches its destination, the receiver
separates the data from the carrier wave.
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2. DATA MODULATION (cont.)
Modulation can be used to make a signal conform to a specific
pathway, as in the case of FM (frequency modulation) radio, in
which the data must travel along a particular frequency.
In FM (frequency modulation), the frequency of the carrier
signal is modified by the application of the data signal.
In AM (amplitude modulation), the amplitude of the carrier
signal is modified by the application of the data signal.
Modulation may also be used to issue multiple signals to the same
communications channel and prevent the signals from interfering
with one another.
Figure below depicts an unaltered carrier wave, a data wave, and
the combined wave as modified through frequency modulation.
Network+ Guide to Networks, 4e 10
DATA MODULATION
Figure 3-5: A carrier wave modified through frequency
modulation
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3. TRANSMISSION
TRANSMISSION
Simplex Full-Duplex
Half-duplex
Simplex transmission: allows data to travel only in a single
direction.
Example of simplex transmission: television broadcast.
Simplex
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Simplex (cont)
Another example of simplex communication is a football coach
calling out orders to his team through a megaphone.
In this example, the coachs voice is the signal, and it travels
in only one directionaway from the megaphones mouthpiece and toward
the team.
Simplex is sometimes called one-way, or unidirectional,
communication.
Half-duplex transmission: messages can move in either direction
, but only one way at a time (walkie-talkie)
Example of half-duplex transmission: walkie-talkie
Half- Duplex
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For example :
intercom system that requires you to press a talk button to
allow your voice to be transmitted uses half-duplex
transmission.
If you visit a friends apartment building, you press the talk
button to send your voice signals to his apartment.
When your friend responds, he presses the talk button in his
apartment to send his voice signal in the opposite direction over
the wire to the speaker in the lobby where you wait.
If you press the talk button while hes talking, you will not be
able to hear his voice transmission.
Full-duplex: signals free to travel in both directions
simultaneously.
Example of full-duplex: telephone conversations.
Full-Duplex
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When signals are free to travel in both directions over a medium
simultaneously, the transmission is considered full-duplex.
Full-duplex may also be called bidirectional transmission or,
sometimes, simply duplex.
When you call a friend on the telephone, your connection is an
example of a full-duplex transmission because your voice signals
can be transmitted to your friend at the same time your friends
voice signals are transmitted in the opposite direction to you.
In other words, both of you can talk and hear each other
simultaneously.
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4. Transmission Direction: Multiplexing
A form of transmission that allows multiple signals to travel
simultaneously over one medium is known as multiplexing. To carry
multiple signals, the mediums channel is logically separated into
multiple smaller channels, or subchannels. Many different types of
multiplexing are available, and the type used in any given
situation depends on what the media, transmission, and reception
equipment can handle.
For each type of multiplexing, a device that can combine many
signals on a channel, a multiplexer (mux), is required at the
transmitting end of the channel.
At the receiving end, a demultiplexer (demux) separates the
combined signals and regenerates them in their original form.
Networks rely on multiplexing to increase the amount of data that
can be transmitted in a given time span over a given bandwidth.
Network+ Guide to Networks, 4e
5.Relationships Between Nodes
Figure 3-10: Point-to-point VS broadcast transmission
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Point to Point Transmission
When a data transmission involves only one
transmitter and one receiver, it is considered a
point-to-point transmission.
The sender only transmits data that is intended to
be used by a specific receiver.
Broadcast transmission
Broadcast transmission involves one transmitter and multiple,
undefined receivers. For example, a TV station indiscriminately
transmitting a signal from its tower to thousands of homes with TV
antennas uses broadcast transmission.
A broadcast transmission sends data to any and all receivers,
without regard for which receiver can use it. Broadcast
transmissions are frequently used on both wired and wireless
networks because they are simple and quick.
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Network+ Guide to Networks, 4e 23
6.Throughput and Bandwidth
Throughput: measure of amount of data transmitted during given
time period
Also called as capacity
Expressed as a quantity of bits transmitted per second, with
prefixes used to designate different throughput amounts. For
example, the prefix kilo combined with the word bit (as in kilobit)
indicates 1000 bits per second.
