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TRANSMISSION MEDIA
Data Transmission
Data transmission occurs between transmitter and receiver over some transmission medium.
Transmission media may be classified as guided or unguided. In both cases, communication is in
the form of electromagnetic waves. With guided media, the waves are guided along a physical
path; examples of guided media are twisted pair, coaxial cable, and optical fiber. Unguided
media, also called wireless, provide a means for transmitting electromagnetic waves but do not
guide them; examples are propagation through air, vacuum, and seawater.
The term direct link is used to refer to the transmission path between two devices in which
signals propagate directly from transmitter to receiver with no intermediate devices, other than
amplifiers or repeaters used to increase signal strength. A guided transmission medium is point
to point if it provides a direct link between two devices and those are the only two devices
sharing the medium. In a multipoint guided configuration, more than two devices share the same
medium.
Transmission Mode
A transmission may be simplex, half duplex, or full duplex. In simplex transmission, signals are
transmitted in only one direction; one station is transmitter and the other is receiver. In halfduplex operation, both stations may transmit, but only one at a time. In full-duplex operation,
both stations may transmit simultaneously, and the medium is carrying signals in both directions
at the same time.
Transmission Media
A transmission medium can be broadly defined as anything that can carry information from a
source to a destination. The transmission media can be classified as guided or unguided.
Guided media provide a physical path along which the signals are propagated; these
include twisted pair, coaxial cable, and optical fiber.
Unguided media transport electromagnetic waves without using a physical conductor.
This type of communication is often referred to as wireless communication. Signals are normally
broadcast through free space and thus are available to anyone who has a device capable of
receiving them. Unguided media employ an antenna for transmitting through air, vacuum, or
water.
The characteristics and quality of a data transmission are determined both by the characteristics
of the medium and the characteristics of the signal. In the case of guided media, the medium
itself is more important in determining the limitations of transmission.
For unguided media,
the bandwidth of the signal produced by the transmitting antenna is more important than the
medium in determining transmission characteristics. One key property of signals transmitted by
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antenna is directionality. In general, signals at lower frequencies are omnidirectional; that is, the
signal propagates in all directions from the antenna. At higher frequencies, it is possible to focus
the signal into a directional beam
A number of design factors relating to the transmission medium and the signal determine the
data rate and distance:
1. Bandwidth: All other factors remaining constant, the greater the bandwidth of a signal,
the higher the data rate that can be achieved.
2. Transmission impairments: Impairments, such as attenuation, limit the distance. For
guided media, twisted pair generally suffers more impairment than coaxial cable, which
in turn suffers more than optical fiber.
3. Interference: Interference from competing signals in overlapping frequency bands can
distort or wipe out a signal. Interference is of particular concern for unguided media, but
is also a problem with guided media. For guided media, interference can be caused by
emanations from nearby cables. Interference can also be experienced from unguided
transmissions. Proper shielding of a guided medium can minimize this problem.
4. Number of receivers: A guided medium can be used to construct a point-to-point link or
a shared link with multiple attachments. In the latter case, each attachment introduces
some attenuation and distortion on the line, limiting distance and/or data rate.
Electromagnetic spectrum and indicates the frequencies at which various guided media and unguided
transmission techniques operate.
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Guided Media
Twisted Pair Cable
Twisted pair consists of two conductors (normally copper), each with its own plastic insulation,
twisted together, as shown in Figure
One of the wires is used to carry signals to the receiver, and the other is used only as a ground
reference. The receiver uses the difference between the two. The twisting tends to decrease the
crosstalk interference between adjacent pairs in a cable. Neighboring pairs in a bundle typically
have somewhat different twist lengths to reduce the crosstalk interference. On long-distance
links, the twist length typically varies from 5 to 15 cm. The wires in a pair have thicknesses of
from 0.4 to 0.9 mm. Twisted pair may be used to transmit both analog and digital transmission.
For analog signals, amplifiers are required about every 5 to 6 km. For digital transmission (using
either analog or digital signals), repeaters are required every 2 or 3 km.
