Transmission Media

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Transmission Media
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Media

• Basic function of media carry flow of
  information in form of bits through a LAN
• In a copper based network, bits will be
  electrical signals
• In a fiber based network, bits will be light
  pulses
• Media considered to be Layer 1 component of a
  LAN
• Physical path between transmitter and receiver
• Wired and Wireless
• Communication is in the form of electromagnetic
  waves
• Characteristics and quality of data transmission
  are determined by characteristics of medium and
  signal
• In wired media, medium characteristics is more
  important, whereas in wireless media, signal
  characteristics is more important

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Transmission Media

• Physical path between transmitter and receiver
• Wired and Wireless
• Communication is in the form of electromagnetic
  waves
• Characteristics and quality of data transmission
  are determined by characteristics of medium and
  signal
• In wired media, medium characteristics is more
  important, whereas in wireless media, signal
  characteristics is more important

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Electromagnetic Spectrum
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Basic limitations

• Attenuation
• Delay Distortion
• Noise
• Thermal/White Noise
• Intermodulation Noise
• Crosstalk
• Echo
• Impulse Noise

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Design Factors for Transmission Media

• Bandwidth All other factors remaining constant,
  the greater the band-width of a signal, the
  higher the data rate that can be achieved.
• Transmission impairments. Limit the distance a
  signal can travel.
• Interference Competing signals in overlapping
  frequency bands can distort or wipe out a signal.
• Number of receivers Each attachment introduces
  some attenuation and distortion, limiting
  distance and/or data rate.

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Classes of Transmission Media

• Conducted or guided media
• use a conductor such as a wire or a fiber optic
  cable to move the signal from sender to receiver
• Wireless or unguided media
• use radio waves of different frequencies and do
  not need a wire or cable conductor to transmit
  signals

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Wire Conductors

• Wire types single conductor, twisted pair,
  shielded multiconductor bundles.
• Large installed base.
• Reasonable cost.
• Relatively low bandwidth, however, recent LAN
  speeds in the 100 Mbps range have been achieved.
• Susceptible to external interference.
• Shielding can reduce external interference.
• Can transmit both analog and digital signals.
  Amplifier required every 5 to 6 km for analog
  signals. For digital signals, repeaters required
  every 2 to 3 km.

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A metric for copper cables
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Wired - Twisted Pair

• The oldest, least expensive, and most commonly
  used media
• Pair of insulated wires twisted together to
  reduce susceptibility to interference ex)
  capacitive coupling, crosstalk
• Skin effect at high frequency
• Up to 250 kHz analog and few Mbps digital
  signaling ( for long-distance point-to-point
  signaling)
• Need repeater every 2-3 km (digital), and
  amplifier every 5-6 km (analog)

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Twisted Pair

• Consists of two insulated copper wires arranged
  in a regular spiral pattern to minimize the
  electromagnetic interference between adjacent
  pairs
• Often used at customer facilities and also over
  distances to carry voice as well as data
  communications
• Low frequency transmission medium
• Telephone (subscriber loop between house and
  local exchange)
• High-speed (10 - 100 Mbps) LAN
• token ring, fast - Ethernet

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Types of Twisted Pair

• STP (shielded twisted pair)
• the pair is wrapped with metallic foil or braid
  to insulate the pair from electromagnetic
  interference
• UTP (unshielded twisted pair)
• each wire is insulated with plastic wrap, but the
  pair is encased in an outer covering

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Ratings of Twisted Pair

• Category 3
• UTP cables and associated connecting hardware
  whose transmission characteristics are specified
  up to 16 MHZ.
• data rates of up to 16mbps are achievable
• Category 4
• UTP cables and associated connecting hardware
  whose transmission characteristics are specified
  up to 20 MHz.
• Category 5
• UTP cables and associated connecting hardware
  whose transmission characteristics are specified
  up to 100 MHz.
• data rates of up to 100mbps are achievable
• more tightly twisted than Category 3 cables
• more expensive, but better performance
• Category 5 enhanced, Cat 6, cat 7
• Fast and giga-ethernet
• STP
• More expensive, harder to work with

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Twisted Pair Advantages

• Advantages
• Inexpensive and readily available
• Flexible and light weight
• Easy to work with and install
• Disvantages
• Susceptibility to interference and noise
• Attenuation problem
• For analog, repeaters needed every 5-6km
• For digital, repeaters needed every 2-3km
• Relatively low bandwidth (MHz)

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Twisted-Pair Cable
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Effect of Noise on Parallel Lines
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Noise on Twisted-Pair Lines
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Unshielded Twisted-Pair Cable
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Shielded Twisted-Pair Cable
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Wired Transmission Media

