Transmission Media

Transmission Media
Transmission media
 Transmission media
are located below
the physical layer
 Computers use
signals to represent
data.
 Signals are
transmitted in form
of electromagnetic
energy.
Transmission Media
Transmission Media and Physical Layer
Transmission Media
 Guided Media (Wired)
 Twisted-Pair Cable
 Coaxial Cable
 Fiber-Optic Cable
 Unguided Media (Wireless)
 Radio Waves
 Microwaves
 Infrared
Classes of transmission media
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.
Overview
 Guided - wire / optical fibre
 Unguided - wireless
 Characteristics and quality determined by
medium and signal
 in unguided media - bandwidth produced by
the antenna is more important
 in guided media - medium is more
important
 Key concerns are data rate and distance
Data Rate and Bandwidth
 Any transmission system has a limited
band of frequencies
 This limits the data rate that can be
carried
Design Factors
 Bandwidth
 higher bandwidth gives higher data rate
 Transmission impairments
 eg. attenuation
 Interference
 Number of receivers in guided media
 more receivers introduces more attenuation
Guided Media – Twisted-pair Cable
Twisted-pair cable
Twisted Pair
Twisted pair
 One of the wires carries signal,
the other is used only as a
ground reference.
 The receiver uses the
difference b/w the two levels.
 Twisting increases the
probability that both wires are
effected by the noise in the
same manner, thus the
difference at the receiver
remains same.
 Therefore, number of twists
per unit length determines the
quality of the cable.
Twisted Pair - Transmission
Characteristics
 analog
 needs amplifiers every 5km to 6km
 digital
 can use either analog or digital signals
 needs a repeater every 2-3km
 limited distance
 limited bandwidth (1MHz)
 limited data rate (100MHz)
 susceptible to interference and noise
Unshielded Versus Shielded Twisted-Pair Cable
UTP and STP cables

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)
Categories of unshielded twisted-pair cables
UTP Categories
Guided Media – UTP
UTP Connector
Guided Media - UTP
 Applications:
 Telephone lines connecting
subscribers to the central office
 DSL lines
 LAN – 10Base-T and 100Base-T
Twisted Pair - Applications
 Most common medium
 Telephone network
 Within buildings
 For local area networks (LAN)
Twisted Pair - Pros and Cons
 Cheap
 Easy to work with
 Low data rate
 Short range
Guided Media – Coaxial Cable
Coaxial Cable
Coaxial Cable
Coaxial cable
 Inner conductor is a
solid wire, outer
conductor serves both
as a shield against
noise
Coaxial Cable Applications
 Most versatile medium
 Television distribution
 Long distance telephone transmission
 Can carry 10,000 voice calls
simultaneously
 Short distance computer
systems links
 Local area networks
Coaxial Cable - Transmission
Characteristics
 superior frequency characteristics to TP
 performance limited by attenuation &
noise
 analog signals
 amplifiers every few km
 closer for higher frequency up to
500MHz
 digital signals
 repeater every 1km
 closer for higher data rates
Categories of coaxial cables
BNC Connectors
BNC connectors
 BNC = Bayone-Neill-
Concelman
 BNC Connector is used to
connect the end of the
cable to a device
 BNC T is used in
networks to branch out a
cable for connection to
a computer or other
 device
BNC Terminator is used
at the end of the cable to
prevent the reflection of
signal.
 Applications:
 Analog telephone networks
 Cable TV networks
 Traditional Ethernet LAN –
10Base2, 10Base5
Guided Media – Fiber-Optic Cable
Fiber-optic cable transmit signals in the form of light
Bending of light ray
 Angle of Incidence
(I): the angle the ray
makes with the line
perpendicular to the
interface between the
two substances
 Critical Angle: the
angle of incidence
which provides an
angle of refraction of
90-degrees.
Optic Fiber
Optical fiber
 Uses reflection to
guide light through a
channel
 Core is of glass or
plastic surrounded by
Cladding
 Cladding is of less
dense glass or plastic
Optical Fiber
Optical Fiber - Benefits
 greater capacity
 data rates of hundreds of Gbps
 smaller size & weight
 lower attenuation
 electromagnetic isolation
 greater repeater spacing
 10s of km at least
Optical Fiber - Transmission
Characteristics
 uses total internal reflection to transmit
light
 effectively acts as wave guide for 1014 to 1015
Hz
 can use several different light sources
 Light Emitting Diode (LED)
 cheaper, wider operating temp range, lasts longer
 Injection Laser Diode (ILD)
 more efficient, has greater data rate
 relation of wavelength, type & data rate
Propagation Modes
Propagation Modes
Optical Fiber Transmission Modes
Fiber Construction
Fiber-optic Cable Connectors
Guided Media – Optical Fiber Cable
 Applications:
 Backbone networks
 Cable TV – backbone
 LAN
 100Base-FX network (Fast Ethernet)
 100Base-X
Comparison of Physical Media
Comparison of Physical Media
Electromagnetic Spectrum
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
 local
Unguided Media
Electromagnetic spectrum for

