4123702
Data Communications System
By
Ajarn Preecha Pangsuban
Chapter 7 – Transmission Media
Relation to Internet Model
 Actually located below the physical layer
 Directly controlled by the physical layer
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Introduction
 Data must be converted into electromagnetic
signals to be transmitted from device to device
 Signals can travel through a vacuum, air, or other
media
 May be in the form of power, voice, radio waves,
infrared light, and X, gamma, and cosmic rays
 Each of these forms constitutes a portion of the
electromagnetic spectrum
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Electromagnetic Spectrum
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Categories of Media
 Guided Media
Twisted pair, coaxial, and fiber-optic
 Unguided Media
Radio waves, infrared light, visible light, and X,
gamma, and cosmic rays
Sent by microwave, satellite, and cellular
transmission



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Guided Media
 Twisted-pair and coaxial use copper conductors
to accept and transport signals in form of
electrical current
 Optical fiber is glass or plastic cable that accepts
and transports signals in form of light
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Twisted-Pair Cable
 Two conductors surrounded by insulating material
 One wire used to carry signals; other used as a
ground reference
 Twisting wires reduces the effect of noise
interference or crosstalk since both wires will likely be
equally affected
 More twists = better quality
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Effect of noise on parallel line
16
Effect of noise on parallel line
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Effect of noise on twisted-pair lines
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Unshielded Twisted Pair (UTP)
 Most common type; suitable for both voice and
data transmission
 Frequency range is from 100 Hz to 5 MHz
 Categories are determined by cable quality


Cat 3 commonly used for telephone systems (up to 10
Mbps)
Cat 5e usually used for data networks (up to 100
Mbps)
 Performance is measured by attenuation versus
frequency and distance
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UTP example
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UTP (cont)
 Adv: cheaper, flexible, easy to install
 UTP connectors attach to wire; most commonly
used is RJ45, which have 8 conductors, one for
each of four twisted pairs
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Shielded Twisted Pair (STP)
 By IBM
 A metal foil or braided-mesh covering encases
each pair of insulated conductors to prevent
electromagnetic noise called crosstalk
 Uses same connectors
 More expensive but less susceptible to noise
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STP example
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Categories of unshielded twisted-pair cables
Category
Max Speed
Digital/Analog
Use
1
< 100 kbps
Analog
Telephone
2
2 Mbps
Analog/digital
T-1 lines
3
10 Mbps
Digital
LANs
4
16 Mbps
Digital
LANs
5e
1 Gbps
Digital
LANs
6
1 Gbps
Digital
LANs
7 (Draft)
10 Gbps
Digital
LANs
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Color code for 4 cable pairs
Pair #
1
2
3
4
Primary color Secondary color (stripe)
Blue
White
Orange
White
Green
Brown
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White
White
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UTP connector
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The T568A and T568B wiring patterns
T568A Jack
1
2
3
4
5
6
7
8
W/G
G
W/O
B
W/B
O
W/Br
Br
Pair 3
Pair 1 Pair 2
Pair 4
8-pin modular jack
(front view)
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T568B Jack
1
2
3
4
5
6
7
8
W/O
O
W/G
B
W/B
G
W/Br
Br
Pair2
Pair 1 Pair 3
Pair 4
8-pin modular jack
(front view)
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UTP performance
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Coaxial Cable
 Has a central core conductor enclosed in an insulating
sheath, encased in an outer conductor of metal foil
 Frequency range is between 100 KHz and 500 MHz
 RG numbers denote physical specs such as




wire gauge
thickness and type of insulator
construction of shield
size/type of outer casing
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RG numbers
 RG-8, RG-9, and RG-11 used in thick
Ethernet (50)
 RG-58 used in thin Ethernet (50)
 RG-59 used for TV (75)
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Coaxial Cable example
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Coaxial Cable Connectors
 Most common is barrel connector (BNC)
 T-connectors are used to branch off to secondary
cables
 Terminators are required for bus topologies to
prevent echoing of signals
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Coaxial Performance
 Higher bandwidth than twisted-pair
 However, attenuation is higher and requires
frequent use of repeaters
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Coaxial Applications
 Analog telephone networks ,

single coaxial network could carry 10,000 voice
signals
 Digital telephone networks ,

single coaxial network could carry digital data up to
600 Mbps
 Cable TV networks
 Traditional Ethernet LANs
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Coaxial cable performance
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Fiber-Optic Cable
 Made of glass or plastic;
signals are transmitted as light pulses from an LED
or laser

