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UNIT
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Transmission Media |
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Unit-05/Lecture-01 |
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Transmission Media A transmission medium is a material substance that can propagate energy waves. For example, the transmission medium for sounds is usually air, but solids and liquids may
also act as transmission media for sound. The absence of a material
medium in vacuum may also constitute a transmission medium
for electromagnetic
waves such as light and radio waves. While material substance is not required
for electromagnetic waves to propagate, such waves are usually affected by the transmission media they pass through, for instance by
absorption or by reflection or refraction at the interfaces between media. The term transmission
medium also refers to a technical device that
employs the material substance to transmit or guide waves. Thus, an optical
fibre or a copper cable is a transmission medium. Not only this but also is
able to guide the transmission of networks. A transmission medium can be classified as a:
Electromagnetic
radiation can be transmitted
through an optical medium, such as optical
fiber, or through twisted
pair wires, coaxial
cable, or dielectric-slab waveguides. It may also pass through any physical material
that is transparent to the specific wavelength, such as water, air, glass, or concrete. Sound is, by definition, the vibration of matter,
so it requires a physical medium for transmission, as do other kinds of
mechanical waves and heat energy. Historically, science incorporated
various aether theories to explain the
transmission medium. However, it is now known that electromagnetic
waves do not require a physical transmission medium, and so can travel
through the "vacuum" of free space. Regions of the insulative vacuum can become conductive for electrical
conduction through the presence of free
electrons, holes, or ions. Transmission and
reception of data is performed in four steps.
Twisted
pair cabling is a type of wiring in which two
conductors of a single circuit are twisted together for the purposes of
cancelling out electromagnetic interference (EMI) from external sources; for instance,
electromagnetic
radiation from unshielded twisted
pair (UTP) cables, and crosstalk between neighbouring pairs. It was
invented by Alexander
Graham Bell. In balanced
pair operation, the two wires
carry equal and opposite signals and the destination detects the difference
between the two. This is known as differential
mode transmission. Noise
sources introduce signals into the wires by coupling of electric or magnetic
fields and tend to couple to both wires equally. The noise thus produces a
common-mode signal which is cancelled at the receiver when the difference
signal is taken. This method starts to
fail when the noise source is close to the signal wires; the closer wire will
couple with the noise more strongly and the common-mode
rejection of the receiver will
fail to eliminate it. This problem is especially apparent in
telecommunication cables where pairs in the same cable lie next to each other
for many miles. One pair can induce crosstalk in another and it is additive along the
length of the cable. Twisting the pairs counters this effect as on each half
twist the wire nearest to the noise-source is exchanged. Providing the interfering
source remains uniform, or nearly so, over the distance of a single twist,
the induced noise will remain common-mode. Differential signaling also
reduces electromagnetic
radiation from the cable, along
with the associated attenuation allowing for greater distance between
exchanges. The twist rate (also
called pitch of the twist,
usually defined in twists per meter) makes up part of the specification for a
given type of cable. Where nearby pairs have equal twist rates, the same
conductors of the different pairs may repeatedly lie next to each other,
partially undoing the benefits of differential mode. For this reason it is
commonly specified that, at least for cables containing small numbers of
pairs, the twist rates must differ.[1] In contrast to ScTP (screened twisted pair), STP (shielded twisted pair), FTP (foiled twisted pair) and other shielded cabling
variations, UTP (unshielded
twisted pair) cable is not surrounded by any shielding. It is the primary
wire type for telephone usage and is very common for computer
networking, especially as patch
cables or temporary network
connections due to the high flexibility of the cables. The earliest telephones
used telegraph lines, or open-wire single-wire
earth return circuits. In the 1880s
electric trams were installed in many cities, which
induced noise into these circuits. Lawsuits being unavailing, the telephone companies
converted to balanced circuits, which had the incidental benefit of
reducing attenuation, hence increasing range. As electrical power
distribution became more commonplace, this measure proved inadequate. Two
wires, strung on either side of cross bars on utility
poles, shared the route with
electrical power lines. Within a few years, the growing use of
electricity again brought an increase of interference, so engineers devised a
method called wire transposition, to cancel out the interference. In wire transposition,
the wires exchange position once every several poles. In this way, the two
wires would receive similar EMI from power lines. This represented an
early implementation of twisting, with a twist rate of about four twists per kilometre, or six per mile. Such open-wire balanced lines with
periodic transpositions still survive today in some rural areas. Twisted pair cables were
invented by Alexander
Graham Bell in 1881. By 1900, the
entire American telephone line network was either twisted pair or open
wire with transposition to guard against interference. Today, most of the
millions of kilometres of twisted pairs in the world are outdoor
landlines, owned by telephone
companies, used for voice service, and only handled or even seen by telephone
workers.
