Wired Media

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Wired Media
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Transmission Flaws

• Transmission signals are adversely affected by a
  number of factors two of which we will discuss
• Noise
• Attenuation

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Noise

• Electrical interference causes spurious
  electrical currents in the media that interfere
  with the signal
• Humans perceive noise during a telephone call
  call as static
• Three sources of noise
• Thermal noise
• Impulse noise
• Crosstalk

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Effect of Noise
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Thermal Noise

• Caused by the thermal agitation of electrons in
  metal
• Present in all electronic equipment and forms an
  upper bound on the potential capacity of a media
• Also called white noise
• Can be reduced by cooling media and equipment but
  this isnt usually practical

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Impulse Noise

• Caused by electromagnetic interference (EMI) and
  radio frequency interference (RFI)
• Electromagnetic interference (EMI)
• Interference that may be caused by motors, power
  lines, television, copiers, fluorescent lights,
  or other sources of electrical activity
• Radiofrequency interference (RFI)
• Interference that may be generated by motors,
  power lines, televisions, copiers, fluorescent
  lights, or broadcast signals from radio or TV
  towers Unlike thermal noise, impulse noise is
  variable and is difficult to plan for
• Care must be taken when cables are laid to avoid
  sources of EMI/RFI

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Crosstalk

• Crosstalk is interference caused in one wire by
  the legitimate signal in an adjacent wire
• Crosstalk can be detected as background
  conversations during phone calls
• Crosstalk is particularly a problem at the
  connectors that connect cables to network devices

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Attenuation

• Loss of signal strength as transmission travels
  away from source
• Analog signals may be boosted by an amplifier,
  which increases not only voltage of a signal but
  also noise accumulated
• Digital signals may be boosted by a repeater
  which regenerates the original signal without
  noise if possible

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Attenuation and Thermal Noise
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Repeaters

• Repeater
• Device used to regenerate a signal

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Effect of Increased Bandwidth

• Increasing the bandwidth of a signal increases
  throughput by increasing the number of bits that
  can be represented in a given time
• At higher frequencies the signal attenuates
  faster
• Smaller bits may be wiped more easily by noise
• Cant turn bandwidth up beyond what media can
  support
• Higher bandwidth signals travel shorter distances
  on a given medium

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Wired Media Types
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Media Characteristics

• Throughput
• Perhaps most significant factor in choosing a
  transmission medium is throughput
• Cost
• Cost of installation
• Cost of new infrastructure versus reusing
  existing infrastructure
• Cost of maintenance and support
• Cost of a lower transmission rate affecting
  productivity
• Cost of obsolescence

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Media Characteristics

• Size and scalability
• Specifications determining size and scalability
• Maximum nodes per segment
• Maximum segment length
• Maximum network length

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Media Characteristics

• Connectors
• Connects wire to network device
• Noise immunity

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Types of Wired Media

• Coax
• Twisted Pair
• Unshielded Twisted Pair (UTP)
• Shielded Twisted Pair (STP)
• Fiber Optics
• Multimode Fiber (MMF)
• Single-Mode Fiber (SMF)

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Coax
Metal Shield
Conductor
Insulation
Cover
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BNC T-Connector
Terminator
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BNC Connector and NIC
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NIC with BNC Connector
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Coaxial Cable
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Coax

• Advantages
• Throughput Very good transmission
  characteristics
• Noise immunity High resistance of EMI/RFI
• Size and Scalability Can support large networks
• Cost Durable so is sometimes used in hostile
  environments
• Disadvantages
• Cost Can be difficult to work with and not used
  often so will need to be replaced if network
  changes
• Connectors Less reliable and more difficult to
  use than the RJ-45 used with twisted pair

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Twisted Pair
Interference in one wire balances interference in
the other. A perfectly balanced pair of wires
with perfectly symmetrical twists would be
totally immune to noise.
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Damaged Twisted Pair
Imperfections reduce the cables resistance to
noise. Imperfections may be due to poor
manufacturing or poor installation practices.
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Four-Pair UTP
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RJ-45 Connector
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NIC with RJ-45 Port
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Categories of UTP
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PreStandard UTP
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Unshielded Twisted Pair (UTP)

• Advantages
• Cost Easy to work with and inexpensive,
  supported by most network standards so can be
  used for many years as network evolves
• Connectors RJ-45 connector is extremely reliable
  when properly constructed
• Size and scalability Individual cable segments
  must be short but structured cabling methods
  support very large networks
• Disadvantages
• Throughput Capable of lower bandwidths than coax
  or fiber
• Noise immunity Have to be careful during
  installation to avoid noise

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Shield Twisted Pair (STP)

• STP is similar to UTP except it has a shielding
  similar to coax around the cable
• Shielding must be grounded as with coax
• Imperfections in shielding can increase noise in
  cable

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STP
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Shield Twisted Pair (STP)

• Advantages
• Cost More expensive and difficult to work with
  than UTP
• Disadvantages
• Throughput Similar to UTP
• Noise Immunity Not significantly better in terms
  of noise resistance than UTP if not installed
  exactly right
• Connectors Reliable but not as easy as RJ-45
  since shielding must be grounded
• Size and scalability Not supported by as many
  standards as UTP

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Fiber Optic Cable
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Fiber Cable
Cladding
Core
Cladding and core are different types of glass
with different reflective indices. Light travels
down the core.
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Fiber-Optic Cable

• Single-mode fiber
• Carries light pulses along single path
• Multimode fiber
• Many pulses of light generated by LED travel at
  different angles

Figure 4-29 Single-mode and multimode
fiber-optic cables
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Fiber Connectors
ST Connector
SC Connector
Ends of cable must be polished smooth to
reduce light loss and fiber must be perfectly
aligned in connector.
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Fiber NIC
ST Connector Port
Requires two fiber strands. One for send and
receive.
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Multimode Fiber (MMF)

• Light bounces off boundary between cladding and
  core
• Causes light loss
• Uses short wavelength LEDs and lasers
• Short wavelength signals attenuate faster
• Comes in 62.5/125 µm and 50/125 µm
• More expensive cable than SMF but less expensive
  connectors and light sources

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Single Mode Fiber (SMF)

• Light is focused down center of core
• Very little light lost
• Uses expensive long wavelength lasers
• Long wavelengths can travel very long distances
• Comes in 8/125 µm
• Cable is cheaper than MMF but connectors and
  light sources are very expensive
• This makes SMF network equipment is very
  expensive

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Graded MMF

• Boundary between cladding and core is not abrupt
  so that light rays are bent toward the center
  rather than reflected
• This causes less light loss

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Fiber Optics

• Advantages
• Noise immunity Totally immune to noise, no
  electrical emissions so almost impossible to tap
• Throughput Capable of very high bandwidths and
  distances
• Size and scalability Capable of very long
  distances
• Disadvantages
• Connectors Fiber connectors require special
  training and care to make
• Cost More expensive cable, more expensive to
  install and much more expensive equipment