 Speed depends on density of medium it is
traveling through;

fastest when in a vacuum, 186,000 miles/second
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Refraction
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Critical Angle
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Reflection
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Fiber Construction (1)
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Fiber Construction (2)
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Propagation Modes
 Method for transmitting optical signals:

Multimode



Multimode step-index fiber
Multimode graded-index fiber
Single Mode
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Propagation Modes (cont)
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Multimode
 Multiple beams from light source move through
core at different paths
 Multimode step-index fiber




Index refers to the index of refraction
Density remains constant from center to edges
Light moves in a straight line until it reaches the
cladding
Some beams penetrate the cladding and are lost,
while others are reflected down the channel to the
destination
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Multimode (cont)

As a result, beams reach the destination at different
times and the signal may not be the same as that
which was transmitted
 Multimode graded-index fiber
May be used to decrease this problem and to allow
for more precise transmissions
Graded-index refers to varying densities of the fiber;
highest at center and decreases at edge


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Multimode (cont)


Since the core density decreases with
distance from the center, the light beams
refract into a curve
Eliminates problem with some of the signals
penetrating the cladding and being lost
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Single Mode
 Only one beam from a light source is transmitted
using a smaller range of angles
 Smaller diameter
 Makes propagation of beams almost horizontal;

delays are negligible
 All beams arrive together and can be recombined
without signal distortion
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Light Sources
 Light source is light-emitting diode (LED) or a
laser


LEDs are cheaper but not as precise (unfocused);
limited to short-distance use
Lasers can have a narrow range, better control over
angle
 Receiving device needs a photosensitive cell
(photodiode) capable of receiving the signal
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Fiber types
Size
Core
Cladding
Mode
50/125
50
125
Multimode, graded-index
62.5/125
62.5
125
Multimode, graded-index
100/125
100
125
Multimode, graded-index
7
125
Single-mode
7/125
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Fiber Sizes
Single mode
7/125
Multimode
50/125
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Multimode
62.5/125
Multimode
100/125
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Packing methods for fiber cable
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Fiber-Optic cable connectors (1)
 Subscriber Channel (SC) Connection
It uses Push/pull locking system mechanism
Used in cable TV
 Straight-tip (ST) Connection
It uses a bayonet locking system
Used for connecting cable to networking devices
More reliable than SC
 MT-RJ
New connector with the same as RJ45






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Fiber-Optic cable connectors (2)
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Optical fiber performance
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Applications of Fiber Optics
 Backbone networks due to wide bandwidth and
cost effectiveness
 Up to 1600 Gbps with WDM (Synchronous
Optical Network : SONET)
 Cable TV
 LANS
100Base-FX (Fast Ethernet)
1000Base-X


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Advantages of Fiber Optics
 Higher bandwidth than twisted-pair and coaxial






cable; not limited by medium, but by equipment
used to generate and receive signals
Noise resistance
Less signal attenuation
Immunity to EMI
More resistant to corrosive materials
Lightweight
Greater security (More immune to tapping)
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Disadvantages of Fiber Optics
 Installation/maintenance
Relatively new technology
need expertise
 Unidirectional, bidirectional communication two