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Unit-05/Lecture-02 |
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Unshielded twisted pair (UTP) (RGPV Dec 2012)
Unshielded
twisted pair For urban outdoor
telephone cables containing hundreds or thousands of pairs, the cable is
divided into smaller but identical bundles. Each bundle consists of twisted
pairs that have different twist rates. The bundles are in turn twisted
together to make up the cable. Pairs having the same twist rate within the
cable can still experience some degree of crosstalk. Wire pairs are selected carefully to
minimize crosstalk within a large cable. Unshielded
twisted pair cable with different twist rates UTP cable is also the most common cable used in computer
networking. Modern Ethernet, the most common data networking standard,
can use UTP cables. Twisted pair cabling is often used in data networks for
short and medium length connections because of its relatively lower costs
compared to optical fiber and coaxial cable. UTP is also finding increasing
use in video applications, primarily in security cameras. Many cameras
include a UTP output with screw terminals; UTP cable bandwidth has improved to match the baseband of television signals. As UTP is a balanced transmission
line, a balun is needed to connect to unbalanced
equipment, for example any using BNC connectors and designed for coaxial cable. Cable Shielding
F/UTP
cable S/FTP
cable U/FTP,
F/UTP and F/FTP are used in Cat.6a cables S/UTP
cable Twisted pair cables are often shielded in an attempt to
prevent electromagnetic interference. Shielding provides an electric conductive barrier to
attenuate electromagnetic waves external to the shield and provides
conduction path by which induced currents can be circulated and returned to
the source, via ground reference connection. This shielding can be applied to individual pairs or
quads, or to the collection of pairs. Individual pairs are foiled, while
overall cable may use braided screen, foil, or braiding with foil. When shielding is applied to the collection of pairs, this
is usually referred to as screening, however different vendors and authors
use different terminology, employing "screening" and
"shielding" interchangeably; for example, STP (shielded twisted
pair) or ScTP (screened twisted pair) has been used to denote U/FTP, S/UTP,
F/UTP, SF/UTP and S/FTP construction). Because the shielding is made of metal, it may also serve
as a ground. Usually a shielded or a screened twisted pair cable has a
special grounding wire added called a drain wire which is electrically
connected to the shield or screen. The drain wire simplifies connection to
ground at the connectors. Common shielded cable types used by Cat. 6a, Cat.7 and
Cat.8 cables include: Shielded twisted pair (U/FTP) Also
pair in metal foil. Individual shielding with foil for each twisted pair or
quad. This type of shielding protects cable from external EMI from entering
or exiting the cable and also protects neighboring pairs from crosstalk. Screened twisted pair (F/UTP, S/UTP and SF/UTP) Also
foiled twisted pair for F/UTP. Overall foil, braided shield or braiding with
foil across all of the pairs within the 100 Ohm twisted pair cable. This type
of shielding protects EMI from entering or exiting the cable. Screened shielded twisted pair (F/FTP and S/FTP) Also
fully shielded twisted pair, shielded screened twisted pair, screened foiled
twisted pair, shielded foiled twisted pair. Individual shielding using foil
between the twisted pair sets, and also an outer metal and/or foil shielding
within the 100 Ohm twisted pair cable. This type of shielding protects EMI
from entering or exiting the cable and also protects neighboring pairs from
crosstalk. The
code before the slash designates the shielding for the cable itself, while
the code after the slash determines the shielding for the individual pairs: TP = twisted pair TQ = twisted pair,
individual shielding in quads U = unshielded F = foil shielding S = braided shielding
(outer layer only)
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Unit-05/Lecture-03 |
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Most
common twisted-pair cables Solid Core Cable Vs Stranded Cable
A solid core cable uses one solid wire per conductor and
in a four pair cable there would be a total of eight solid wires. Stranded
conductor uses multiple wires wrapped around each other in each conductor and
in a four pair with seven strands per conductor cable, there would be a total
of 56 wires (2 per pair x 4 pairs x 7 strands). Solid core cable is supposed to be used for permanently
installed runs. It is less flexible than stranded cable and is more prone to
failure if repeatedly flexed. Stranded cable is used for fly leads at patch
panel and for connections from wall-ports to end devices, as it resists
cracking of the conductors. Connectors need to be designed differently for solid core
than for stranded. Use of a connector with the wrong cable type is likely to
lead to unreliable cabling. Plugs designed for solid and stranded core are
