Download Fiber-Optic Cable

Transmission Medium
• Media for serial Data transmission
Selection Criteria
• When selecting which medium is suitable,
several factors should be kept in mind:
- costs and installation effort,
- transmission safety - susceptibility to tapping,
interference susceptibility, error probability, etc.
- maximum data rate,
- distances and topological position of the
participants, etc.
Good Signal Quality and
Low Interference Susceptibility
• No medium has all the optimum properties
so that the signals are more or less
attenuated with increasing distance. To
achieve high data rates, the transmission
medium must fulfill specific requirements.
• Another negative effect is the risk of data
being corrupted by interference signals.
Common Types of Physical Cable
Straight Cable
• This is the simplest type of cable. It
consists of copper wires surrounded by an
insulator. The wire comes in bundles or as
flat “ribbon” cables and is used to connect
various peripheral devices over short
distances. Cables for internal disk drives
are typically flat cables with multiple
transmission wires running in parallel.
Properties of Wired Transmission
Media
• Bbbb
Twisted-pair Cable
• This cable consists of copper-core wires surrounded by an insulator.
Two wires are twisted together to form a pair, and the pair forms a
balanced circuit (voltages in each pair have the same amplitude but
are opposite in phase).
• The twisting protects against EMI (electromagnetic interference) and
RFI (radio frequency interference). A typical cable has multiple
twisted pairs, each color-coded to differentiate it from other pairs.
• UTP (unshielded twisted-pair) has been used in the telephone
network and is commonly used for data networking in the United
States.
• STP (shielded twisted-pair) cable has a foil shield around the wire
pairs in a cable to provide superior immunity to RFI. Traditional
twisted-pair LANs use two pairs, one for transmit and one for
receive, but newer Gigabit Ethernet networks use four pairs to
transmit and receive simultaneously. UTP and STP are constructed
of 100-ohm, 24-AWG solid conductors.
Coaxial Cable
• This cable consists of a solid copper core surrounded by
an insulator, a combination shield and ground wire, and
an outer protective jacket.
• In the early days of LANs, coaxial cable was used for its
high bit rates. An Ethernet Thinnet (10Base-2) network
has a data rate of 10Mbits/sec and implements a bus
topology in which each station is attached to a single
strand of cable.
• Today, hierarchical wiring schemes are considered more
practical, and even though more twisted pair wire is
required to cable such a network, cost has dropped,
making such networks very practical.
Fiber-optic Cable
• This cable consists of a center glass core
through which light waves propagate. This core
is surrounded by a glass cladding that basically
reflects the inner light of the core back into the
core. A thick plastic outer jacket surrounds this
assembly, along with special fibers to add
strength.
• Fiber-optic cable is available with a metal core
for strength if the cable will be hung over
distances.
Electric Lines
• A great advantage of electric lines is their simple
and cost-effective preparation (cutting to length
and termination). However, there are some
disadvantages which include the attenuation of
signals and interference susceptibility.
• These drawbacks are not only influenced by the
type of cable used - twisted-pair, coaxial, etc. but also by the interface specification
Transmission Behavior
of Electric Lines
• To be able to determine the electric
properties of a cable, the line is described
as a sequence of sub-networks consisting
of resistors, capacitors, and inductors (See
Figure on the next slide). While the
resistors change the static signal level,
capacitors and inductors create low
passes which have a negative effect on
the edge steepness.
Equivalent Circuit Diagram of a
Transmission Cable
• Equivalent Circuit
Transmission Cable Characteristics
• Bbb
Attenuation and Signal Distortion
•
The cable must therefore be selected to meet the following criteria:
- The line resistance must be low enough so that a sufficiently high signal
amplitude can be guaranteed on the receiver side.
- The cable capacitances and inductances must not distort the signal edges
to an extent that the original information is lost.
Both criteria are influenced by the electric line parameters and the influence
increases with the length of the line as well as with the number of
participants connected. As a result, each cable type is limited in its line
length and maximum number of participants.
The higher the signal frequency, the stronger the effect the capacitances and
inductances have on the signal. An increasing transmission frequency has
therefore a limiting effect on the maximum line length.
To limit the signal distortion occurring in long-distance lines and at high data
rates, such applications frequently use low-inductance and low-capacitance
cables, e.g. Ethernet with coaxial cable.