• Coaxial Cable
• Most versatile medium
• gt LANs, Cable TV, Long-distance
  telephones, VCR-to-TV connections
• Noise immunity is good
• Very high channel capacity
  
• gt few 100 MHz / few 100 Mbps
• Need repeater/amplifier every few kilometer or so
  (about the same as with twisted pair)
• Has an inner conductor surrounded by a braided
  mesh
• Both conductors share a common center axial,
  hence the term co-axial

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Coaxial cable

• Signal and ground wire
• Solid center conductor running coaxially inside a
  solid (usually braided) outer circular conductor.
• Center conductor is shielded from external
  interference signals.

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Properties of coaxial cable

• Better shielding allows for longer cables and
  higher transfer rates.
• 100 m cables
• 1 to 2 Gbps feasible (modulation used)
• 10 Mbps typical
• Higher bandwidth
• 400 to 600Mhz
• up to 10,800 voice conversations
• Can be tapped easily stations easily added (pros
  and cons)
• Much less susceptible to interference than
  twisted pair
• Used for long haul routes by Phone Co.
• Mostly replaced now by optical fiber.
• High attenuation rate makes it expensive over
  long distance
• Bulky
• Baseband vs. broadband coax.

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Wired Transmission Media

• Optical Fiber
• Flexible, thin (few to few hundred ?m), very pure
  glass/plastic fiber capable of conducting optical
  rays
• Extremely high bandwidth capable of ? 2 Gbps
• Very high noise immunity, resistant to
  electromagnetic interference
• Does not radiate energy/cause interference
• Very light
• Need repeaters only 10s or 100 km apart
• Very difficult to tap Better security but
  multipoint not easy
• Require a light source with injection laser diode
  (ILD) or light-emitting diodes (LED)

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Wired Transmission Media

• Optical Fiber (Contd)
• Need optical-electrical interface (more expensive
  than electrical interface)

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Wired Transmission Media
Optical Fiber

• Principle of optical fiber transmission Based on
  the principle of total internal reflection
  
  
  
  
• If ?gt?, medium B (water) has a higher optical
  density than medium A (air)
• In case the index of refractionlt1 (?gt?), if ?
  is less than a certain critical angle, there is
  no refracted light i.e., all the light is
  reflected. This is what makes fiber optics work.

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Fiber optics Physics 101

• Refractive indexmaterial (Speed of light in
  vacuum)/(Speed of light in material)
• Light is bent as it passes through a surface
  where the refractive index changes. This bending
  depends on the angle and refractive index.
  Frequency does not change, but because it slows
  down, the wave length gets shorter, causing wave
  to bend.
• In case of fiber optic media, refractive index of
  core gt refractive index of cladding thereby
  causing internal reflection.

cladding
core
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Fiber Optic Layers

• consists of three concentric sections

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Modes of fiber

• Fiber consists of two parts the glass core and
  glass cladding with a lower refractive index.
• Light propagates in 1 of 3 ways depending on the
  type and width of the core material.
• Multimode stepped index fiber
• Both core and cladding have different but uniform
  refractive index.
• Relies on total internal reflection Wide pulse
  width.
• Multimode graded index fiber
• Core has variable refractive index (light bends
  as it moves away from core).
• Narrow pulse width resulting in higher bit rate.
• Singlemode fiber (gt 100 Mbs)
• Width of core diameter equal to a single
  wavelenth.

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Mode
Multimode
Single mode
Step index
Graded-index
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Fiber Optic Types

• multimode step-index fiber
• the reflective walls of the fiber move the light
  pulses to the receiver
• multimode graded-index fiber
• acts to refract the light toward the center of
  the fiber by variations in the density
• single mode fiber
• the light is guided down the center of an
  extremely narrow core

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Types of optical fiber

• Modes, bundles of light rays enter the fiber at a
  particular angle
• Single-mode
• Also known as mono-mode
• Only one mode propagates through fiber
• Higher bandwidth than multi-mode
• Longer cable runs than multi-mode
• Lasers generate light signals
• Used for inter-building connectivity

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Types of optical fiber

• Multi-mode
• Multiple modes propagate through fiber
• Different angles mean different distances to
  travel
• Transmissions arrive at different times
• Modal dispersion
• LEDs as light source
• Used for intra-building connectivity

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Fiber Optic Signals
fiber optic multimode step-index
fiber optic multimode graded-index
fiber optic single mode
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Optical Fiber Transmission Mode
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Fiber Optic