readily available, and some vendors even offer plugs designed for use with
both types. The punch-down blocks on patch-panel and wall port jacks are
designed for use with solid core cable. Advantages
Disadvantages
Minor twisted
pair variants
Loaded Twisted Pair A twisted pair that has intentionally added
inductance, formerly common practice on
telecommunication lines. The added inductors are known as load
coils and reduce attenuation
for voiceband frequencies but increase it on higher
frequencies. Load coils cause distortion in voiceband on very long lines.[10] In this context a line without load coils
is referred to as an unloaded line. Bonded Twisted Pair A twisted pair variant in which the pairs
are individually bonded to increase robustness of the cable. Pioneered by Belden, it means the electrical specifications of
the cable are maintained despite rough handling. Twisted Ribbon Cable A variant of standard ribbon
cable in which adjacent pairs
of conductors are bonded and twisted together. The twisted pairs are then
lightly bonded to each other in a ribbon format. Periodically along the
ribbon there are short sections with no twisting to enable connectors and PCB headers to be terminated using the usual ribbon cable IDC techniques. |
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Unit-05/Lecture-04 |
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Coaxial Cable Coaxial
cable, or coax is a type of cable that has an inner conductor
surrounded by a tubular insulating layer,
surrounded by a tubular conducting shield. Many coaxial cables also have an
insulating outer sheath or jacket. The term coaxial comes from the inner conductor and the
outer shield sharing a geometric axis. Coaxial
cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880. Coaxial
cable differs from other shielded cable used for carrying lower-frequency signals,
such as audio signals, in that the dimensions of the cable are
controlled to give a precise, constant conductor spacing, which is needed for
it to function efficiently as a radio frequency transmission
line. Coaxial cable conducts electrical signal using
an inner conductor surrounded by an insulating layer and all enclosed by a
shield, typically one to four layers of woven metallic braid and metallic
tape. The cable is protected by an outer insulating jacket. Normally, the
shield is kept at ground potential and a voltage is applied to the center
conductor to carry electrical signals. The advantage of coaxial design is
that electric and magnetic fields are confined to the dielectric with little leakage outside the shield. Conversely, electric
and magnetic fields outside the cable are largely kept from causing
interference to signals inside the cable. Larger diameter cables and cables
with multiple shields have less leakage. This property makes coaxial cable a
good choice for carrying weak signals that cannot tolerate interference from
the environment or for higher electrical signals that must not be allowed to
radiate or couple into adjacent structures or circuits. Common applications of
coaxial cable include video and CATV distribution, RF and microwave
transmission, and computer and instrumentation data connections. The characteristic
impedance of the cable is
determined by the dielectric
constant of the inner insulator
and the radii of the inner and outer conductors. A controlled cable
characteristic impedance is important because the source and load impedance
should be matched to ensure maximum power transfer and minimum standing
wave ratio. Other important
properties of coaxial cable include attenuation as a function of frequency,
voltage handling capability, and shield quality. Construction
Coaxial cable design choices affect physical size,
frequency performance, attenuation, power handling capabilities, flexibility,
strength, and cost. The inner conductor might be solid or stranded; stranded
is more flexible. To get better high-frequency performance, the inner
conductor may be silver-plated. Copper-plated steel wire is often used as an
inner conductor for cable used in the cable TV industry. The insulator surrounding
the inner conductor may be solid plastic, a foam plastic, or air with spacers
supporting the inner wire. The properties of dielectric control some electrical
properties of the cable. A common choice is a solid polyethylene (PE) insulator, used in lower-loss cables.
Solid Teflon (PTFE) is also used as an insulator. Some
coaxial lines use air and have spacers to keep the inner conductor from
touching the shield. Many conventional coaxial
cables use braided copper wire forming the shield. This allows the cable to
be flexible, but it also means there are gaps in the shield layer, and the
inner dimension of the shield varies slightly because the braid cannot be
flat. Sometimes the braid is silver-plated. For better shield performance,
some cables have a double-layer shield. The shield might be just two braids,
but it is more common now to have a thin foil shield covered by a wire braid.