Interference Caused by
Line Reflection
• Signals transmitted over electric lines are subject to yet
another phenomenon, which is important to be aware of when
installing a line. The electric properties of a line can be
influenced by
- changing the cable type,
- branching the cable,
- connecting devices or
- a line that is not terminated at the beginning or at the end.
• This causes so-called line reflections. The term means that
transient reactions take place on the line, that are caused by
the finite signal propagation speed. Since transient reactions
distort the signal levels, a signal can only be read accurately,
when
- the transient reactions have largely died out or
- the effects of the transient reactions are small.
Terminating Resistors
• To enable the use of long lines even for high data rates,
the formation of line reflections must be prevented. This
is achieved when the electric properties remain constant
across the entire line. The line properties must be
imitated as precisely as possible at the beginning and at
the end of the line by connecting a terminating resistor.
• The line properties are described by means of the socalled characteristic wave impedance of the cable.
Typical values for the characteristic wave impedance
and, hence, the terminating resistor are as follows:
- twisted-pair line: 100 to 150 ohms
- coaxial cable (RG 58): 50 ohms
Terminating Resistors for Different
Lines
• a) twisted two-wire line
• b) RS 485 standard
• c) IEC 61158-2
Fiber Optics
• An optical fiber consists of a light-transmitting core fiber
embedded in a glass cladding and an external plastic
cladding. When light hits the boundary layer in a small
angle of incidence, the different densities of the core and
the glass cladding cause total reflection. The light beam
is reflected almost free of any loss and transmitted within
the core fiber only.
• The diameter of an optical fiber is approx. 0.1 mm.
Depending on the version, the diameter of the lighttransmitting core lies between 9 µm and 60 µm. Usually,
several - up to a thousand - of such fibers and a strain
relief are grouped into a cable.
• Multimode and monomode optical fiber
Cross Section of a Fiber-Optic
Cable
• A typical cable
Profiles and refractive indices of
optical fibers
• Bbbb
• Light from a source enters the cylindrical glass or plastic core. Rays
at shallow angles are reflected and propagated along the fiber; other
rays are absorbed by the surrounding material. This form of
propagation is called step-index multimode, referring to the variety
of angles that will reflect.
• When the fiber core radius is reduced, fewer angles will reflect. By
reducing the radius of the core to the order of a wavelength, only a
single angle or mode can pass: the axial ray. This single-mode
propagation provides superior performance for the following reason.
Because there is a single transmission path with single-mode
transmission, the distortion found in multimode cannot occur. Singlemode is typically used for long-distance applications, including
telephone and cable television.
• By varying the index of refraction of the core, a third type of
transmission, known as graded-index multimode, is possible. This
type is intermediate between the other two in characteristics.
Sizes
• A fiber is thinner than a human hair but
stronger than a steel fiber of similar
thickness. The sizes of the fiber have been
standardized nationally and internationally.
For example, when expressed as
62.5/125, the first number is the core
diameter and the second number is the
cladding diameter in microns or μm.
Types of material make up fiberoptic cables
•
•
•
•
Glass
Plastic
Plastic-clad silica (PCS)
These three cable types differ with respect to
attenuation. Attenuation is principally caused by
two physical effects: absorption and scattering.
Absorption removes signal energy in the
interaction between the propagating light
(photons) and molecules in the core. Scattering
redirects light out of the core to the cladding.
Glass Fiber-Optic Cable
• Glass fiber-optic cable has the lowest attenuation. A pure-glass,
fiber-optic cable has a glass core and a glass cladding. This cable
type has, by far, the most widespread use. It has been the most
popular with link installers, and it is the type of cable with which
installers have the most experience. The glass used in a fiber-optic
cable is ultra-pure, ultra-transparent, silicon dioxide, or fused quartz.
During the glass fiber-optic cable fabrication process, impurities are
purposely added to the pure glass to obtain the desired indices of
refraction needed to guide light.
• Germanium, titanium, or phosphorous is added to increase the index
of refraction. Boron or fluorine is added to decrease the index of
refraction. Other impurities might somehow remain in the glass
cable after fabrication. These residual impurities can increase the
attenuation by either scattering or absorbing light.
Plastic Fiber-Optic Cable
• Plastic fiber-optic cable has the highest attenuation among the three
types of cable. Plastic fiber-optic cable has a plastic core and
cladding. This fiber-optic cable is quite thick.