Bandwidth: difference between highest and lowest frequencies
that a medium can transmit
Range of frequencies is directly related to throughput.
Network+ Guide to Networks, 4e 24
7.Baseband and Broadband
Baseband: digital signals sent through direct current (DC)
pulses applied to a wire
Requires exclusive use of wires capacity
Baseband systems can transmit one signal at a time
Ethernet
Broadband: signals modulated as radiofrequency (RF) analog
waves that use different frequency ranges
Does not encode information as digital pulses
broadband transmission is used to bring cable TV to your
home.
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By the end of this subtopic, student should be able to :
Describe common transmission flaws (kecacatan
penghantaran):
Noise
Attenuation
Latency
Outcomes 2
Transmission flaws
1 Attenuation
2 Noise
3 Latency
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1. Attenuation
Attenuation (pengurangan/penyusutan):
the loss of signal strength over long distances
when signals travel along cabling.
Measured in decibels (dB).
Copper cabling has much greater attenuation
than fiber-optic cabling, which makes copper
suitable only for relatively short cable runs.
A digital device, known as a remodulator,
provides better signal quality by removing all of
the accumulated noise and attenuation and
transmitting a cleaned-up signal.
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Loss of signal strength as transmission travel away
from source
Analog signals pass through an amplifier,which
increase not only voltage of a signal but also noise
accumulated.
Noise :
interference in cabling by proximity to electrical equipment
that generates electromagnetic interference (EMI).
Any undersirable influence degrading or distorting signal
Noise is generated by all electrical and electronic devices,
including : motors
fluorescent lamps
power lines, and office equipment.
2. Noise (Hingar)
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Noise can usually be reduced (but never entirely
eliminated) by using higher-quality components,
lowering the temperature of components, or
using shielded cabling
noise
An analog signal distorted by noise
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A digital signal distorted by noise
Latency (masa pendam):
Delay between transmission and receipt of a signal
Many possible causes: Cable length
Intervening connectivity device (e.g., modems and routers)
3. Latency
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By the end of this subtopic, student should be able to :
Describe Transmission Media in network
Explain physical characteristics of :
1. coaxial cable,
2. STP,
3. UTP, and
4. fiber-optic media.
Outcomes 3
Transmisson Media
Guided
Unshielded Twisted Pair
Cable
Shielded Twisted Pair
Cable Coaxial Cable Fiber Optic
Cabel
Unguided
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Twisted pair cables
Twisted pair cables consist of one or more pairs of insulated
copper wires that are twisted together and housed in a protective
jacket. Like all copper cables, twisted pair uses pulses of
electricity to transmit data.
Data transmission is sensitive to interference or noise, which
can reduce the data rate that a cable can provide. A twisted pair
cable is susceptible to electromagnetic interference (EMI), a type
of noise.
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A source of interference, known as crosstalk, occurs when cables
are bundled together for long lengths. The signal from one cable
can leak out and enter adjacent cables.
When data transmission is corrupted due to interference such as
crosstalk, the data must be retransmitted. This can degrade the
data carrying capacity of the medium.
In twisted pair cabling, the number of twists per unit length
affects the amount of resistance that the cable has to
interference. Twisted pair cable suitable for carrying telephone
traffic, referred to as CAT3, has 3-4 turns per foot making it less
resistant. Cable suitable for data transmission, known as CAT5, has
3-4 turns per inch, making it more resistant to interference.
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UTP
Unshielded twisted pair (UTP) is the most commonly encountered
type of network cable in North America and many other areas.
Shielded cables (ScTP and F-UTP) are used almost exclusively in
European countries.
UTP cable is inexpensive, offers a high bandwidth, and is easy
to install. This type of cable is used to connect workstations,
hosts and network devices. It can come with many different numbers
of pairs inside the jacket, but the most common number of pairs is
four. Each pair is identified by a specific color code.
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Many different categories of UTP cables have been developed over
time. Each category of cable was developed to support a specific
technology and most are no longer encountered in homes or offices.
The cable types which are still commonly found include Categories
3, 5, 5e and 6.