Some cables may invest in more than two shield layers, such as
"quad-shield", which uses four alternating layers of foil and
braid. Other shield designs sacrifice flexibility for better performance;
some shields are a solid metal tube. Those cables cannot be bent sharply, as
the shield will kink, causing losses in the cable. For high-power
radio-frequency transmission up to about 1 GHz, coaxial cable with a solid
copper outer conductor is available in sizes of 0.25 inch upward. The outer
conductor is rippled like a bellows to permit flexibility and the inner
conductor is held in position by a plastic spiral to approximate an air
dielectric. Coaxial cables require an
internal structure of an insulating (dielectric) material to maintain the
spacing between the center conductor and shield. The dielectric losses increase in this order: Ideal
dielectric, vacuum, air, polytetrafluoroethylene (PTFE), polyethylene foam, and solid polyethylene.
A low relative permittivity allows for higher-frequency usage. An
inhomogeneous dielectric needs to be compensated by a non-circular conductor
to avoid current hot-spots. While many cables have a
solid dielectric, many others have a foam dielectric that contains as much
air or other gas as possible to reduce the losses by allowing the use of a
larger diameter center conductor. Foam coax will have about 15% less
attenuation but some types of foam dielectric can absorb moisture—especially
at its many surfaces — in humid environments, significantly increasing the
loss. Supports shaped like stars or spokes are even better but more expensive
and very susceptible to moisture infiltration. Still more expensive were the
air-spaced coaxials used for some inter-city communications in the mid-20th
century. The center conductor was suspended by polyethylene discs every few
centimeters. In some low-loss coaxial cables such as the RG-62 type, the
inner conductor is supported by a spiral strand of polyethylene, so that an
air space exists between most of the conductor and the inside of the jacket.
The lower dielectric constant of air allows for a greater inner diameter
at the same impedance and a greater outer diameter at the same cutoff
frequency, lowering ohmic losses. Inner conductors are sometimes
silver-plated to smooth the surface and reduce losses due to skin
effect. A rough surface
prolongs the path for the current and concentrates the current at peaks and,
thus, increases ohmic losses. The insulating jacket can
be made from many materials. A common choice is PVC, but some applications may require
fire-resistant materials. Outdoor applications may require the jacket resist ultraviolet
light, oxidation and rodent damage. Flooded coaxial cables
use a water blocking gel to protect the cable from water infiltration through
minor cuts in the jacket. For internal chassis connections the insulating
jacket may be omitted. Applications
Coaxial cable is used as a transmission line for radio
frequency signals. Its
applications include feedlines connecting radio transmitters and receivers with their antennas, computer network (Internet) connections, and distributing cable
television signals. One advantage
of coaxial over other types of radio transmission line is that in an ideal coaxial cable the electromagnetic field carrying the signal exists only in the
space between the inner and outer conductors. This allows coaxial
cable runs to be installed next to metal objects such as gutters
without the power losses that occur in other types of transmission lines.
Coaxial cable also provides protection of the signal from external electromagnetic
interference |
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S.NO |
RGPV
QUESTIONS |
Year |
Marks |
|
Q.1 |
Discuss
the parameters of ultimate analysis of coal. |
DEC2013 Dec 2011 |
4 4 |
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Unit-05/Lecture-05 |
|
Uses
of coaxial cable Short coaxial cables are
commonly used to connect home video equipment, in ham
radio setups, and in measurement
electronics. They used to be common
for implementing computer networks, in particular Ethernet, but twisted pair cables have replaced them in most
applications except in the growing consumer cable modem market for broadband
Internet access. Micro coaxial cables are
used in a range of consumer devices, military equipment, and also in
ultra-sound scanning equipment. The most common
impedances that are widely used are 50 or 52 ohms, and 75 ohms, although
other impedances are available for specific applications. The 50 / 52 ohm cables
are widely used for industrial and commercial two-way
radio frequency applications
(including radio, and telecommunications), although 75 ohms is commonly used
for broadcast television and radio. Coax cable is often used
to carry data or signals from an antenna to a receiver from a satellite
dish to a satellite receiver,
from a television
antenna to a television
receiver, from a radio
mast to a radio
receiver, etc. In many cases, the
same single coax cable carries power in the opposite direction, to the
antenna, to power the low-noise
amplifier. In some cases a single
coax cable carries (unidirectional) power and bidirectional data or signals. Fiber
Optical Cable (RGPV Dec2013) An optical fiber cable is a cable containing one or more optical
fibers that are used to carry
light. The optical fiber elements are typically individually coated with
plastic layers and contained in a protective tube suitable for the
environment where the cable will be deployed. Different types of cable are
used for different applications, for example long distance telecommunication, or providing a high-speed data connection
between different parts of a building. Optical fiber consists of
a core and a cladding layer, selected for total
internal reflection due to the difference in
the refractive index between the two. In practical fibers, the
cladding is usually coated with a layer of acrylate
polymer or polyimide. This coating protects the fiber from
damage but does not contribute to its optical
waveguide properties. Individual
coated fibers (or fibers formed into ribbons or bundles) then have a tough resin buffer layer and/or core tube(s) extruded around
them to form the cable core. Several layers of protective sheathing,
depending on the application, are added to form the cable. Rigid fiber
assemblies sometimes put light-absorbing glass between the fibers, to prevent
light that leaks out of one fiber from entering another. This reduces cross-talk between the fibers, or reduces flare in fiber bundle imaging applications.