• Typical dimensions are 480/500, 735/750, and 980/1000. The core
generally consists of polymethylmethacrylate (PMMA) coated with a
fluropolymer. Plastic fiber-optic cable was pioneered principally for
use in the automotive industry. The higher attenuation relative to
glass might not be a serious obstacle with the short cable runs often
required in premise data networks. The cost advantage of plastic
fiber-optic cable is of interest to network architects when they are
faced with budget decisions.
• Plastic fiber-optic cable does have a problem with flammability.
Because of this, it might not be appropriate for certain environments
and care has to be taken when it is run through a plenum.
Otherwise, plastic fiber is considered extremely rugged with a tight
bend radius and the capability to withstand abuse.
Plastic-Clad Silica (PCS) FiberOptic Cable
• The attenuation of PCS fiber-optic cable falls between
that of glass and plastic. PCS fiber-optic cable has a
glass core, which is often vitreous silica, and the
cladding is plastic, usually a silicone elastomer with a
lower refractive index.
• PCS fabricated with a silicone elastomer cladding suffers
from three major defects. First, it has considerable
plasticity, which makes connector application difficult.
Second, adhesive bonding is not possible. And third, it is
practically insoluble in organic solvents. These three
factors keep this type of fiber-optic cable from being
particularly popular with link installers. However, some
improvements have been made in recent years.
• The light signals are usually supplied to the fiber via a laser
LED and analyzed by photo-sensitive semiconductors on
the receiver side. Since signals transmitted in optical fibers
are resistant to electromagnetic interferences and only
slightly attenuated, this medium can be used to cover
extremely long distances and achieve high data rates. The
advantages of optical data transmission are summarized in
the following:
- suitable for extremely high data rates and very long
distances,
- resistant to electromagnetic interference,
- no electromagnetic radiation,
- suitable for hazardous environments and
- electrical isolation between the transmitter and receiver
stations
Monomode fibers
• Monomode fibers help achieve the best
pulse repeat accuracy. The core diameter
of these fibers is so small that only the
paraxial light beam (mode 0) can be
formed. The small diameter, however,
requires particularly high precision when
the light beam is supplied to the fiber.
Multimode Fibers
• If multimode fibers with a larger diameter are used, the
number of possible propagation paths increases and,
hence, the distortion of the pulses. However, this effect
can be reduced by using specially manufactured fibers.
These special fibers do not have a step index profile, i.e.
a constant refractive index, but a so-called grade index
profile. In this case, the refractive index of the core
increases with the radius. The propagation rate which
changes with the refractive index largely compensates
for the different propagation times in the core, thus
enabling higher pulse accuracy.
Cross Section
• mmm
Digital Data Transmission
• High Speed Transmission Over Optical Fiber
Fiber-Optic Communications
System
• Information (voice, data, and video) from the source is encoded into
electrical signals that can drive the transmitter.
Optical fiber communications link
• Simplex optical fiber
communications link
Source
Analog or
digital
interface
Transmitter
Voltage-tocurrent
converter
Light
source
Source-tofiber
interface
Optical fiber cable
Signal
regenerato
r
Optical fiber cable
Fiber-tolight detector
interface
Light
detector
Receiver
Current-tovoltage
converter
Analog-todigital
interface
Destination
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.
• Characteristics:
• 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. Currently, data rates and
bandwidth utilization over fiber-optic cable are limited not by the
medium but by the signal generation and reception technology
available. Modern optical fiber communications systems are capable
of transmitting several gigabits per second over hundreds of miles,
allowing literally millions of individual voice and data channels to be
combined and propagated over one optical fiber cable.
Applications (Cont.)
• Smaller size and lighter weight: Optical fibers are considerably
thinner than coaxial cable or bundled twisted-pair cable – at least an
order of magnitude thinner for comparable information transmission
capacity. 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.
• Lower attenuation: Attenuation is significantly lower for optical fiber
than for coaxial cable or twisted pair and is constant over a wide
range. Fiber-optic transmission distance is significantly greater than
that of other guided media. A signal can run for miles without
requiring regeneration.
• Electromagnetic isolation: Because optical fiber cables are
nonconductors of electrical current, they are not affected by external
electromagnetic fields. Thus the system is not vulnerable to
interference, impulse noise, or crosstalk.
Applications (Cont.)