There are electrical environments in which EMI and RFI are so
strong that shielding is a requirement to make communication
possible, such as in a noisy factory. In this instance, it may be
necessary to use a cable that contains shielding, such as Shielded
twisted-pair (STP) and Screened twisted-pair (ScTP).
Unfortunately both STP and ScTP are very expensive, not as
flexible, and have additional requirements due to the shielding
that make them difficult to work with.
All Categories of data grade UTP cable are traditionally
terminated into an RJ-45 connector.
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Shielded cables and screened twisted pair are
used almost exclusively in European
countries.
Individual pair are wrapped in a shield and
then the entire four pairs are wrapped in
another shield
Used for data transmission (in a noisy
factory)
Not as flexible (bulky), and have additional
requirements due to the shielding that make
them difficult to work with.
STP & ScTP
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SHIELDED TWISTED PAIR
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Coaxial Cable
Like twisted pair, coaxial cable (or coax) also carries data in
the form of electrical signals. It provides improved shielding
compared to UTP, so has a lower signal-to-noise ratio and can
therefore carry more data. It is often used to connect a TV set to
the signal source, be it a cable TV outlet, satellite TV, or
conventional antenna. It is also used at NOCs to connect to the
cable modem termination system (CMTS) and to connect to some
high-speed interfaces.
Although coax has improved data carrying characteristics,
twisted pair cabling has replaced coax in local area networking
uses. Among the reasons for the replacement is that - compared to
UTP - coax is physically harder to install, more expensive, and
harder to troubleshoot.
Coaxial cable
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Coaxial cable
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Fiber optic cables
Unlike TP and coax, fiber optic cables transmit data using
pulses of light. Although not normally found in home or small
business environments, fiber optic cabling is widely used in
enterprise environments and large data centers.
Fiber optic cable is constructed of either glass or plastic,
neither of which conducts electricity. This means that it is immune
to EMI and is suitable for installation in environments where
interference is a problem.
In addition to its resistance to EMI, fiber optic cables support
a large amount of bandwidth making them ideally suited for
high-speed data backbones. Fiber optic backbones are found in many
corporations and are also used to connect ISPs on the Internet.
Each fiber optic circuit is actually two fiber cables. One is
used to transmit data; the other is used to receive data.
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Multimode
Of the two forms of fiber optic, multimode is the less expensive
and more widely used. The light source that produces the pulses of
light is usually an LED.
It is referred to as multimode because there are multiple rays
of light, each carrying data, being transmitted through the cable
simultaneously. Each ray of light takes a separate path through the
multimode core.
Multimode fiber optical cables are generally suitable for links
of up to 2000 meters. However, improvements in technology are
continually improving this distance.
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Single Mode
Single mode fiber optic cables are constructed in such a way
that light can follow only a single path through the fiber.
The light source for single mode fiber optic cables is usually a
LED laser, which is significantly more expensive and intense than
ordinary LEDs. Due to the intensity of the LED laser, much higher
data rates and longer ranges can be obtained.
Single mode fibers can transmit data for approximately 3000
meters and are used for backbone cabling including the
interconnection of various NOCs. Again, improvements in technology
are continually improving this distance.
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Test Yourself
FO UTP
A company must provide network connectivity between
three buildings on a single campus. The cables must be run
outside and there is a high probability of lighting storms
in
the area.
A company must provide network connectivity between
two buildings located 1 km apart.
3. A company must provide 100Mbps connectivity to users
located in their main office by running cables from the
central switch to the individual desktops. The maximum
distance from the switch to a workstation is 60 meters.
By the end of this subtopic, student should be able to :
Describe benefit and limitation of different
networking media in terms of :
1. Throughput
2. Noise Immunity
3. Size and Scalability
4. Cost
Outcomes 4
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Network+ Guide to Networks, 4e 73
1.Throughput
Probably most significant factor in choosing transmission
method
Limited by signaling and multiplexing techniques used in
given
transmission method
Transmission methods using fiber-optic cables achieve faster
throughput than those using copper or wireless connections
Noise and devices connected to transmission medium can limit
throughput
UTP STP Fiber Optic Coaxial Cable
STP and UTP can both transmit data
at 10 Mbps, 100 Mbps, 1 Gbps,
and 10 Gbps, depending on the
grade of cabling and the
transmission method in use.