wireless communication
Unguided Media
Propagation Methods
Bands
Unguided Media
Wireless transmission waves
Broadcast Radio
 radio is 3kHz to 300GHz
 use broadcast radio, 30MHz - 1GHz, for:
 FM radio
 UHF and VHF television
 is omnidirectional
 still need line of sight
 suffers from multipath interference
 reflections from land, water, other objects
Unguided Media – Radio Waves
Omnidirectional Antenna
 Frequencies between 3 KHz and

1 GHz.
 are used for multicasts
communications, such as radio and
television, and paging system.
Terrestrial Microwave
 used for long haul telecommunications
 and short point-to-point links
 requires fewer repeaters but line of sight
 use a parabolic dish to focus a narrow beam
onto a receiver antenna
 1-40GHz frequencies
 higher frequencies give higher data rates
 main source of loss is attenuation
 distance, rainfall
 also interference
Unguided Media – Microwaves
 Frequencies between 1 and 300 GHz.
 Used for unicast communication such as cellular phones, satellite
networks and wireless LANs.
Unidirectional Antenna
Satellite Microwave
 satellite is relay station
 receives on one frequency, amplifies or repeats
signal and transmits on another frequency
 eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz
 typically requires geo-stationary orbit
 height of 35,784km
 spaced at least 3-4° apart
 typical uses
 television
 long distance telephone
 private business networks
 global positioning
Unguided Media – Infrared
 Frequencies between 300 GHz to 400 THz.

 Can not penetrate walls.
Used for short-range communication in a
closed area using line-of-sight propagation.
Infrared
 modulate noncoherent infrared light
 end line of sight (or reflection)
 are blocked by walls
 no licenses required
 typical uses
 TV remote control
 IRD port
Antennas
 electrical conductor used to radiate or collect
electromagnetic energy
 transmission antenna
 radio frequency energy from transmitter
 converted to electromagnetic energy by antenna
 radiated into surrounding environment
 reception antenna
 electromagnetic energy impinging on antenna
 converted to radio frequency electrical energy
 fed to receiver
 same antenna is often used for both purposes
Radiation Pattern
 power radiated in all directions
 not same performance in all directions
 as seen in a radiation pattern diagram
 an isotropic antenna is a (theoretical)
point in space
 radiates in all directions equally
 with a spherical radiation pattern
Antenna Gain
 measure of directionality of antenna
 power output in particular direction verses
that produced by an isotropic antenna
 measured in decibels (dB)
 results in loss in power in another
direction
 effective area relates to size and
shape
 related to gain
Satellite Point to Point Link
Satellite Broadcast Link
Wireless Propagation
Ground Wave
Sky Wave
Line of Sight
Refraction
 velocity of electromagnetic wave is a function of
density of material
~3 x 108 m/s in vacuum, less in anything else
 speed changes as move between media
 Index of refraction (refractive index) is
 sin(incidence)/sin(refraction)
 varies with wavelength
 have gradual bending if medium density varies
 density of atmosphere decreases with height
 results in bending towards earth of radio waves
 hence o ptical and radio horizons
differ
Line of Sight Transmission
 Free space loss
 loss of signal with distance
 Atmospheric Absorption
 from water vapour and oxygen absorption
 Multipath
 multiple interfering signals from reflections
 Refraction
 bending signal away from receiver
Multipath Interference
Comparison of Media
 Medium Cost Speed Atten Interfere
 SecurityLow 1-100M
UTP High
 STP High Low Medium 1-150M High
 Coax Medium Low Medium 1M–1G
 Fibre Medium Medium
 Radio Low High 10M–2G Low
 Microwv High Low 1M–10G Varies High
High Medium
Medium 1-10M
 SatelliteVaries
High 1 M–10G Varies
High High
Medium Low
 Cellular High 9.6–19.2K Low Medium
THE - END

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Transmission Media

Guided media, which are those that provide a conduit

UTP and STP cables

Electromagnetic spectrum for

 Frequencies between 3 KHz and

 Frequencies between 300 GHz to 400 THz.

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