Guided Media

1
Guided Media

• Media that provide a conduit from one device to
  another.
• Signals travelling along any of these media is
  directed and contained by the physical limits of
  the medium.
• Examples are
• Twisted-pair cable
• Coaxial cable
• Fiber-optic cable

2
Twisted-Pair Cable

• Uses two metallic (copper) conductors with
  individual plastic insulator each.
• Accepts and transports signals in the form of
  electric current.
• One wire is used to carry signals to the receiver
  and the other is used as a ground reference.
• The receiver uses the difference between the two
  signal levels in the wires.
• Noise or crosstalk effect is balanced by twisting
  the wires.

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Figure 7.3 Twisted-pair cable
4
Twisted-Pair (Continued)

• Twisted-pair cable can be divided into
• Unshielded (UTP) The most common twisted-pair
  cable used in data communication.
• Shielded (STP) IBMs version for its use. It
  has a metal foil of braided-mesh covering
  encasing each pair of the insulated conductors.
  It improves the quality of the cable, but it is
  bulkier.

5
Figure 7.4 UTP and STP
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Twisted-Pair (Continued)

• Standards
• The Electronic Industries Association (EIA) has
  developed standards to classify UTP cable into
  seven categories based on their quality.
• Category 1 is the lowest in quality and category
  7 is the highest.
• Connectors
• The most common is RJ45, which is a keyed
  connector.

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Table 7.1 Categories of unshielded twisted-pair
cables
Category Bandwidth Data Rate Digital/Analog Use
1 very low lt 100 kbps Analog Telephone
2 lt 2 MHz 2 Mbps Analog/digital T-1 lines
3 16 MHz 10 Mbps Digital LANs
4 20 MHz 20 Mbps Digital LANs
5 100 MHz 100 Mbps Digital LANs
6 (draft) 200 MHz 200 Mbps Digital LANs
7 (draft) 600 MHz 600 Mbps Digital LANs
8
Figure 7.5 UTP connector
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Twisted-Pair (Continued)

• Performance
• A twisted-pair cable can pass a wide range of
  frequencies. However, the attenuation (dB/mi)
  sharply increases with frequencies above 100KHz.
• Applications
• Voice and data channels in telephone lines.
• High data rate connections in DSL lines.
• 10Base-T and 100Base-T LANs.

10
Figure 7.6 UTP performance
11
Coaxial Cable

• Uses central core conductor of solid or stranded
  copper wire enclosed in an insulating sheath.
• The insulating sheath is then encased in an outer
  conductor of metal foil, braid or a combination
  of the two.
• The metallic wrapping serves both as a shield
  against noise and as a second conductor that
  completes the circuit.
• This outer conductor is also enclosed in another
  insulating sheath and the whole thing is
  protected by a plastic cover.

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Figure 7.7 Coaxial cable
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Coaxial Cable (Continued)

• Standards
• Categorised by their radio government (RG)
  ratings with each number denotes a unique set of
  physical specifications.

Category Impedance Use
RG-59 75 W Cable TV
RG-58 50 W Thin Ethernet
RG-11 50 W Thick Ethernet
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Coaxial Cable (Continued)

• Connectors
• The most commonly used connectors are the
  Bayone-Neill-Concelman (BNC) connectors.
• The three popular types of BNC are
• BNC used to connect the end of the cable to a
  device, such as TV set.
• BNC T used in Ethernet networks to branch out a
  cable for connection to a computer or other
  devices.
• BNC terminator used at the end of the cable to
  prevent the signal reflection.

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Figure 7.8 BNC connectors
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Coaxial Cable (Continued)

• Performance
• Coaxial cable has a much higher bandwidth
  compared to the twisted-pair cable. However, the
  signal weakens rapidly and it requires frequent
  use of repeaters.
• Applications
• Analogue and digital telephone networks.
• Cable TV.
• Traditional Ethernet LANs.

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Figure 7.9 Coaxial cable performance
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Fiber-Optic Cable

• Made of glass or plastic and transmits signals in
  the form of light.
• Making use of the property of light that changes
  direction when travelling through substances of
  differing density.
• 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.

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Figure 7.14 Fiber construction
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Figure 7.10 Bending of light ray
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Figure 7.11 Optical fiber
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Fiber-Optic Cable (Continued)

• Sizes
• Defined by the ratio of the diameter of their
  core to the diameter of their cladding, both
  expressed in micrometers.

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Fiber-Optic (Continued)

• Connectors
• Uses three types of connectors
• Subscriber channel (SC) uses push/pull locking
  system.
• Straight-tip (ST) uses bayonet locking system
  and more reliable than SC.
• MT-RJ a new connector with the same size as
  RJ45.
• Performance
• Attenuation is flatter than in the case of TP and
  coaxial cables. Therefore, less repeater is
  needed for fiber-optic cable.

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Figure 7.15 Fiber-optic cable connectors
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Figure 7.16 Optical fiber performance
26
Fiber-Optic (Continued)

• Applications
• Used often in the backbone networks.
• Used in hybrid cable TV network as the backbone.
• Used in 100Base-FX and 1000Base-X LANs