Fiber has proved reliable
in transmitting data at
rates that can reach 100
gigabits (or 100,000
megabits) per second per
channel.
Each type of coax is suited to a
different purpose. When discussing
the size of the conducting core in a
coaxial cable, we refer to its
American Wire Gauge (AWG) size.
The larger the AWG size, the
smaller the diameter of a piece of
wire. RG-6 coaxial cables are used,
for example, to deliver broadband
cable Internet service and cable TV,
particularly over long distances.
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Network+ Guide to Networks, 4e 75
2.Noise Immunity
Some types of media are more susceptible to noise than
others
Fiber-optic cable least susceptible
Install cabling away from powerful electromagnetic forces
May need to use metal conduit to contain and protect cabling
Possible to use antinoise algorithms
UTP STP Fiber Optic Coaxial Cable
signals transmitted over
UTP may be subject to
filtering and balancing
techniques to offset the
effects of noise.
Because of its shielding,
STP is more noise
resistant than UTP.
Because fiber does not
conduct electrical
current to transmit
signals,
it is unaffected by EMI.
Its impressive noise
resistance is one reason
why fiber can span
such long distances
before it requires
repeaters to regenerate
its signal.
Because of its shielding,
most coaxial cable has a
high resistance to noise.
It can also carry signals
farther than twisted pair
cabling before
amplification of the
signals becomes
necessary (although not
as far as fiber-optic
cabling). On the other
hand, coaxial cable is
more expensive than
twisted pair cable
because it requires
significantly more raw
materials to
manufacture.
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Network+ Guide to Networks, 4e 77
3.Size and Scalability
Three specifications determine size and scalability of
networking media:
Maximum nodes per segment
Depends on attenuation and latency
Maximum segment length
Depends on attenuation, latency, and segment type
Populated segment contains end nodes
Maximum network length
Sum of networks segment lengths
UTP STP Fiber Optic Coaxial Cable
The maximum segment length
for both STP and UTP is 100 m,
or 328 feet, on Ethernet
networks that support data rates
from 1 Mbps to 10 Gbps.
These accommodate a maximum
of 1024 nodes. (However,
attaching so many nodes to a
segment is very impractical, as it
would slow traffic and make
management nearly
impossible.)
Depending on the type of
fiber-optic cable used,
segment lengths vary
from 150 to 40,000
meters. This limit is due
primarily to optical loss,
or the degradation of the
light signal after it travels
a certain distance away
from its source.
The maximum
segment length of 185
meters (or roughly
200).
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Network+ Guide to Networks, 4e 79
4.Cost
Many variables can influence final cost of implementing
specific
type of media:
Cost of installation
Cost of new infrastructure versus reusing existing
infrastructure
Cost of maintenance and support
Cost of a lower transmission rate affecting productivity
Cost of obsolescence
UTP STP Fiber Optic Coaxial Cable
Inexpensive.
High-grade UTP, can
be expensive too,
however.
For example, Cat 6e
costs more per foot
than Cat 5 cabling
Typically, STP is
more expensive than
UTP because it
contains more
materials and it has a
lower demand. It also
requires grounding,
which
can lead to more
expensive
installation.
Fiber-optic cable is
the most expensive
transmission
medium. Because of
its cost, most
organizations find it
impractical to run
fiber to every
desktop.
In addition, hiring
skilled fiber cable
installers costs more
than hiring twisted
pair cable
installers.
The sheath, which
protects the cable
from physical
damage, may be
PVC or a more
expensive, fire-
resistant plastic.
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Network+ Guide to Networks, 4e 81
5.Connectors and Media Converters
Connectors: pieces of hardware connecting wire to network
device
Every networking medium requires specific kind of connector
Media converter: hardware enabling networks or segments
running on different media to interconnect and exchange
signals
Type of transceiver
Device that transmits and receives signals
UTP STP Fiber Optic Coaxial Cable
STP and UTP use RJ-45
(Registered Jack 45) modular
connectors and
data jacks, which look similar
to analog telephone connectors
and jacks. However, telephone
connections follow the RJ-11
(Registered Jack 11) standard.
