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CS601 - Data Communication - Lecture Handout 25

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Shielded Twisted Pair (STP)

  • Has a metal foil or braided-mesh covering that encases each pair of insulated conductors

Shielded Twisted Pair (STP)

  • The metal casing prevents the penetration of EM noise
  • It also can eliminate a phenomenon called Crosstalk, which is the undesired effect of one circuit (or channel) on another circuit (or channel)
  • It occurs when one one line picks up some of the signals traveling down another line.
  • This effect can be experienced during telephone conversations when one can hear other conversations in the background
  • Shielding each pair of twisted pair can eliminate most crosstalk
  • STP Cable has the same quality considerations and uses the same connectors as UTP but the shield must be connected to a ground
  • STP is more expensive than UTP but is less susceptible to noise

Coaxial Cable

  • Instead of having two wires, coaxial cable has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath
  • This 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

Coaxial Cable

Frequency Range of Coaxial Cable

Coax carries signals signals of higher frequency ranges than twisted pair cable

Frequency Range of Coaxial Cable

Coaxial Cable Standards

Different coaxial cable designs are categorized by their Radio government ( RG ) ratings
Each RG number denotes a unique set of physical specifications, including

  • the wire gauge of inner conductor
  • the thickness and type of inner insulator
  • Construction of the shield
  • Size and type of outer casing

Each cable defined by RG rating is adapted for a specialized function:

  • RG-8
    • Used in Thick Ethernet
  • RG-9
    • Used in Thick Ethernet
  • RG-11
    • Used in Thick Ethernet
  • RG-58
    • Used in Thin Ethernet
  • RG-59
    • Used for TV

Coaxial Cable Connectors

  • Over the years, a no. of connectors have been designed for use with coaxial cable
  • Most common of the connectors is called “BARREL connector” because of its shape
  • Of the barrel connectors, the most popular is the Bayonet Network Connector (BNC)
  • BNC connector pushes on and locks into place with half turn
  • Other types of barrel connectors either screw together and so require more effort to install or push on w/o locking which is less secure
  • Coaxial cables are familiar in Cable TV and VCR hookups that employ both threaded and alip on style
  • Two other commonly used connectors are T-connectors and Terminators

 

  • A T-connector (used in Thin Ethernet) allows a secondary cable or cables to branch off from a main line
  • Terminators are required for bus topologies where one main cable acts as a backbone with branches to several devices but does not itself terminate in a device

If main cable is left un terminated, any signal tx over the line echoes back and interferes with the original signal
A terminated absorbs the wave at the end and eliminates this echo

Optical Fiber

  • Until this point we have discussed conductive (metal) cables that transmit signals in the form of current
    • Optical fiber is made of glass or plastic
    • It transmits signals in the form of light

The Nature of Light

The speed of light

  • 300,000 Km/sec in a vacuum
  • Depends on the density of the medium through which it is traveling
  • The higher the density, the slower the speed

Refraction

  • Light travels in a straight line as long as it is moving through a single uniform structure
  • If a ray of light traveling through one substance enters another (more or less dense) substance, its speed changes abruptly causing the ray to change direction
  • This phenomenon is called Refraction
    • Example of Refraction
      • A pencil sticking out of a glass of water appears bent because the light by which we see it changes direction as it moves from air to water

Direction of Refraction

Direction in which a light is refracted depends upon the density of a medium

Direction of Refraction

  • A beam of light moves from a less dense into a more dense medium bend towards vertical axis
  • Incident angle is ‘I’ and Refracted angle is ‘R’

Critical Angle

Critical Angle

  • We have a beam of light moving from a more dense to a less dense medium
  • We gradually increase the angle of incidence measured from vertical axis
  • As angle of incidence increases, so does the angle of refraction
  • The angle at which refracted line lies on the horizontal axis is called

Critical Angle

Reflection

Reflection

  • When angle of incidence becomes greater than critical angle, reflection occurs
  • Light no longer passes into the less denser medium but is reflected back into the same medium
  • The Angle of Incidence (I) = Angle of Reflection (R)

Optical Fibers & Reflection

  • 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 the density of CORE and CLADDING is such that the beam of light moving through the core is reflected off the cladding
  • Information is encoded onto a beam of light as a series of ON-OFF flashes that represent 1 and 0 bits

Propagation Modes

Propagation Modes

  • Fiber Technology supports two modes for the propagation of light
    • Multimode
    • Single Mode

Each of these modes require fiber with different physical characteristics

  • There are two further sub categories of Multimode Fiber:
    • Multimode Step-Index Fiber
    • Multimode Graded-Index Fiber

Multimode Fiber

  • Multiple beams from a light source move through the core in different paths
  • Two types of the Multimode fiber:
    • Multimode Step Index Fiber
    • Multimode Graded Index Fiber

Multimode Step-Index Fiber

Multimode Step-Index Fiber

  • 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 to lower density, that alters the angle of the beam’s motion
  • Step Index
    • Suddenness of this change
  • Some beams travel straight and reach the destination without reflecting
  • Some strike the interface of core and cladding at an angle smaller than critical angle and penetrate cladding and are lost
  • Others hit edge of the core at angles greater than critical angle and bounce back and forth to the destination

Disadvantage of Multimode Step-Index Fiber

  • Each beams angle is equal to its angle of reflection
  • If I is small, R is small and the beam will require more bounces and it will take more time to reach the destination
  • If I is large, R is large and beam will reach destination quickly
  • In other words there is a difference is Path Lengths that results into a distortion at the receiver
  • This distortion limits the data rate and make Multimode Step index fiber inadequate for precise applications

Multimode Graded-Index Fiber

  • The solution to the above problem is Multimode Graded Index Fiber
  • A grade index fiber is the one with varying densities
  • Density is highest at the center of the core and decreases gradually to its lowest at the edge

Multimode Graded-Index Fiber

  • The signal is introduced at the center of the core
  • The horiz beams move straight to the receiver
  • Beams at other angles moves through the series of constantly changing densities
  • Each density difference causes each beam to refract into a curve
  • Signal can be reconstructed with far greater precision as all the beams reach the receiver at almost the same time

Summary

  • Transmission Media and its Types
  • Guided Media
    • Twisted Pair
    • Coaxial Cable
    • Optical Fiber

Reading Sections

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