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CS601 - Data Communication - Lecture Handout 26

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Single Mode Fiber

  • 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
  • Single Mode fiber is manufactured with a much smaller Diameter than Multimode
  • All of the beams arrive at the destination together and can be recombined without distortion to the signal

Single Mode Fiber

Fiber Sizes

  • Optical Fibers are defined by the ratio of the diameter of their Core to the diameter of their Cladding
  • Both the diameters are expressed in Microns (Micrometers)

Fiber Sizes

  • A core is surrounded by cladding forming the Fiber.
  • In most cases, fiber is covered by a Buffer layer that protects it from moisture.
  • Finally the entire cable is encased in an outer jacket
  • Both core and cladding can be made of either glass or plastic but must be of different densities
  • In addition the inner core must be ultra pure and completely regular in size and shape
  • Chemical differences in material and even small variations in the size or shape of the core alter the angle of reflection and distort the signals
  • Some applications can handle a certain amount of distortion and their cables can be made cheaply but others depend on complete uniformity
  • The outer jacket can be made of several materials including Teflon, Plastic, Fibrous Plastic, metal tubing

Each of these materials have a purpose:

  • Plastic is lightweight and cheap but do not provide structural strength and can emit fumes when burnt
  • Metal tubing provides strength but is costly
  • Teflon is lightweight and can be used in open air but it is expensive and does not increase cable strength

Light Sources for Optical Cable

  • For tx to occur the sending device must have a light source and the receiving device with a photosensitive cell (Photodiode)
  • Photodiode converts the light into current usable by the computer

The Light source can either be an LED or an ILD

LED:

  • Cheaper but provide Unfocused light that strikes the boundaries of channel at uncontrollable angles
  • Limited to short distance use

LASER:

  • Can be focused to a narrow range allowing control over angle of incidence

Fiber Optic Connectors

  • Importance of Connectors
  • If connector is over tight, two cores can be compressed and angle of reflection of the signal will be altered
  • All of popular connectors are Barrel shaped that come in male and female versions
  • The cable has a male connector that fixes into a female connector attached to the device to be connected

Advantages of Optical Fiber

The major advantages of Fiber over twisted pair and coaxial cable are:

Noise Resistance:

  • Because fiber uses light rather than electricity, noise is not a factor
  • External light the only form of possible interference is blocked from the channel by the Outer jacket

Less Signal Attenuation

  • Fiber optic Transmission distance is significantly greater than other media
  • A signal can run miles w/o regeneration

Higher Bandwidth

  • Can support higher BWs and higher data rates
  • High rates are not utilized by absence of signal generation and reception technology

Disadvantages of Optical Fiber

  • COST

    • Expensive
    • No impurities or imperfections can be tolerated, so manufacturing is costly
    • Laser light sources can be expensive
  • INSTALLATION

    • Roughness & Cracking of core cannot be tolerated
    • All connections must be perfectly alligned

Disadvantages of Optical Fiber

Fragility

  • Glass fiber is very fragile
  • Can not be used in extreme conditions where hardware portability is required

Unguided Media

  • Unguided Media or Wireless Communication transport Electromagnetic waves without a physical conductor
  • Instead signals are broadcast through Air and are available to anyone who has a receiver capable of receiving them

Radio Frequency Allocation

  • The section of EM spectrum defined as Radio Communication is divided into 8 ranges called BANDS
  • BANDS are rated from very low frequency (VLF) to extremely high frequency (EHF)

Radio Frequency Allocation

Propagation of Radio Waves

Radio Wave transmission utilizes five different types of propagation:

Types of Propagation:

  • Surface
  • Tropospheric
  • Ionospheric
  • Line-of-Sight
  • Space

Radio Technology considers the earth as surrounded by two layers of atmosphere:

Space

TROPOSHERE

  • It is the portion of the atmosphere extending outwards approx. 30 miles from the earth’s surface
  • It contains what we call as AIR
  • Clouds, wind, Temp. variation and weather in general occur in the Troposhere as does jet plane travel

IONOSPHERE

  • It is the layer of atmosphere above the troposhere but below space
  • It is beyond what we think of as atmosphere

Surface Propagation

  • In surface propagation, radio waves travel through the lowest layer of the atmosphere, hugging the earth
  • At the lowest frequencies signal emanate in all direction from the tx antenna and follow the curvature of the planet
  • Distance depends on the amount of power in the signal

Troposhpheric Propagation

This can work in two ways

Line-of-Sight:

  • A signal can be directed in a straight from Antenna to antenna

Broadcast:

  • Signal is broadcasted at an angle into the upper layers of the troposphere from where it is reflected back to earth’s surface

 

  • The first method demands that both transmitter and receiver be placed within line-of-sight distances and is limited by the curvature of the earth
  • The second method allows greater distances to be covered

Ionoshpheric Propagation

  • Higher frequency radio waves are radiated towards the ionosphere where they are reflected back to the earth
  • The difference in density between troposhphere and ionosphere causes each radio wave to speed up and change direction bending back to earth
  • Allows greater distances to be covered by lower power output

Line of Sight Propagation

  • Very high frequencies signals are transmitted in straight line from antenna to antenna
  • Antennas must be directional facing each other or either tall enough or close enough to each other to avoid earth’s curvature
  • Radio waves can reflect of the objects in the middle and can reach late to the receiver. These late signals distort signal

Space Propagation

  • A broadcast signal is received by the orbiting satellites which rebroadcast the signal to the intended receiver on the earth

Summary

  • Guided Media
  • Optical Fiber Cable
  • Unguided Media
  • Radio Frequency Allocation
  • Propagation of Radio Waves

Readig Sections

  • Section 7.1,“Data Communications and Networking” 4th Edition by Behrouz A. Forouzan