Transmission medium

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Transmission medium

• We can say that transmission media belong to
  layer zero

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Transmission medium

• Transmission media can be divided into two broad
  categories
• Guided media include twisted-pair cable, coaxial
  cable, and fiber-optic
• Unguided media is usually air

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Guided media

• Guided media, which are those that provide a
  conduit from one device to another, include
  twisted-pair cable, coaxial cable, and
  fiber-optic cable.
• A signal traveling along guided media is directed
  and contained by the physical limits of the
  medium
• Twisted-pair and coaxial cable use metallic
  (copper) conductors that accept an transport
  signals in the form of electric current
• Optical fiber is a glass cable that accepts and
  transports signals in the form of light

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Twisted-Pair Cable

• Consists of
• two conductors (normally copper)
• each with its own plastic insulation
• twisted together

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Twisted-Pair Cable

• One of the wires is used to carry signals to the
  receiver
• And the other is used as a ground reference
• Receiver uses the difference between the two
  level
• Interference (noise) and crosstalk may affect
  both wires and create unwanted signals
• Receiver operates only on the difference between
  these unwanted signals
• If the two wires are affected by noise or
  crosstalk equally
• Receiver is immune (the difference is zero)
• If the two wires are parallel
• The effect of these unwanted signals is not the
  same in both wires (one close and one farther)
• By twisting the pairs, a balance is maintained

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Twisted-Pair Cable

• The number of twists per unit of the length (e.g.
  inch) determines the quality of the cable
• More twists mean better quality

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Unshielded versus Shielded

• Two common twisted-pair cable used in
  communications
• Unshielded twisted-pair (UTP)
• Shielded twisted-pair (STP)
• STP has a metal foil or braided-mesh covering
• Preventing the penetration of noise or crosstalk
• It is bulkier and more expensive

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Categories

• EIA the Electronic Industries association
  classifies unshielded twisted-pair cable into
  seven categories which determined by quality (1
  lowest and 7 highest)

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Connectors

• Most common UTP connector is RJ45
• R J stands for Registered Jack

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Performance

• One way measure performance is to compare
• attenuation vs. frequency and distance.
• A twisted-pair can pass a wide range of
  frequencies.
• Figure 7.6 shows with increasing frequency,
  attenuation in decibels per kilometer (dB/km),
  sharply increase with frequency above 100 kHz.
• Gauge is a measure of the thickness of the wire.

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Application

• Used in telephone Lines (UTP)
• DSL lines (UTP)
• Local area networks

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Coaxial Cable

• Carries signals of higher frequency ranges than
  twisted-pair cable
• Coax has a central core conductor of solid or
  stranded wire (usually copper)
• Encased in an outer conductor of metal foil,
  braid, or a combination of the two
• The outer metallic wrapping services as
• Shield against noise
• Second conductor (completes the circuit)
• Outer conductor is also enclosed in an insulating
  sheath
• Whole cable is protected by a plastic cover

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Coaxial Cable Standards

• Coaxial cables are categorized by their radio
  government (RG) ratings
• Each RG number denotes a unique set of physical
  specifications
• Wire gauge of the inner conductor
• Thickness and type of the inner insulator
• Construction of the shield
• Size and type of the outer casing
• Each RG ratings is adapted for a specialized
  function

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Coaxial Cable Connectors

• Coaxial cable connector is BNC (Bayone-Neill-Conce
  lman)
• Figure shows the BNC connector, the BNC T
  connector, and the BNC terminator

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Performance
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Applications

• It was used in analog and digital telephone
  networks (replaced by fiber-optic)
• Cable TV networks (replaced by fiber-optic)
• Traditional Ethernet Lans

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Fiber-Optic Cable

• It is made of glass or plastic and transmits
  signals in the form of light
• If a ray pf light traveling through one substance
  and enters another (more or less dense), the ray
  changes directions
• Optical fibers use reflection to guide light
  through a channel

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Fiber-Optic Cable

• Glass or plastic core is surrounded by a cladding
  of less dense glass or plastic

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Propagation Modes

• Current technology supports two modes (multimode
  and single mode) for propagating light along
  optical channels
• Multimode multiple beams from a light source
  move through the core in different paths
• Multimode can be implemented in two forms
• Step-index
• Graded-index

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Propagation Modes
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Fiber Sizes

• By the ratio of the diameter of their core to the
  diameter of their cladding micrometers

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Cable compostion
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Fiber-Optic Connections