With fiber cabling, you can
use any of 10 different types
of connectors.
Most common connector
types:
the ST (straight tip), SC
(subscriber connector or
standard connector), LC
(local connector), and MT-
RJ (mechanical transfer
registered jack).
F-type connectors attach to coaxial
cable so that the pin in the center of
the connector is the conducting
core of the cable. Therefore, F-type
connectors require that the cable
contain a solid
metal core. A BNC connector is
crimped, compressed, or twisted
onto a coaxial cable. It connects to
another BNC connector via a turning
and locking mechanism.
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By the end of this subtopic, student should be able to :
Explain the best practices for cabling buildings and
work areas.
Outcomes 5
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The best practice for installing cable is to follow the TIA/EIA
568 specifications and the manufacturers recommendations.
Be careful not to exceed a cables bend radius, untwist wire
pairs more than one-half inch, or remove more than one inch of
insulation from copper wire.
Install plenum-rated cable in ceilings and floors, and run
cabling away from where it might suffer physical damage. Maintain
clear, comprehensive documentation on your cable plant.
TIA/EIAs 568 Commercial Building Wiring Standard, also known as
structured cabling, provides guidelines for uniform,
enterprise-wide, multivendor cabling systems.
Structured cabling is based on a hierarchical design that begins
with a service providers facilities and end at users
workstations.
Network+ Guide to Networks, 4e 86
Cable Design and Management
Cable plant: hardware making up enterprise-wide cabling
system
Structured cabling: TIA/EIAs 568 Commercial Building
Wiring Standard
Entrance facilities point where buildings internal cabling
plant
begins
Demarcation point: division between service carriers network
and
internal network
Backbone wiring: interconnection between telecommunications
closets, equipment rooms, and entrance facilities
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Network+ Guide to Networks, 4e 87
Cable Design and Management
(continued)
Structured cabling (continued):
Equipment room: location of significant networking hardware,
such as servers and mainframe hosts
Telecommunications closet: contains connectivity for groups of
workstations in area, plus cross connections to equipment rooms
Horizontal wiring: wiring connecting workstations to closest
telecommunications closet
Work area: encompasses all patch cables and horizontal wiring
necessary to connect workstations, printers, and other network
devices from NICs to telecommunications closet
Network+ Guide to Networks, 4e 88
Installing Cable
Many network problems can be traced to poor cable
installation techniques
Two methods of inserting UTP twisted pairs into RJ-45 plugs:
TIA/EIA 568A and TIA/EIA 568B
Straight-through cable allows signals to pass straight
through
between terminations
Crossover cable: termination locations of transmit and
receive
wires on one end of cable reversed
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By the end of this subtopic, student should be able to
define the characteristics of wireless transmission 1) Signal
Propagation (Penyebaran isyarat)
2) Signal Degradation (Penurunan isyarat)
3) Antenna
4) Narrowband, broadband and spread spectrum signals
5) Fixed and mobile wireless communication
Outcomes 6
Wireless Network?
Networks that transmit signals through the
atmosphere via radio frequency (RF) waves are
known as wireless networks or WLANs (wireless
local area networks).
Wireless transmission media is now common in
business and home networks and necessary in
some specialized network environments.
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The Wireless Spectrum
All wireless signals are carried through the air by
electromagnetic waves.
The wireless spectrum is a continuum of the electromagnetic
waves used for data and voice communication. On the spectrum, waves
are arranged according to their frequencies, from lowest to
highest.
The wireless spectrum (as defined by the FCC, which controls its
use) spans frequencies between 9 KHz and 300 GHz.
Each type of wireless service can be associated with one area of
the wireless spectrum.
AM broadcasting, for example, sits near the low-frequency end of
the wireless communications spectrum, using frequencies between 535
and 1605 KHz.
Infrared waves belong to a wide band of frequencies at the
high-frequency end of the spectrum, between 300 GHz and 300,000
GHz.