For indoor
applications, the jacketed fiber is generally enclosed, with a bundle
of flexible fibrous polymer strength
members like aramid , in a lightweight plastic cover to form a
simple cable. Each end of the cable may be terminated with a specialized optical
fiber connector to allow it to be easily
connected and disconnected from transmitting and receiving equipment. Fibre-optic
cable in a Telstra pit An
optical fiber breakout cable For use in more strenuous environments, a much more robust cable
construction is required. In loose-tube
construction the fiber is laid helically into semi-rigid tubes, allowing the cable
to stretch without stretching the fiber itself. This protects the fiber from
tension during laying and due to temperature changes. Loose-tube fiber may be
"dry block" or gel-filled. Dry block
offers less protection to the fibers than gel-filled, but costs considerably
less. Instead of a loose tube, the fiber may be
embedded in a heavy polymer jacket, commonly
called "tight buffer" construction. Tight buffer
cables are offered for a variety of applications, but the two most
common are "Breakout" and "Distribution". Breakout cables normally contain a
ripcord, two non-conductive dielectric strengthening members an aramid yarn,
and 3 mm buffer tubing with an additional layer of Kevlar
surrounding each fiber. The ripcord is a parallel cord of strong yarn that is
situated under the jacket of the cable for jacket removal. Distribution
cables have an overall Kevlar wrapping, a ripcord,
and a 900 micrometer buffer coating surrounding each fiber. These fiber units are commonly bundled
with additional steel strength members, again with a helical twist to allow
for stretching. A critical concern in
outdoor cabling is to protect the fiber from contamination by water. This is
accomplished by use of solid barriers such as copper tubes, and
water-repellent jelly or water-absorbing powder surrounding the fiber. Finally, the cable may be
armored to protect it from environmental hazards, such as construction work
or gnawing animals. Undersea cables are more heavily armored in their
near-shore portions to protect them from boat anchors, fishing gear, and even
sharks, which may be attracted to the electrical power that is carried to power amplifiers or
repeaters in the cable. Modern cables come in a
wide variety of sheathings and armor, designed for applications such as
direct burial in trenches, dual use as power lines, installation in conduit,
lashing to aerial telephone poles, submarine installation, and insertion in paved streets. Color
Coding |
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S.NO |
RGPV
QUESTIONS |
Year |
Marks |
|
Q.1 |
How
signal travel in optical fiber cable |
Dec
2013 |
7 |
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UNIT 5/LECTURE 6 |
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Radio wave
transmission In the radio signal. The electrons in our
wire are moving, but not in one direction. These electrons are moving back
and forth. Actually, the wave displayed in the activity is a representation
of the back and forth movement of electrons. If the wave has a frequency of
200,000 Hz (cycles per second), the electrons in the wire are moving back and
forth 200,000 times a second. When electrons move in a wire, an
electromagnetic field is created around that wire. There's no magic behind
this; it's just the way things work. Just as the electrons move in the wire,
they move in the transmitter's antenna. And just as an electromagnetic field
is created around the wire, a field is created around the antenna. But there
is a difference between the wire and the antenna. The wire is shielded
(surrounded by another wire) to keep the electromagnetic field in. The
antenna, on the other hand, is designed to radiate the electromagnetic field.
