COE 341: Data

1
COE 341 Data Computer Communications
(T081)Dr. Marwan Abu-Amara

• Chapter 4
• Transmission Media

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Agenda

• Overview
• Guided Transmission Media
• Twisted Pair
• Coaxial Cable
• Optical Fiber
• Wireless Transmission
• Antennas
• Terrestrial Microwave
• Satellite Microwave
• Broadcast Radio
• Infrared

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Overview

• Media
• Guided - wire
• Unguided - wireless
• Transmission characteristics and quality
  determined by
• Medium
• Signal
• For guided, the medium is more important
• For unguided, the bandwidth produced by the
  antenna is more important
• Key concerns are data rate and distance

4
Design Issues

• Key communication objectives are
• High data rate
• Low error rate
• Long distance
• Bandwidth Tradeoff - Larger for higher data
  rates
• - But smaller for economy
• Transmission impairments
• Attenuation Twisted Pair gt Cable gt Fiber (best)
• Interference Worse with unguided (medium is
  shared!)
• Number of receivers
• In multi-point links of guided media
• More connected receivers introduce more
  attenuation

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Electromagnetic Spectrum
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Study of Transmission Media

• Physical description
• Main applications
• Main transmission characteristics

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Guided Transmission Media

• Twisted Pair
• Coaxial cable
• Optical fiber

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Transmission Characteristics of Guided Media
 
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Twisted Pair
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UTP Cables
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UTP Connectors
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Note Pairs of Wires

• It is important to note that these wires work in
  pairs (a transmission line)
• Hence, for a bidirectional link
• One pair is used for TX
• One pair is used for RX

13
Twisted Pair - Applications

• Most commonly used guided medium
• Telephone network (Analog Signaling)
• Between house and local exchange (subscriber
  loop)
• Within buildings (Digital Signaling)
• To private branch exchange (PBX)
• For local area networks (LAN)
• 10Mbps or 100Mbps
• Range 100m

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Twisted Pair - Pros and Cons

• Pros
• Cheap
• Easy to work with
• Cons
• Limited bandwidth
• Low data rate
• Short range
• Susceptible to interference and noise

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Twisted Pair - Transmission Characteristics

• Analog Transmission
• Amplifiers every 5km to 6km
• Digital Transmission
• Use either analog or digital signals
• Repeater every 2km or 3km
• Limited distance
• Limited bandwidth (1MHz)
• Limited data rate (100Mbps)
• Susceptible to interference and noise

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Attenuation in Guided Media
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Ways to reduce EM interference

• Shielding the TP with a metallic braid or
  sheathing
• Twisting reduces low frequency interference
• Different twisting lengths for adjacent pairs
  help reduce crosstalk

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Unshielded and Shielded TP

• Unshielded Twisted Pair (UTP)
• Ordinary telephone wire
• Cheapest
• Easiest to install
• Suffers from external EM interference
• Shielded Twisted Pair (STP)
• Metal braid or sheathing that reduces
  interference
• More expensive
• Harder to handle (thick, heavy)

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STP Metal Shield
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UTP Categories

• Cat 3
• up to 16MHz
• Voice grade found in most offices
• Twist length of 7.5 cm to 10 cm
• Cat 4
• up to 20 MHz
• Cat 5
• up to 100MHz
• Commonly pre-installed in new office buildings
• Twist length 0.6 cm to 0.85 cm
• Cat 5E (Enhanced) see tables
• Cat 6
• Cat 7

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Near End Crosstalk (NEXT)

• Coupling of signal from one wire pair to another
• Coupling takes place when a transmitted signal
  entering a pair couples back to an adjacent
  receiving pair at the same end
• i.e. near transmitted signal is picked up by near
  receiving pair

NEXT Attenuation 10 log P1/P2 dBs The larger
the smaller the crosstalk (i.e. the better the
performance)
NEXT attenuation is a desirable attenuation - The
larger the better!
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Transmission Properties for Shielded Unshielded
TP
Undesirable Attenuation- Smaller is better
Desirable Attenuation- Larger is better!
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Twisted Pair Categories and Classes
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Coaxial Cable
Physical Description
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Physical Description
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Coaxial Cable Applications

• Most versatile medium
• Television distribution
• Cable TV
• Long distance telephone transmission
• Can carry 10,000 voice calls simultaneously
  (though FDM multiplexing)
• Being replaced by fiber optic
• Short distance computer systems links
• Local area networks (thickwire Ethernet cable)