Most cordless telephones and many wireless LANs use frequencies
around 2.4 GHz. Other wireless LANs use a range of frequencies near
5 GHz.
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Network+ Guide to Networks, 4e 93
The Wireless Spectrum
Figure 3-37: The wireless spectrum
1. Signal propagation
A wireless signal would travel directly in a straight line
from
its transmitter to its intended receiver. This type of
propagation, known as LOS (line-of-sight), uses the least
amount of energy and results in the reception of the
clearest
possible signal.
When an obstacle stands in a signals way, the signal may
pass
through the object or be absorbed by the object, or it may
be
subject to any of the following phenomena: reflection,
diffraction, or scattering. (Pantulan, pembelauan, atau
berselerak.)
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Phenomena 1 : Reflection
Reflection in wireless signaling is no different from reflection
of other electromagnetic waves, such as light. The wave encounters
an obstacle and reflectsor bounces backtoward its source.
A wireless signal will bounce off objects whose dimensions are
large compared to the signals average wavelength. In the context of
a wireless LAN, which may use signals with wavelengths between one
and 10 meters, such objects include walls, floors, ceilings, and
the Earth. In addition, signals reflect more readily off conductive
materials, like metal, than insulators, like concrete.
Phenomena 2 : Diffraction
In diffraction, a wireless signal splits into
secondary waves when it encounters an
obstruction. The secondary waves continue to
propagate in the direction in which they were
split.
If you could see wireless signals being diffracted,
they would appear to be bending around the
obstacle. Objects with sharp edgesincluding
the corners of walls and deskscause diffraction.
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Phenomena 3 : Scattering
Scattering is the diffusion, or the reflection in multiple
different directions, of a signal. Scattering occurs when a
wireless signal encounters an object that has small dimensions
compared to the signals wavelength.
Scattering is also related to the roughness of the surface a
wireless signal encounters. The rougher the surface, the more
likely a signal is to scatter when it hits that surface. In an
office building, objects such as chairs, books, and computers cause
scattering of wireless LAN signals. For signals traveling outdoors,
rain, mist, hail, and snow may all cause scattering.
Because of reflection, diffraction, and scattering,
wireless signals follow a number of different
paths to their destination. Such signals are known
as multipath signals.
Figure below illustrates multipath signals caused
by these three phenomena.
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Network+ Guide to Networks, 4e Figure 3-39: Multipath signal
propagation
2. Signal degradation
No matter what paths wireless signals take, they are bound
to run into obstacles.
When they do, the original signal issued by the transmitter
will experience fading, or a change in signal strength as a
result of some of the electromagnetic energy being
scattered, reflected, or diffracted after being issued by
the
transmitter.
Because of fading, the strength of the signal that reaches
the
receiver is lower than the transmitted signals strength.
This
makes sense because as more waves are reflected, diffracted,
or scattered by obstacles, fewer are likely to reach their
destination.
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Attenuation is not the most severe flaw affecting
wireless signals.
Wireless signals are also susceptible to noise
(more often called electromagnetic interference
or simply, interference, in the context of wireless
communications).
Interference is a significant problem for wireless
communications because the atmosphere is
saturated with electromagnetic waves.
For example, wireless LANs may be affected by cellular phones,
mobile phones, or overhead lights. Interference can distort and
weaken a wireless signal in the same way that noise distorts and
weakens a wired signal. However, because wireless signals cannot
depend on a conduit or shielding to protect them from extraneous
EMI, they are more vulnerable to noise.
The extent of interference that a wireless signal experiences
depends partly on the density of signals within a geographical
area. Signals traveling through areas in which many wireless
communications systems are in usefor example, the center of a
metropolitan areaare the most apt to suffer interference.
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3. Antenna
Just as with wired signals, wireless signals originate from
electrical current traveling along a conductor. The electrical
signal travels from the transmitter to an antenna, which then emits
the signal, as a series of electromagnetic waves, to the
atmosphere. The signal propagates through the air until it reaches
its destination.
At the destination, another antenna accepts the signal, and a
receiver converts it back to current. Figure below illustrates this
process.
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Each type of wireless service requires an antenna specifically
designed for that service.
The services specifications determine the antennas power output,
frequency, and radiation pattern. An antennas radiation pattern
describes the relative strength over a three-dimensional area of
all the electromagnetic energy the antenna sends or receives.