The electromagnetic field travels from the antenna in all directions and at
the speed of light. It travels until it hits your radio's antenna as well as
hundreds of other receiving antennas. And what happens at the receiving
antenna? Just as a current in a wire produces an electromagnetic field, an
electromagnetic field produces current in a wire (or antenna). This current
is then amplified and processed by the radio. For the propagation and
interception of radio waves, a transmitter and receiver are employed. A radio
wave acts as a carrier of information-bearing signals; the information may be
encoded directly on the wave by periodically interrupting its transmission
(as in dot-and-dash telegraphy) or impressed on it by a process called modulation. The actual information in a modulated
signal is contained in its sidebands, or frequencies added to the carrier wave,
rather than in the carrier wave itself. The two most common types of
modulation used in radio are amplitude modulation (AM) and frequency
modulation (FM). Frequency modulation minimizes noise and provides greater fidelity than
amplitude modulation, which is the older method of broadcasting. Both AM and FM are analog transmission
systems, that is, they process sounds into continuously varying patterns of
electrical signals which resemble sound waves. Digital
radio uses a transmission system in which the signals propagate as
discrete voltage pulses, that is, as patterns of numbers; before
transmission, an analog audio signal is converted into a digital signal,
which may be transmitted in the AM or FM frequency range.
A digital radio broadcast offers compact-disc-quality reception and
reproduction on the FM band and FM-quality
reception and reproduction on the AM band. In its most common form,
radio is used for the transmission of sounds (voice and music) and pictures
(television). The sounds and images are converted
into electrical signals by a microphone (sounds) or video camera (images),
amplified, and used to modulate a carrier wave that has been generated by an oscillator circuit in a transmitter. The modulated
carrier is also amplified, then applied to an antenna that converts the electrical signals to
electromagnetic waves for radiation into space. Such waves radiate at the
speed of light and are transmitted not only by line of sight but also by
deflection from the ionosphere. Receiving antennas
intercept part of this radiation, change it back to the form of electrical
signals, and feed it to a receiver. The most efficient and most common
circuit for radio-frequency selection and amplification used in radio
receivers is the superheterodyne. In that system, incoming signals are mixed
with a signal from a local oscillator to produce intermediate
frequencies (IF) that are equal to the arithmetical sum and difference
of the incoming and local frequencies. One of those frequencies is applied to
an amplifier. Because the IF amplifier operates at
a single frequency, namely the intermediate frequency, it can be
built for optimum selectivity and gain. The tuning
control on a radio receiver adjusts the local oscillator frequency. If the
incoming signals are above the threshold of sensitivity of the receiver and
if the receiver is tuned to the frequency of the signal, it will amplify the
signal and feed it to circuits that demodulate it, i.e., separate the signal
wave itself from the carrier wave. There are certain differences between AM and FM receivers.
In an AM transmission the carrier wave is constant in frequency and varies in
amplitude (strength) according to the sounds present at the microphone; in FM
the carrier is constant in amplitude and varies in frequency. Because the
noise that affects radio signals is partly, but not completely, manifested in
amplitude variations, wideband FM receivers are inherently less sensitive to
noise. In an FM receiver, the limiter and discriminator stages are circuits
that respond solely to changes in frequency. The other stages of the FM receiver are similar to those of the AM receiver
but require more care in design and assembly to make full
use of FM's advantages. FM is also used in television sound systems.