• Fiber-optic use three different type of
  connectors
• Subscriber Channel (SC) connector
• Used in cable TV and it uses a push/pull locking
  system
• The Straight-Tip (ST) connector
• Used for connecting cable to networking devices
• MT-RJ in new connector with the same size as RJ45

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Optical fiber performance
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Fiber-Optic Connections

• Applications
• It used in backbone networks
• For cable TV with coaxial cable (a hybrid
  network)

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Advantages and Disadvantages of Optical

• Advantages over twisted-pair and coaxial
• Higher bandwidth
• Less signal attenuation
• Signal with fiber-optic can run for 50 km with
  requiring regeneration
• 5 km for coaxial or twisted-pair cable
• Immunity to electromagnetic interference
• Electromagnetic noise cannot affect fiber-optic
  cables
• Resistance to corrosive materials
• Glass is more resistant to corrosive materials
  than copper
• Light weight
• More immune to tapping
• Fiber-optic cables definitely more immune to
  tapping than copper cables

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Advantages and Disadvantages of Optical

• Disadvantages
• Installation/maintenance
• Because it is new technology, need expertise
• Unidirectional
• If we need bidirectional we need two fibers
• Cost
• The cables and interfaces are expensive

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UNGUIDED MEDIA WIRELESS

• Unguided media
• transport electromagnetic waves without using a
  physical conductor (wireless communication)
• Signal broadcast through air

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UNGUIDED MEDIA WIRELESS

• Unguided signals can travel from the source to
  destination in several ways
• Ground propagation
• Sky propagation
• Line-of-sight propagation

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UNGUIDED MEDIA WIRELESS

• Ground propagation (below 2 MHz)
• Radio waves travel through the lowest portion of
  atmosphere
• Waves emanate in all directions from the
  transmitting antenna
• Distance depends on the amount of power in the
  signal
• Sky propagation (2 - 30 MHz)
• Higher frequency radio waves radiate upward into
  ionosphere where they reflected back to earth
• This type of transmission allows for greater
  distance with lower power output

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UNGUIDED MEDIA WIRELESS

• Line-of-sight propagation (above 30 MHz)
• Very high-frequency signals are transmitted in
  straight lines directly from antenna to antenna
• Antennas must be directional
• Facing each other
• Either tall enough or close enough

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UNGUIDED MEDIA WIRELESS

• Electromagnetic spectrum defined as radio waves
  and microwaves is divided into eight ranges,
  called bands

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UNGUIDED MEDIA WIRELESS

• Wireless transmission can be divided into
• Radio waves
• Microwaves
• Infrared waves

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UNGUIDED MEDIA WIRELESS

• Radio waves
• Radio waves waves range in frequencies between 3
  KHz and 1 GHz
• Microwaves waves ranging in frequencies between
  1 and 300 GHz
• Radio waves are omnidirectional (propagated in
  all directions)
• The sending and receiving antennas do not have to
  be aligned
• Disadvantage
• One antenna interferences another antenna when
  they using same frequency
• Radio waves that propagate in the sky mode, can
  travel long distance AM radio
• Radio waves low and medium frequencies can
  penetrate walls inside a building
• Advantage an AM radio can be received inside a
  building
• Disadvantage we cannot isolate a communication
  to just inside or outside a building
• Applications
• Useful for multicasting Radio, television.
  Cordless phones and paging system

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UNGUIDED MEDIA WIRELESS

• Microwaves
• Microwaves are unidirectional
• Antenna need to be aligned
• Advantage pair of antenna can be aligned without
  interfering with another pair
• Microwave propagation is line-of-sight
• For long distance communication
• Very tall towers
• Repeater
• Very high-frequency microwaves cannot penetrate
  walls
• Disadvantage if receiver inside a building

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UNGUIDED MEDIA WIRELESS

• Applications
• Due to unidirectional properties, microwaves
  useful when unicasting (one-to-one) communication
• Cellular phones
• Satellite networks
• Wireles LANs

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UNGUIDED MEDIA WIRELESS

• Infrared
• From 300 GHz to 400 THz (wavelengths from 1 mm to
  770 nm)
• Use for short-range communication
• It has high frequency, cannot penetrate walls
• Prevents interference between one system and
  another
• Remote control not interfere with our neighbors
• Can not be used outside because suns rays
  contain infrared waves (interference)
• (IrDA) Infrared Data Association established
  standards for communicating between devices
• Keyboards, mice, PCs and printers