A directional antenna issues wireless signals along a single
direction. This type of antenna is used when the source needs to
communicate with one destination, as in a point-to-point link. A
satellite downlink (for example, the kind used to receive digital
TV signals) uses directional antennas.
In contrast, an omnidirectional antenna issues and receives
wireless signals with equal strength and clarity in all directions.
This type of antenna is used when many different receivers must be
able to pick up the signal, or when the receivers location is
highly mobile.TV and radio stations use omnidirectional antennas,
as do most towers that transmit cellular telephone signals.
The geographical area that an antenna or wireless system can
reach is known as its range. Receivers must be within the range to
receive accurate signals consistently. Even within an antennas
range, however, signals may be hampered by obstacles and rendered
unintelligible.
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4. Narrowband, broadband and spread
spectrum signals.
Narrowband : A transmitter concentrates the signal energy at
a
single frequency or in a very small range of frequencies.
Broadband : Uses a relatively wide band of the wireless
spectrum. Broadband technologies, as a result of their wider
frequency bands, offer higher throughputs than narrowband
technologies.
Spread-spectrum : The use of multiple frequencies to
transmit
a signal is known as spread-spectrum technology (because the
signal is spread out over the Wireless spectrum).
In other words, a signal never stays continuously
within one frequency range during its transmission.
One result of spreading a signal over a wide
frequency band is that it requires less power per
frequency than narrowband signaling. This
distribution of signal strength makes spread-
spectrum signals less likely to interfere with
narrowband signals traveling in the same frequency
band.
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Wireless
Communication
Fixed and mobile wireless communication
Fixed VS Mobile?
In fixed wireless systems, the locations of the transmitter and
receiver
do not move. The transmitting antenna focuses its energy
directly
toward the receiving antenna. This results in a point-to-point
link.
One advantage of fixed wireless is that because the receivers
location
is predictable, energy need not be wasted issuing signals across
a large
geographical area. Thus, more energy can be used for the
signal.
Fixed wireless links are used in some data and voice
applications.For
example, a service provider may obtain data services through a
fixed
link with a satellite. In cases in which a long distance or
difficult
terrain must be traversed, fixed wireless links are more
economical
than cabling.
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However, many types of communications are unsuited to
fixed wireless, For example, a waiter who uses a wireless
handheld computer to transmit orders to the restaurants
kitchen could not use a service that requires him to remain
in one spot to send and receive signals. Instead, wireless
LANs, along with cellular telephone, paging, and many
other services use mobile wireless systems.
In mobile wireless, the receiver can be located anywhere
within the transmitters range. This allows the receiver to
roam from one place to another while continuing to pick up
its signal.
Fixed VS Mobile?
Network+ Guide to Networks, 4e 112
Summary
Information can be transmitted via two methods: analog or
digital
In multiplexing, the single medium is logically separated into
multiple channels, or subchannels
Throughput is the amount of data that the medium can transmit
during a given period of time
Baseband is a form of transmission in which digital signals are
sent through direct current pulses applied to the wire
Noise is interference that distorts an analog or digital
signal
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Network+ Guide to Networks, 4e 113
Summary (continued)
Analog and digital signals may suffer attenuation
Cable length contributes to latency, as does the presence of
any
intervening connectivity device
Coaxial cable consists of a central copper core surrounded by
a
plastic insulator, a braided metal shielding, and an outer
plastic
cover (sheath)
Twisted-pair cable consists of color-coded pairs of
insulated
copper wires
There are two types of twisted-pair cables: STP and UTP
Network+ Guide to Networks, 4e 114
Summary (continued)
There are a number of Physical layer specifications for
Ethernet
networks
Fiber-optic cable provides the benefits of very high
throughput,
very high resistance to noise, and excellent security
Fiber cable variations fall into two categories: single-mode
and
multimode
Structured cabling is based on a hierarchical design that
divides
cabling into six subsystems
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Network+ Guide to Networks, 4e 115
Summary (continued)
The best practice for installing cable is to follow the
TIA/EIA
568 specifications and the manufacturers recommendations
Wireless transmission requires an antenna connected to a
transceiver
Infrared transmission can be used for short-distance
transmissions