• 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. Coaxial and twisted-pair systems
generally have repeaters every few kilometers.
Disadvantages
• Cost. Fiber-optic cable is expensive. Because any impurities or
imperfections in the core can throw off the signal, manufacturing
must be painstakingly precise. Also, a laser light source can cost
thousands of dollars, compared to hundreds of dollars for electrical
signal generators.
• Installation/maintenance. Any roughness or cracking in the core of
an optical cable diffuses light and alters the signal. All splices must
be polished and precisely fused. All connections must be perfectly
aligned and matched for core size, and must provide a completely
light-tight seal. Metallic media connections, on the other hand, can
be made by cutting and crimping using relatively unsophisticated
tools.
• Fragility. Glass fiber is more easily broken than wire, making it less
useful for applications where hardware portability is required.
Unguided Media
• Unguided or wireless, media transport
electromagnetic waves without using a
physical conductor. Instead, signals are
broadcast either through air (or, in a few
cases, water), and thus are available to
anyone who has a device capable of
receiving them.
Wireless Data Transmission
• Wireless transmission in communications systems is
well-suited to extremely long distances (radio relay
systems, satellite technology, etc.) and remote controlled
and/or mobile applications.
• When the participants communicate while in sight of
each other and when the distances to be covered are
small and the data rates low, the comparably simple
optical transmission via infrared radiation can be used
successfully.
• Radio-based communication can be used for a lot more
applications. In everyday life, mobile phones are a good
example of the widespread use of radio-based
communication. Radio communications extend not only
to the field of telecommunications.
Types of Propagation
Telecommunication Link
• Wireless communication is usually
combined with wired communication.
Antennas
• An antenna can be defined as an electrical
conductor or system of conductors used either
for radiating electromagnetic energy or for
collecting electromagnetic energy.
• For transmission of a signal, electrical energy
from the transmitter is converted into
electromagnetic energy by the antenna and
radiated into the surrounding environment
(atmosphere, space, water).
• For reception of a signal, electromagnetic
energy impinging on the antenna is converted
into electrical energy and fed into the receiver.
Antennas (Cont.)
• In two-way communication, the same antenna
can be and often is used for both transmission
and reception. This is possible because any
antenna transfers energy from the surrounding
environment to its input receiver terminals with
the same efficiency that it transfers energy from
the output transmitter terminals into the
surrounding environment, assuming that the
same frequency is used in both directions. Put
another way, antenna characteristics are
essentially the same whether an antenna is
sending or receiving electromagnetic energy.
Parabolic Reflective Antenna
• An important type of antenna is the
parabolic reflective antenna, which is used
in terrestrial microwave and satellite
applications. You may recall from your
precollege geometry studies that a
parabola is the locus of all points
equidistant from a fixed line and a fixed
point not on the line.
Microwave Antenna
• The most common type of microwave antenna is the
parabolic “dish.” A typical size is about 3 m in diameter.
The antenna is fixed rigidly and focuses a narrow beam
to achieve line-of-sight transmission to the receiving
antenna.
• Microwave antennas are usually located at substantial
heights above ground level to extend the range between
antennas and to be able to transmit over intervening
obstacles.
• To achieve long-distance transmission, a series of
microwave relay towers is used, and point-to-point
microwave links are strung together over the desired
distance.
Microwave Antenna
• To increase the distance served by terrestrial microwave, a system
of repeaters can be installed with each antenna.
Earth
Line of sight to the horizon
• Microwave is commonly used for both voice and television
transmission. The transmit station must be in visible contact with the
receive station
Atmosphere
50 km
Earth
Transmission Characteristics
•
Microwave transmission covers a substantial portion of the
electromagnetic spectrum. Common frequencies used for transmission
are in the range 1 to 40 GHz.
Band (GHz)
Bandwidth (MHz)
Data Rate (Mbps)
2
7
12
6
30
90
11
40
135
18
220
274
Satellite Microwave
• The communication satellite
Transponders
Satellite
Solar
panel
35 863 km orbit
Antenna
Downlink
Uplink
Earth Stations
Footpoint
Downlink
Geosynchronous Satellites
• To remain stationary, the satellite must have a period of rotation
equal to the earth’s period of rotation. This match occurs at a height
of 35 863 km at the equator
North
Pole
Equator
Satellite
35 863 km
orbit