In both radio and television receivers, once the basic signals have been
separated from the carrier wave they are fed to a loudspeaker or a display
device (usually a cathode-ray tube), where they are converted into sound and
visual images, respectively. Microwave
transmission (RGPV DEC 2013) Microwave
transmission refers to the technology
of transmitting
information or energy by the use of radio
waves whose wavelengths are conveniently measured in small numbers
of centimetre; these are called microwaves. This part of the radio
spectrum ranges across frequencies of roughly 1.0 gigahertz (GHz) to 30 GHz. These correspond to wavelengths from 30
centimeters down to 1.0 cm. Microwaves are widely
used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can
be pointed directly at the receiving antenna. This allows nearby microwave
equipment to use the same frequencies without interfering with each other, as
lower frequency radio waves do. Another advantage is that the high frequency
of microwaves gives the microwave band a very large information-carrying
capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio
spectrum below it. A disadvantage
is that microwaves are limited to line
of sight propagation; they cannot
pass around hills or mountains as lower frequency radio waves can. Microwave radio transmission is commonly used in point-to-point communication
systems on the surface of the
Earth, in satellite
communications, and in deep
space radio communications. Other parts of the
microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy. The next higher part of
the radio electromagnetic
spectrum, where the frequencies are above 30 GHz and below 100 GHz, are
called "millimeter waves" because their wavelengths are
conveniently measured in millimeters, and their wavelengths range from 10 mm
down to 3.0 mm. Radio waves in this band are
usually strongly attenuated by the Earthly
atmosphere and particles contained
in it, especially during wet weather. Also, in wide band of frequencies
around 60 GHz, the radio waves are strongly attenuated by molecular
oxygen in the atmosphere. The
electronic technologies needed in the millimeter wave band are also much more
difficult to utilize than those of the microwave band. Wireless transmission of information
A parabolic satellite
antenna for Erdfunkstelle Raisting,
C
band horn-reflector antennas on the roof of a telephone switching
center
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UNIT 5/LECTURE 7 |
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Infrared Transmission Infrared transmission refers to energy in the
region of the electromagnetic radiation spectrum at wavelengths longer than those of visible light, but
shorter than those of radio waves. Correspondingly, infrared frequencies are
higher than those of microwaves, but lower than those of visible light. Scientists divide the
infrared radiation (IR) spectrum into three regions. The wavelengths are
specified in microns (symbolized µ, where 1 µ = 10-6 meter) or in
nanometers (abbreviated nm, where 1 nm = 10-9 meter = 0.001 5).
The near IR band
contains energy in the range of wavelengths closest to the visible, from
approximately 0.750 to 1.300 5 (750 to 1300 nm). The intermediate IR band
(also called the middle IR
band) consists of energy in the range 1.300 to 3.000 5 (1300 to
3000 nm). The far IR band extends
from 2.000 to 14.000 5 (3000 nm to 1.4000 x 104 nm). Infrared is used in a
variety of wirelesscommunications, monitoring, and control
applications. Here are some examples: ·
Home-entertainment
remote-control boxes ·
Wireless (local area
networks) ·
Links between notebook computers
and desktop computers ·
Cordless modem ·
Intrusion detectors ·
Motion detectors ·
Fire sensors ·
Night-vision systems ·
Medical diagnostic
equipment ·
Missile guidance systems ·
Geological monitoring
devices Transmitting IR data from one device to
another is sometimes referred to as beaming. RJ 45 (RGPV Dec2013) A registered jack (RJ)
is a standardize ]physical network
interface—both jack construction
and wiring pattern—for connecting telecommunications or data equipment to a
service provided by a local
exchange carrier or long
distance carrier. The standard designs for
these connectors and their wiring are named RJ11, RJ14, RJ21,RJ35, RJ45, RJ48, etc. Many of these interface
standards are commonly used in North America,
though some interfaces are used world-wide. It is common to find a dash
(hyphen) between the RJ and the number, but the
actual standard has no dash or hyphen. The physical connectors
that registered jacks use are mainly of the modular
connector and 50-pin miniature
ribbon connector types. For
example, RJ11 uses a six-position two-conductor (6P2C), RJ14 uses a six-position four-conductor
(6P4C) modular plug and jack, while RJ21 uses a
25-pair (50-pin) miniature ribbon connector. The RJ45 physical
connector is standardised as the IEC 60603-7 8P8C
modular connector with different
"categories" of performance, with all eight conductors present. A
similar standard jack once used for modem/data connections, the RJ45S, used a
"keyed" variety of the 8P8C body with an extra tab that prevents it
mating with other connectors; the visual difference compared to the more
common 8P8C is subtle, but it is a different connector. The original RJ45S [6][7] keyed 8P2C modular connector had pins 5
and 4 wired for tip and ring of a single telephone line and pins 7 and 8
shorting a programming resistor, but is obsolete today. Electronics catalogs
commonly advertise 8P8C modular connectors as "RJ45". An installer
can wire the jack to any pin-out or use it as part of a generic structured
cabling system such as ISO/IEC
15018 or ISO/IEC 11801 using 8P8C patch panels for both phone and
data. Virtually all electronic equipment which uses an 8P8C connector (or
possibly any 8P connector at all) will document it as an "RJ45"
connector. Network interface card (RGPV Dec2013) A network interface card (NIC) is a circuit
board or cardthat is installed in a computer so that it
can be connected to a network. A network interface card provides the computer
with a dedicated, full-time connection to a network. Personal computers and workstations on a local area network (LAN) typically contain a network interface
card specifically designed for the LAN transmissiontechnology. Straight
Connection Cable Coding Standards The information listed here is to assist Network
Administrators in the color coding of Ethernet cables. Please be aware that
modifying Ethernet cables improperly may cause loss of network connectivity.