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Coaxial Cable - Transmission Characteristics

• Analog
• Amplifiers every few km
• Closer if higher frequency
• Up to 500MHz
• Digital
• Repeater every 1km
• Closer for higher data rates

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Attenuation in Guided Media
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Optical Fibers

• An optical fiber is a very thin strand of silica
  glass
• It is a very narrow, very long glass cylinder
  with special characteristics. When light enters
  one end of the fiber it travels (confined within
  the fiber) until it leaves the fiber at the other
  end
• Two critical factors stand out
• Very little light is lost in its journey along
  the fiber
• Fiber can bend around corners and the light will
  stay within it and be guided around the corners
• An optical fiber consists of three parts
• The core
• Narrow cylindrical strand of glass with
  refractive index n1
• The cladding
• Tubular jacket surrounding the core with
  refractive index n2
• The core must have a higher refractive index than
  the cladding for the propagation to happen

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Optical Fibers (Contd.)

• Protective outer jacket
• Protects against moisture, abrasion, and crushing

Individual Fibers (Each having core Cladding)
Multiple Fiber Cable
Single Fiber Cable
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Reflection and Refraction

• At a boundary between a denser (n1) and a rarer
  (n2) medium, n1 gt n2 (e.g. water-air, optical
  fiber core-cladding) a ray of light will be
  refracted or reflected depending on the incidence
  angle

Increasing Incidence angle,
?1
?2
rarer
v2 c/n2
n2
denser
?1
?2
n1
?critical
?1
n1 gt n2
v1 c/n1
Total internal reflection
Critical angle refraction
Refraction
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Optical Fiber
Refraction at boundary for .
Escaping light is absorbed in jacket
?i lt
?critical
n2
Rarer
n1
Denser
Denser
n1
Rarer
?i
Total Internal Reflection at boundary for
?i gt
?critical
n1 gt n2
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Attenuation in Guided Media
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Optical Fiber - Benefits

• Greater capacity
• Data rates of hundreds of Gbps
• Smaller size weight
• Lower attenuation
• An order of magnitude lower
• Relatively constant over a larger frequency
  interval
• Electromagnetic isolation
• Not affected by external EM fields
• No interference, impulse noise, crosstalk
• Does not radiate
• Not a source of interference
• Difficult to tap (data security)
• Greater repeater spacing
• 10s of km at least

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Optical Fiber - Applications

• Long-haul trunks
• Metropolitan trunks
• Rural exchange trunks
• Subscriber loops
• LANs

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Optical Fiber - Transmission Characteristics

• Act as wave guide for light (1014 to 1015 Hz)
• Covers portions of infrared and visible spectrum
• Light Emitting Diode (LED)
• Cheaper
• Wider operating temp range
• Last longer
• Injection Laser Diode (ILD)
• More efficient
• Greater data rate

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Optical Fiber Transmission Modes
Dispersion Spread in arrival time
Refraction
Shallow reflection
Deep reflection
n2
n1
Large
Core
Cladding
2 ways
Smaller

• v c/n
• n1 lower away from centerthis speeds up deeper
  rays
• and compensates for their larger distances,
  arrive together with shallower rays

Smallest
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Optical Fiber Transmission modes

• Spread of received light pulse in time
  (dispersion) is bad
• Causes inter-symbol interference ? bit errors
• Limits usable data rate and usable distance
• Caused by propagation through multiple
  reflections at different angles of incidence
• Dispersion increases with
• Larger distance traveled
• Thicker fibers with step index
• Can be reduced by
• Limiting the distance
• Thinner fibers and a highly focused light source
• ? Single mode High data rates, very long
  distances
• Graded-index thicker fibers The half-way solution

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Optical Fiber Wavelength Division Multiplexing
(WDM)

• A form of FDM (channels sharing the medium by
  occupying different frequency bands)
• Multiple light beams at different frequencies
  (wavelengths) transmitted on the same fiber
• Each beam forms a separate communication channel
• Example
• 256 channels _at_ 40 Gbps each ? 10 Tbps total data
  rate

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Optical Fiber Four Transmission bands (windows)
in the Infrared (IR) region

• Selection based on
• Attenuation of the fiber
• Properties of the light sources
• Properties of the light receivers