• Advantages
• greater capacity (bandwidth Gbps)
• smaller size and lighter weight
• lower attenuation
• immunity to environmental interference
• highly secure due to tap difficulty and lack of
  signal radiation
• Disvantages
• expensive over short distance
• requires highly skilled installers
• adding additional nodes is difficult

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Fiber Channel Requirements

• Full duplex links with 2 fibers/link
• 100 Mbps 800 Mbps
• Distances up to 10 km
• Small connectors
• high-capacity
• Greater connectivity than existing multidrop
  channels
• Broad availability
• Support for multiple cost/performance levels
• Support for multiple existing interface command
  sets

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Components of an optical transmission system

• 3 components
• 1. Light source
• 2. Transmission medium
• 3. The detector
• Light means a 1 bit, no light means a 0 bit.
• Transmitter LED or injection laser diode.
• Detector (photodiode or photo transistor)
  generates an electrical pulse when light falls on
  it.
• Unidirectional data transmission system.
• Electrical signal to light signal and back again.

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Fiber cables

• Multimode diameter of core is 50 microns.
• About the same as a human hair.
• Single mode diameter of core 8-10 microns.
• They can be connnected by connectors, or by
  splicing, or by fusion.

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Fiber vs. copper

• Fiber (pros)
• Higher bandwidth,
• Lower attenuation,
• Immune to electromagnetic noise and corrosive
  chemicals,
• Thin and lightweight,
• Security (does not leak light, difficult to tap).
• Fiber (cons)
• Not many skilled fiber engineers,
• Inherently unidirectional,
• Fiber interfaces are expensive.

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Wireless (Unguided Media) Transmission

• transmission and reception are achieved by means
  of an antenna
• directional
• transmitting antenna puts out focused beam
• transmitter and receiver must be aligned
• omnidirectional
• signal spreads out in all directions
• can be received by many antennas

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The Radio Spectrum

• Radio wave
• Wavelength l c/f
• Speed of light c3x108 m/s
• Frequency f

VUSEHF VeryUltraSuperExtra High
Frequency
f 900 MHz ? l 33 cm
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Atmospheric Transmission Media

• Infrared Transmission
• Infrared networks use infrared light signals to
  transmit data
• Direct infrared transmission depends on
  transmitter and receiver remaining within line of
  sight
• In indirect infrared transmission, signals can
  bounce off of walls, ceilings, and any other
  objects in their path

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Atmospheric Transmission Media

• RF Transmission
• Radio frequency (RF) transmission relies on
  signals broadcast over specific frequencies
• Narrowband concentrates significant RF energy at
  a single frequency
• Spread spectrum uses lower-level signals
  distributed over several frequencies
  simultaneously

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Infrared
Wireless Transmission

• For short-range communication
• Remote controls for TVs, VCRs and stereos
• IRD port
• Indoor wireless LANs
• Do not pass through solid walls
• Better security and no interference (with a
  similar system in adjacent rooms)
• No government license is needed
• Cannot be used outdoors

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Infrared

• Transceivers must be within line of sight of each
  other (directly or via reflection)
• Unlike microwaves, infrared does not penetrate
  walls
• Fairly low bandwidth (4 Mbps).
• Uses wavelengths between microwave and visible
  light.
• Uses transmitters/receivers (transceivers) that
  modulate noncoherent infrared light.
• No frequency allocation issue since not
  regulated.
• Uses include local building connections, wireless
  LANs, and new wireless peripherals.

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Infrared Waves

• Short range communication.
• e.g. Remotes on VCRs and TVs.
• Directional.
• Do not pass through walls.
• Behaves more like visible light.
• Can be used for LANs
• indoors only.
• Can just use visible unguided light (lasers).

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Wireless Transmission
Frequencies

• 2GHz to 40GHz
• Microwave
• Highly directional
• Point to point
• Satellite
• 30MHz to 1GHz
• Omnidirectional
• Broadcast radio
• 3 x 1011 to 2 x 1014
• Infrared

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Wireless Transmission
Terrestrial Microwave

• Parabolic dish
• Focused beam
• Line of sight
• Long haul telecommunications
• Higher frequencies give higher data rates

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Terrestrial Microwave

• used for long-distance telephone service
• uses radio frequency spectrum, from 2 to 40 Ghz
• parabolic dish transmitter, mounted high
• used by common carriers as well as private
  networks
• requires unobstructed line of sight between
  source and receiver
• curvature of the earth requires stations
  (repeaters) 30 miles apart

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Radio Transmission

• Radio waves
• Easy to generate, travel long distances, and
  penetrate buildings easily.
• Omnidirectional.
• Low frequencies
• Pass through obstacles well,
• Quick power drop off (e.g. 1/r3 in air).
• High frequencies
• Travel in straight lines and bounce off
  obstacles.
• Absorbed by rain.
• Subject to electrical interference

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Media Broadcast Radio

• Covers 30MHz to 1 GHz
• Omindirectional
• Enables mobile communication computing!
• Broadcast mechanisms cellular radio, radio nets,
  low-orbit satellites.
• Low bandwidth.
• Lack of security.
• Susceptible to interference (primarily multipath
  interference).
• Reallocation of limited frequencies may be
  required for wireless communication growth.