Categories
The most common twisted-pair cable used in communications is referred to as unshielded
twisted-pair (UTP). IBM has also produced a version of twisted-pair cable for its use called
shielded twisted-pair (STP). STP cable has a metal foil or braidedmesh covering that encases
each pair of insulated conductors. Although metal casing improves the quality of cable by
preventing the penetration of noise or crosstalk, it is bulkier and more expensive. The Electronic
Industries Association (EIA) has developed standards to classify unshielded twisted-pair cable
into seven categories. Categories are determined by cable quality, with 1 as the lowest and 7 as
the highest.
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EIA-568-A recognizes three categories of UTP cabling:
•Category 3: UTP cables and associated connecting hardware whose transmission
characteristics are specified up to 16 MHz
•Category 4: UTP cables and associated connecting hardware whose transmission
characteristics are specified up to 20 MHz
•Category 5: UTP cables and associated connecting hardware whose transmission
characteristics are specified up to 100 MHz
Of these, it is Category 3 and Category 5 cable that have received the most attention for
LAN applications. Category 3 corresponds to the voice-grade cable found in abundance in most
office buildings. Over limited distances, and with proper design, data rates of up to 16 Mbps
should be achievable with Category 3. Category 5 is a data-grade cable that is becoming standard
for pre installation in new office buildings. Over limited distances, and with proper design, data
rates of up to 100 Mbps are achievable with Category 5.
Compared to other commonly used guided transmission media (coaxial cable, optical
fiber), twisted pair is limited in distance, bandwidth, and data rate. The attenuation for twisted
pair is a very strong function of frequency. Other impairments are also severe for twisted pair.
The medium is quite susceptible to interference and noise because of its easy coupling with
electromagnetic fields. Shielding the wire with metallic braid or sheathing reduces interference.
The twisting of the wire reduces low-frequency interference, and the use of different twist
lengths in adjacent pairs reduces crosstalk.
For point-to-point analog signaling, a bandwidth of up to about 1 MHz is possible. For
very short distances, data rates of up to 10 Gbps have been achieved in commercially available
products.
Performance
One way to measure the performance of twisted-pair cable is to compare attenuation versus
frequency and distance. A twisted-pair cable can pass a wide range of frequencies. However,
Figure shows that with increasing frequency, the attenuation, measured in decibels per kilometer
(dB/km), sharply increases with frequencies above 100 kHz.
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Coaxial Cable
Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted pair
cable. Coaxial cable has a central core conductor of solid or stranded wire (usually copper)
enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil,
braid, or a combination of the two. The outer metallic wrapping serves both as a shield against
noise and as the second conductor, which completes the circuit. This outer conductor is also
enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.
Categories
Coaxial cables are categorized by their radio government (RG) ratings. Each RG number denotes
a unique set of physical specifications, including the wire gauge of the inner conductor, the
thickness and type of the inner insulator, the construction of the shield, and the size and type of
the outer casing.
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The most common type of coaxial cable connector used is the Bayone-Neill-Concelman (BNe),
connector.: the BNC connector, the BNC T connector, and the BNC terminator. The BNC
connector is used to connect the end of the cable to a device, such as a TV set. The BNC T
connector is used in Ethernet networks to branch out to a connection to a computer or other
device. The BNC terminator is used at the end of the cable to prevent the reflection of the signal.
Performance
The attenuation is much higher in coaxial cables than in twisted-pair cable. Although coaxial
cable has a much higher bandwidth, the signal weakens rapidly and requires the frequent use of
repeaters.
Frequency(KHz)
Applications
Coaxial cable can be used over longer distances and support more stations on a shared line than
twisted pair.