S
L
C
Bandwidth, THz
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12
4
7
Note l in fiber v/f (c/n)/f (c/f)/n l in
vacuum/n i.e. l in fiber lt l in vacuum
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Attenuation in Guided Media
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Wireless Transmission

• Free-space is the transmission medium
• Need efficient radiators, called antenna, to take
  signal from transmission line (wireline) and
  radiate it into free-space (wireless)
• Famous applications
• Radio TV broadcast
• Cellular Communications
• Microwave Links
• Wireless Networks

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Wireless Transmission Frequencies

• Radio 30MHz to 1GHz
• Omni-directional
• Broadcast radio
• Microwave 2GHz to 40GHz
• Microwave
• Highly directional
• Point to point
• Satellite
• Infrared Light 3 x 1011 to 2 x 1014
• Localized communications

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Antennas

• Electrical conductor (or system of..) used to
  radiate/collect electromagnetic energy
• Transmission
• Radio frequency electrical energy from
  transmitter
• Converted to electromagnetic energy by antenna
• Radiated into surrounding environment
• Reception
• Electromagnetic energy impinging on antenna
• Converted to radio frequency electrical energy
• Fed to receiver
• Same antenna often used for both TX and RX in
  2-way communication systems

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Radiation Pattern

• Power radiated in all directions
• Not same performance in all directions
• Isotropic antenna is (theoretical) point in space
• Radiates in all directions equally
• Gives spherical radiation pattern
• Used as a reference for other antennae
• Directional Antenna
• Concentrates radiation in a given desired
  direction
• Used for point-to-point, line of sight
  communications
• Gives gain in that direction
• relative to isotropic

Radiation Patterns
Isotropic
Directional
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Parabolic Reflective Antenna

• Used for terrestrial and satellite microwave
• Source placed at focus will produce waves
  reflected from parabola parallel to axis
• Creates (theoretical) parallel beam of
  light/sound/radio
• In practice, some divergence (dispersion) occurs,
  because source at focus has a finite size (not
  exactly a point!)
• On reception, signal is concentrated at focus,
  where detector is placed
• The larger the antenna (in wavelengths) the
  better
• the directionality

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Parabolic Reflective Antenna
Axis
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Antenna Gain, G

• Measure of directionality of antenna
• Power output in particular direction compared
  with that produced by isotropic antenna
• Measured in decibels (dB)
• Increased power radiated in one direction causes
  less power radiated in another direction (Total
  power is fixed)
• Effective area, Ae, relates to size and shape of
  antenna
• Determines antenna gain

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Antenna Gain, G Effective Areas

• An isotropic antenna has a gain G 1 (0 dBi)
• i.e.
• A parabolic antenna has
• Substituting we get
• Gain in dBi 10 log G
• Important Gains apply to both TX and RX antennas

A Actual Area p r2
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Terrestrial Microwave

• Parabolic dish
• Focused beam
• Line of sight
• Curvature of earth limits maximum range ? Use
  relays to increase range (multi-hop link)
• Long haul telecommunications
• Higher frequencies give higher data rates but
  suffers from larger attenuation

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Terrestrial Microwave Propagation Attenuation

• As signal propagates in space, its power drops
  with distance according to the inverse square law

While with a guided medium, signal drops
exponentially with distance giving larger
attenuation and lower repeater spacing
d distance in ls
i.e. loss in signal power over distance traveled,
d

• Show that L increases by 6 dBs for every
  doubling of distance d.
• For guided medium, corresponding attenuation a
  d dBs, a dBs/km

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Satellite Microwave

• Satellite is relay station
• Satellite receives on one frequency (uplink),
  amplifies or repeats signal and transmits on
  another frequency (downlink)
• Requires geo-stationary orbit
• Height of 35,784km
• Applications
• Television
• Long distance telephone
• Private business networks

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Satellite Point to Point Link
Relay
Downlink
Uplink
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Satellite Broadcast Link
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Broadcast Radio

• Omni-directional
• No dishes
• No line of sight requirement
• No antenna alignment
• Applications
• FM radio
• UHF and VHF television
• Suffers from multipath interference
• Reflections (e.g. TV ghost images)

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Infrared

• Modulate non-coherent infrared light
• Line of sight (or reflection)
• Blocked by walls
• No licensing required for frequency allocation
• Applications
• TV remote control
• IRD port