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Microwave transmission

• Microwave waves
• Travel in straight lines and thus can be narrowly
  focused.
• Easy to avoid interference with other microwaves.
• Parabolic antenna is used to concentrate the
  energy (improves SNR).
• More popular before fiber.
• Waves do not pass through buildings.
• Multiple towers used as repeaters.

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Media Terrestrial Microwave

• High bandwidth (45 Mbps).
• No cabling between sites.
• Clear line-of-sight required (30 miles).
• Susceptible to radio interference.
• Attenuation increases with rainfall
• Lack of security.
• Up-front investment in towers repeaters.
• Low power used to minimize effects on people
• line of sight requirement
• expensive towers and repeaters
• subject to interference such as passing airplanes
  and rain

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Propagation Types
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Satellite Microwave
Wireless Transmission

• Satellite is relay station
• Satellite receives on one frequency, amplifies or
  repeats signal and transmits on another frequency
• Requires geo-stationary orbit
• Height of 35,784km
• Optimum transmission in 1 - 10 GHz range
• Bandwidth of 100s MHz
• Significant propagation delay (?270 ms)
• Application Television, long distance telephone,
  Private business networks

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Satellite Transmission Process
satellite transponder
dish
dish
22,300 miles
uplink station
downlink station
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Satellite Transmission Applications

• television distribution
• a network provides programming from a central
  location
• direct broadcast satellite (DBS)
• long-distance telephone transmission
• high-usage international trunks
• private business networks

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Principal Satellite Transmission Bands

• C band 4(downlink) - 6(uplink) GHz
• the first to be designated
• Ku band 12(downlink) -14(uplink) GHz
• rain interference is the major problem
• Ka band 19(downlink) - 29(uplink) GHz
• equipment needed to use the band is still very
  expensive

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Physical media and their applications
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Physical Media

• Twisted Pair (TP)
• two insulated copper wires
• Category 3 traditional phone wires, 10 Mbps
  ethernet
• Category 5 TP 100Mbps ethernet

• physical link transmitted data bit propagates
  across link
• guided media
• signals propagate in solid media copper, fiber
• unguided media
• signals propagate freely e.g., radio

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Physical Media coax, fiber

• Coaxial cable
• wire (signal carrier) within a wire (shield)
• baseband single channel on cable
• broadband multiple channel on cable
• bidirectional
• common use in 10Mbs Ethernet

• Fiber optic cable
• glass fiber carrying light pulses
• high-speed operation
• 100Mbps Ethernet
• high-speed point-to-point transmission (e.g., 5
  Gbps)
• low error rate

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Physical media radio

• Radio link types
• microwave
• e.g. up to 45 Mbps channels
• LAN (e.g., waveLAN)
• 2Mbps, 11Mbps, 54 Mbps
• wide-area (e.g., cellular)
• e.g. CDPD, 10s Kbps
• satellite
• up to 50Mbps channel (or multiple smaller
  channels)
• 270 msec end-end delay
• geosynchronous versus LEOS

• signal carried in electromagnetic spectrum
• no physical wire
• bidirectional
• propagation environment effects
• reflection
• obstruction by objects
• interference

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Cables, at least 16 types described

• Cat 3, 5, 6, 7
• Screened and unscreened
• USB cable
• IEEE 1394 cable
• Plastic Optical Fiber
• 50/125, 62.5/125 and singlemode fibre
• 75 ohm 3-GHz coax
• speaker wire - two grades

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IEEE 1394 Firewire
22 AWG
28 AWG
IEEE 1394b 800, 1600, 3200 Mb/s over POF 3.2 Gb/s
over glass fibre 100 Mb/s over UTP
400 Mb/s over 4.5 m
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Universal Serial Bus
1.5, 12 or 480 Mb/s, up to 5 m, cascade 5 devices
up to 30 m
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Fiber vs Satellite
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Choosing the Right Transmission Media

• Areas of high EMI or RFI
• Corners and small spaces
• Distance
• Security
• Existing infrastructure
• Growth