Coaxial cable is a versatile transmission medium, used in a wide variety of
applications, including:
•Television distribution - aerial to TV & CATV systems
•Long-distance telephone transmission - traditionally used for inter-exchange links, now being
replaced by optical fiber/microwave/satellite
•Short-run computer system links
•Local area networks
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Fiber-Optic Cable
An optical fiber is a thin (2 to 125 µm), flexible medium capable of guiding an optical
ray. Various glasses and plastics can be used to make optical fibers. An optical fiber cable has a
cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket
The core is the innermost section and consists of one or more very thin strands, or fibers, made
of glass or plastic; the core has a diameter in the range of 8 to 50 µm. Each fiber is surrounded
by its own cladding, a glass or plastic coating that has optical properties different from those of
the core( less dense glass or plastic) and a diameter of 125 µm. The interface between the core
and cladding acts as a reflector to confine light that would otherwise escape the core. The
outermost layer, surrounding one or a bundle of cladded fibers, is the jacket. The jacket is
composed of plastic and other material (inside the jacket are Kevlar strands to strengthen the cable)
layered to protect against moisture, abrasion, crushing, and other environmental dangers.
Optical fiber transmits a signal-encoded beam of light by means of total internal
reflection. Total internal reflection can occur in any transparent medium that has a higher index
of refraction than the surrounding medium. 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 suddenly
enters another substance (of a different density), the ray changes direction.
As the figure shows, if the angle of incidence (the argle (I), the ray makes with the line
perpendicular to the interface between the two substances) is less than the critical angle, the ray
refracts and moves closer to the surface. When the light travels from a denser medium to a less
dense medium the angle of incidence is greater than the angle of refraction. If the angle of
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incidence is equal to the critical angle, the light bends along the interface. If the angle of
incidence is greater than the critical angle, the ray reflects (makes a turn) and travels again in the
denser substance. In this case angle of incidence is equal to the angle reflection. The critical
angle is a property of the substance, and its value differs from one substance to another.
Optical fibers use reflection to guide light through a channel. A glass or plastic core is
surrounded by a cladding of less dense glass or plastic. The difference in density of the two
materials must be such that a beam of light moving through the core is reflected off the cladding
instead of being refracted into it. Information is encoded onto a beam of light as a series of onoff flashes that represent 1 and 0 bits.
Propagation Modes
There are two modes (multimode and single mode) for propagating light along optical channels,
each requiring fiber with different physical characteristics. Multimode can be implemented in
two forms: step-index or graded-index.
Multimode :-Multimode is so named because multiple beams from a light source move through
the core in different paths. How these beams move within the cable depends on the structure of
the core.
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Multimode step-index :- In Multimode step-index fiber, the density of the core remains constant
from the center to the edges. A beam of light moves through this constant density in a straight
line until it reaches the interface of the core and the cladding. At the interface, there is an abrupt
change due to a lower density; this alters the angle of the beam's motion. The term step index
refers to the suddenness of this change.
Figure shows various beams travelling through a step index fiber. Some beams in the middle
travels in the straight line through the core and reach the destination. Some beams strike the
interface of the core and the cladding at an angle less than the critical angle, penetrate the
cladding and lost. The beams that hit at an angle greater than the critical angle, bouncing back
and forth down the channel until they reach the destination. A beam with smaller angle of
incidence will require more bounces to travel the same distance than a beam with larger angle of
incidence. Consequently, the beam with smaller angle of incident must travel farther to reach the
destination. This difference in path length means that different beams arrive at the destination at
different times. As these beams are combined at the receiver the resultant signal may not be the
exact replica of the signal that was transmitted. Such a signal has been distorted by propagation
delays.
The arrival of different modes of the light at different times is called Modal Dispersion.
Modal Dispersion is also called modal distortion, multimode dispersion, intermodal distortion,
intermodal dispersion, and intermodal delay distortion.