Use this information at your own risk, and insure all connectors and cables
are modified in accordance with standards. The Internet Centre and its
affiliates cannot be held liable for the use of this information in whole or
in part.
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UNIT 5/LECTURE 8 |
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T-568A Straight-Through Ethernet Cable The TIA/EIA 568-A standard which was ratified in 1995, was
replaced by the TIA/EIA 568-B standard in 2002 and has been updated since.
Both standards define the T-568A and T-568B pin-outs for using Unshielded
Twisted Pair cable and RJ-45 connectors for Ethernet connectivity. The
standards and pin-out specification appear to be related and interchangeable,
but are not the same and should not be used interchangeably. T-568B Straight-Through Ethernet Cable Both the T-568A and the T-568B standard Straight-Through
cables are used most often as patch cords for your Ethernet connections. If
you require a cable to connect two Ethernet devices directly together without
a hub or when you connect two hubs together, you will need to use a Crossover
cable instead. RJ-45 Crossover Ethernet Cable A good way of remembering how to wire a Crossover Ethernet
cable is to wire one end using the T-568A standard and the other end using
the T-568B standard. Another way of remembering the color coding is to simply
switch the Green set of wires in place with the Orange set of wires.
Specifically, switch the solid Green (G) with the solid Orange, and switch
the green/white with the orange/white. Ethernet Cable Instructions: ·
Pull the cable off the reel to the desired length and cut. If
you are pulling cables through holes, its easier to attach the RJ-45 plugs
after the cable is pulled. The total length of wire segments between a PC and
a hub or between two PC's cannot exceed 100 Meters (328 feet) for 100BASE-TX
and 300 Meters for 10BASE-T. ·
Start on one end and strip the cable jacket off (about
1") using a stripper or a knife. Be extra careful not to nick the wires,
otherwise you will need to start over. ·
Spread, untwist the pairs, and arrange the wires in the order
of the desired cable end. Flatten the end between your thumb and forefinger.
Trim the ends of the wires so they are even with one another, leaving only
1/2" in wire length. If it is longer than 1/2" it will be
out-of-spec and susceptible to crosstalk. Flatten and insure there are no
spaces between wires. ·
Hold the RJ-45 plug with the clip facing down or away from
you. Push the wires firmly into the plug. Inspect each wire is flat even at
the front of the plug. Check the order of the wires. Double check again.
Check that the jacket is fitted right against the stop of the plug. Carefully
hold the wire and firmly crimp the RJ-45 with the crimper. ·
Check the color orientation, check that the crimped connection
is not about to come apart, and check to see if the wires are flat against
the front of the plug. If even one of these are incorrect, you will have to
start over. Test the Ethernet cable. |
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UNIT 5/LECTURE 9 |
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Ethernet Cable Tips: ·
A straight-thru cable has identical
ends. ·
A crossover cable has different
ends. ·
A straight-thru is used as a patch
cord in Ethernet connections. ·
A crossover is used to connect two
Ethernet devices without a hub or for connecting two hubs. ·
A crossover has one end with the
Orange set of wires switched with the Green set. ·
Odd numbered pins are always
striped, even numbered pins are always solid colored. ·
Looking at the RJ-45 with the clip
facing away from you, Brown is always on the right, and pin 1 is on the left. ·
No more than 1/2" of the
Ethernet cable should be untwisted otherwise it will be susceptible to
crosstalk. ·
Do not deform, do not bend, do not
stretch, do not staple, do not run parallel with power cables, and do not run
Ethernet cables near noise inducing components. Basic Theory: By looking at a T-568A UTP Ethernet straight-thru cable and an
Ethernet crossover cable with a T-568B end, we see that the TX (transmitter)
pins are connected to the corresponding RX (receiver) pins, plus to plus and
minus to minus. You can also see that both the blue and brown wire pairs on
pins 4, 5, 7, and 8 are not used in either standard. What you may not realize
is that, these same pins 4, 5, 7, and 8 are not used or required in
100BASE-TX as well. So why bother using these wires, well for one thing its
simply easier to make a connection with all the wires grouped together.
Otherwise you'll be spending time trying to fit those tiny little wires into
each of the corresponding holes in the RJ-45 connector. |
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