Transmission Medium - PowerPoint PPT Presentation


Title: Transmission Medium


1
Transmission Medium
  • Media for serial Data transmission

2
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.

3
Good Signal Quality andLow 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.

4
Common Types of Physical Cable
5
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.

6
Properties of Wired Transmission Media
  • Bbbb

7
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.

8
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.

9
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.

10
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

11
Transmission Behaviorof 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.

12
Equivalent Circuit Diagram of a Transmission Cable
  • Equivalent Circuit

13
Transmission Cable Characteristics
  • Bbb

14
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.

15
Interference Caused byLine 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.

16
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
    so-called 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

17
Terminating Resistors for Different Lines
  • a) twisted two-wire line
  • b) RS 485 standard
  • c) IEC 61158-2

18
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
    light-transmitting 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.

19
  • Multimode and monomode optical fiber

20
Cross Section of a Fiber-Optic Cable
  • A typical cable

21
Profiles and refractive indices of optical fibers
  • Bbbb

22
  • 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. Single-mode 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.

23
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.

24
Types of material make up fiber-optic 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.

25
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.

26
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.

27
Plastic-Clad Silica (PCS) Fiber-Optic 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.

28
  • 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

29
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.

30
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.

31
Cross Section
  • mmm

32
Digital Data Transmission
  • High Speed Transmission Over Optical Fiber

33
Fiber-Optic Communications System
  • Information (voice, data, and video) from the
    source is encoded into electrical signals that
    can drive the transmitter.

34
Optical fiber communications link
  • Simplex optical fiber communications link

35
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.

36
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.

37
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.

38
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.

39
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.

40
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.

41
Types of Propagation
42
Telecommunication Link
  • Wireless communication is usually combined with
    wired communication.

43
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.

44
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.

45
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.

46
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.

47
Microwave Antenna
  • To increase the distance served by terrestrial
    microwave, a system of repeaters can be installed
    with each antenna.

48
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

49
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
50
Satellite Microwave
  • The communication satellite

51
Geosynchronous Satellites
  • To remain stationary, the satellite must have a
    period of rotation equal to the earths period of
    rotation. This match occurs at a height of 35 863
    km at the equator
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Transmission Medium

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Transcript and Presenter's Notes

Title: Transmission Medium


1
Transmission Medium
  • Media for serial Data transmission

2
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.

3
Good Signal Quality andLow 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.

4
Common Types of Physical Cable
5
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.

6
Properties of Wired Transmission Media
  • Bbbb

7
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.

8
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.

9
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.

10
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

11
Transmission Behaviorof 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.

12
Equivalent Circuit Diagram of a Transmission Cable
  • Equivalent Circuit

13
Transmission Cable Characteristics
  • Bbb

14
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.

15
Interference Caused byLine 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.

16
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
    so-called 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

17
Terminating Resistors for Different Lines
  • a) twisted two-wire line
  • b) RS 485 standard
  • c) IEC 61158-2

18
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
    light-transmitting 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.

19
  • Multimode and monomode optical fiber

20
Cross Section of a Fiber-Optic Cable
  • A typical cable

21
Profiles and refractive indices of optical fibers
  • Bbbb

22
  • 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. Single-mode 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.

23
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.

24
Types of material make up fiber-optic 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.

25
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.

26
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.

27
Plastic-Clad Silica (PCS) Fiber-Optic 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.

28
  • 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

29
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.

30
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.

31
Cross Section
  • mmm

32
Digital Data Transmission
  • High Speed Transmission Over Optical Fiber

33
Fiber-Optic Communications System
  • Information (voice, data, and video) from the
    source is encoded into electrical signals that
    can drive the transmitter.

34
Optical fiber communications link
  • Simplex optical fiber communications link

35
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.

36
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.

37
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.

38
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.

39
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.

40
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.

41
Types of Propagation
42
Telecommunication Link
  • Wireless communication is usually combined with
    wired communication.

43
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.

44
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.

45
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.

46
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.

47
Microwave Antenna
  • To increase the distance served by terrestrial
    microwave, a system of repeaters can be installed
    with each antenna.

48
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

49
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
50
Satellite Microwave
  • The communication satellite

51
Geosynchronous Satellites
  • To remain stationary, the satellite must have a
    period of rotation equal to the earths period of
    rotation. This match occurs at a height of 35 863
    km at the equator
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