Multimode graded-index :- In multimode graded-index fiber, decreases this distortion of the
signal through the cable. Graded-index fiber’s refractive index decreases gradually away from its
center, finally dropping to the same value as the cladding at the edge of the core. The change in
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refractive index causes refraction, instead of total internal reflection, which bends light rays back
toward the fiber axis as they pass through layers with lower refractive index. No total internal
reflection happens because refraction bends light rays back into the fiber axis before they reach
the cladding boundary. Different light modes in a graded-index multimode fiber still follow
different lengths along the fiber, as in step-index multimode fiber. However their speeds differ
because the speed of guided light changes with fiber core’s refractive index. So the farther the
light goes from the center of the fiber, the faster its speed. So the speed difference compensate
for the longer paths followed by the light rays that go farthest from the center of the fiber. The
variation in the distance travelled by each beam travels in a given period of time resulting
different beams intersecting at regular intervals. By placing the receiver at one of these
intersecting points can reconstruct the original signal with far greater precision. This equalizing
of transit times of different modes greatly reduces modal dispersion.
Single-Mode :- Single-mode uses step-index fiber and a highly focused source of light that
limits beams to a small range of angles, all close to the horizontal. The single mode fiber itself is
manufactured with a much smaller diameter than that of multimode fiber, and with substantially
lower density (index of refraction). The decrease in density results in a critical angle that is close
enough to 90° to make the propagation of beams almost horizontal. In this case, propagation of
different beams is almost identical, and delays are negligible. All the beams arrive at the
destination "together" and can be recombined with little distortion to the signal.
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Light Sources
Two different types of light source are used in fiber optic systems: the light-emitting diode
(LED) and the injection laser diode (ILD). Both are semiconductor devices that emit a beam of
light when a voltage is applied. The LED is less costly, operates over a greater temperature
range, and has a longer operational life. The ILD, which operates on the laser principle, is more
efficient and can sustain greater data rates.
There is a relationship among the wavelength employed, the type of transmission, and the
achievable data rate. Both single mode and multimode can support several different wavelengths
of light and can employ laser or LED light sources.
Fiber-Optic Cable Connectors
There are three types of connectors for fiber-optic cables- the subscriber channel (SC) connector
is used for cable TV. It uses a push/pull locking system. The straight-tip (ST) connector is used
for connecting cable to networking devices. It uses a bayonet locking system and is more reliable
than SC. MT-RJ is a connector that is the same size as RJ45.
Fiber Sizes
Optical fiber are defined by the ratio of the diameter of their core to the diameter of their
cladding both expressed in microns. Common sizes are
Advantages
The following characteristics distinguish optical fiber from twisted pair or coaxial cable:
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Greater capacity: The potential bandwidth, and hence data rate, of optical fiber is
immense; data rates of hundreds of Gbps over tens of kilometers have been demonstrated.
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Smaller size and lighter weight: Optical fibers are considerably thinner than coaxial
cable or bundled twisted-pair cable. For cramped conduits in buildings and underground along
public rights-of-way, the advantage of small size is considerable. The corresponding reduction in
weight reduces structural support requirements.
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Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial
cable or twisted pair, and is constant over a wide range.
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Electromagnetic isolation: Optical fiber systems are not affected by external
electromagnetic fields. Thus the system is not vulnerable to interference, impulse noise, or
crosstalk. By the same token, fibers do not radiate energy, so there is little interference with other
equipment and there is a high degree of security from eavesdropping. In addition, fiber is
inherently difficult to tap.
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Greater repeater spacing: Fewer repeaters mean lower cost and fewer sources of error.
The performance of optical fiber systems from this point of view has been steadily improving.
Repeater spacing in the tens of kilometers for optical fiber is common, and repeater spacings of
hundreds of kilometers have been demonstrated.
Performance
Figure shows the attenuation vs wavelength for a typical optical fiber. The unusual shape of the
curve is due to the combination of a variety of factors that contribute to attenuation. The two
most important of these are absorption and scattering, which is the change in direction of light
rays after they strike small particles or impurities in the medium.
Applications
Optical fiber already enjoys considerable use in long-distance telecommunications, and
its use in military applications is growing. The continuing improvements in performance and
decline in prices, together with the inherent advantages of optical fiber, have made it increasingly
attractive for local area networking. Five basic categories of application have become important
for optical fiber: Long-haul trunks, Metropolitan trunks, Rural exchange trunks, Subscriber loops
& Local area networks.