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Media Selection Criteria

• Cost (Initial, Expansion, Maintenance)
• Speed (Data Rate Response Time)
• Availability
• Expandability
• Error Rates
• Security
• Distance (Geography Number of Sites)
• Environment
• Application-Specific Constraints
• Maintenance

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Media Selection Criteria
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From Signals to Packets
Analog Signal
Digital Signal
0 0 1 0 1 1 1 0 0 0 1
Bit Stream
01000101010111001010101010111011100000011110101011
10101010101101011010111001
Packets
Header/Body
Header/Body
Header/Body
Packet Transmission
Receiver
Sender
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Modulation

• Sender changes the nature of the signal in a way
  that the receiver can recognize.
• Similar to radio AM or FM
• Digital transmission encodes the values 0 or 1
  in the signal.
• It is also possible to encode multi-valued
  symbols
• Amplitude modulation change the strength of the
  signal, typically between on and off.
• Sender and receiver agree on a rate
• On means 1, Off means 0
• Similar frequency or phase modulation.
• Can also combine method modulation types.

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Amplitude and FrequencyModulation
0 0 1 1 0 0 1 1 0 0 0 1 1 1 0 0 0 1 1 0 0 0 1 1
1 0
0 1 1 0 1 1 0
0 0 1
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Assumptions

• We use two discrete signals, high and low, to
  encode 0 and 1
• The transmission is synchronous, i.e., there is a
  clock used to sample the signal
• In general, the duration of one bit is equal to
  one or more clock ticks

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Encoding

• Goal Send bits from one node to another node on
  the same physical media
• Problem Specify a robust and efficient encoding
  scheme to achieve this goal

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Encoding Schemes

• Non Return to Zero (NRZ)
• Non Return to Zero Inverted (NRZI)
• Manchester Encoding
• 4B/5B Encoding

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Modulation

• Non-Return to Zero (NRZ)
• Used by Synchronous Optical Network (SONET)
• 1high signal, 0low signal
• Long sequence of same bit cause difficulty
• DC bias hard to detect low and high detected by
  difference from average voltage
• Clock recovery difficult

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Show the NRZ encoding for the following pattern
1
0
0
0
1
0
0
0
1
1
1
1
1
0
0
1
Bits
clock
NRZ
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Modulation

• Non-Return to Zero Inverted (NRZI)
• 1inversion of current value, 0same value
• No problem with string of 1s
• NRZ-like problem with string of 0s

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Show the NRZI encoding for the following pattern
1
0
0
0
1
0
0
0
1
1
1
1
1
0
0
1
Bits
clock
NRZI
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Modulation

• Manchester
• Used by Ethernet
• 1low to high transition, 0high to low
  transition
• Transition for every bit simplifies clock
  recovery
• Not very efficient
• Doubles the number of transitions
• Circuitry must run twice as fast

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Modulation

• 4b/5b
• Used by FDDI
• Uses 5bits to encode every 4bits
• Encoding ensures no more than 3 consecutive 0s
• Uses NRZI to encode resulting sequence
• 16 data values, 3 special illegal values, 6
  extra values, 7 illegal values

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Chapter Summary

• Information can be transmitted via analog or
  digitally
• Both signals suffer attenuation
• Throughput is the amount of data a medium can
  transmit during a given period of time
• Costs depend on many factors
• Three specifications dictating networking media
• Length of a network segment is limited due to
  attenuation
• Connectors connect wire to the network device
• Coaxial cable consists of central copper core
  surrounded by an insulator and a sheath
• In baseband transmission, digital signals are
  sent through direct current pulse applied to the
  wire

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Chapter Summary

• Twisted-pair cable consists of color-coded pairs
  of insulated copper wires, twisted around each
  other and encased in plastic coating
• The more twists per inch in a pair of wires, the
  more resistant to noise
• STP cable consists of twisted pair wires
  individually insulated and surrounded by a
  shielding
• UTP cabling consists of one or more insulated
  wire pairs encased in a plastic sheath
• UTP comes in a variety of specifications
• Fiber-optic cable contains one or several glass
  fibers in its core
• On todays networks, fiber is used primarily as
  backbone cable

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Chapter Summary

• Best practice for installing cable is to follow
  the TIA/EIA 568 (see structured cabling)
  specifications and manufacturers recommendations
• Wireless LANs can use radio frequency (RF) or
  infrared transmission
• Infrared transmission can be indirect or direct
• RF transmission can be narrowband or spread
  spectrum
• To make correct media transmission choices,
  consider, throughput, cabling, noise resistance,
  security/flexibility, and plans for growth