fibers are needed
 Cost


Cable and interface are relatively more expensive
Bandwidth is not high, often the use of fiber cannot
justified
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Unguided Media: Wireless
 Wireless communication; transporting
electromagnetic waves without a physical conductor
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Wireless Propagation Methods
 Ground – radio waves travel through lowest
portion of atmosphere, hugging the Earth
 Sky – higher-frequency radio waves radiate
upward into ionosphere and then reflect back to
Earth
 Line-of-sight – high-frequency signals transmitted
in straight lines directly from antenna to antenna
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Propagation Methods
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Bands
Band
VLF
Range
Propagation
Application
3–30 KHz
Ground
Long-range radio navigation
LF
30–300 KHz
Ground
Radio beacons and
navigational locators
MF
300 KHz–3 MHz
Sky
AM radio
HF
3–30 MHz
Sky
Citizens band (CB),
ship/aircraft communication
VHF
30–300 MHz
Sky and
line-of-sight
VHF TV, FM radio
UHF
300 MHz–3 GHz
Line-of-sight
UHF TV, cellular phones,
paging, satellite
SHF
3–30 GHz
Line-of-sight
Satellite communication
EHF
30–300 GHz
Line-of-sight
Long-range radio navigation
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Wireless Transmission Waves
 Radio Waves
 Microwave
 Infrared
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Radio Waves
 Frequency ranges: 3 KHz to 1 GHz
 Omnidirectional
 Susceptible to interference by other antennas
using same frequency or band
 Ideal for long-distance broadcasting
 May penetrate walls
 Apps: AM and FM radio, TV, maritime radio,
cordless phones, paging
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Omnidirectional antennas
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Microwaves
 Frequencies between 1 and 300 GHz
 Unidirectional
 Narrow focus requires sending and receiving
antennas to be aligned
 Issues:


Line-of-sight (curvature of the Earth; obstacles)
Cannot penetrate walls
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Terrestrial Microwave
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Parabolic Dish Antenna
 Incoming signals - Signal
bounces off of dish and is
directed to focus
 Outgoing signals –
transmission is broadcast
through horn aimed at
dish and are deflected
outward
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Horn Antenna
 Outgoing transmissions
broadcast through a stem
and deflected outward
 Received transmissions
collected by a scooped
part of the horn and
deflected downward into
the stem
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Microwave Applications
 Unicasting – one-to-one communication between
sender and receiver



Cellular phones
Satellite networks
Wireless LANs
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Satellite Microwave
 Similar to terrestrial microwave except the signal





travels from a ground station on earth to a satellite
and back to another ground station.
Satellite receives on one frequency, amplifies or
repeats signal and transmits on another frequency
Satellite is relay station
Television
Long distance telephone
Private business networks
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Satellite as a repeater
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Satellite frequency band
Band
Downlink
(GHz)
Uplink
(GHz)
C
3.7 to 4.2 GHz
Ku
11.7 to 12.2 GHz
14 to 14.5 GHz
500
Ka
17.7 to 21 GHz
27.5 to 31 GHz
3500
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5.925 to 6.425 GHz
Bandwidth,
(MHz)
500
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Infrared
 Frequencies between 300 GHz and 400 THz
 Short-range communication
 High frequencies cannot penetrate walls
 Requires line-of-sight propagation
 Adv: prevents interference between systems in
adjacent rooms
 Disadv: cannot use for long-range communication
or outside a building due to sun’s rays
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Infrared Applications
 Wide bandwidth available for data transmission
 Communication between keyboards, mice, PCs,
and printers
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Bluetooth
 Bluetooth is a Radio Frequency specification for
short-range, point-to-multipoint voice and data
transfer.
 Bluetooth can transmit through solid, non-metal
objects.
 Its typical link range is from 10 cm to 10 m, but
can be extended to 100 m by increasing the
power.
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Bluetooth
 Bluetooth will enable users to connect to a wide
range of computing and telecommunication
devices without the need of connecting cables.
 Typical uses include phones and pagers,
modems, LAN access devices, headsets,
notebooks, desktop computers, and PDAs.
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WAP (Wireless Application Protocol)
 WAP allows wireless devices such as mobile
telephones, PDAs, pagers, and two-way radios to
access the Internet.
 WAP is designed to work with small screens and
with limited interactive controls.
 WAP incorporates Wireless Markup Language
(WML) which is used to specify the format and
presentation of text on the screen.
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WAP
 WAP may be used for applications such as:
travel directions
sports scores
e-mail
online address books
traffic alerts
banking
news







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Broadband Wireless Systems
 Delivers Internet services into homes and
businesses.
 Designed to bypass the local loop telephone line.
 Transmits voice, data and video over high
frequency radio signals.
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Credits
 All figures obtained from publisher-provided
instructor downloads
Data Communications and Networking, 3rd edition by
Behrouz A. Forouzan. McGraw Hill Publishing, 2004
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