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Department of Computer Science
Southern Illinois University Carbondale
Wireless & Network Security
Lecture 3: Radio Basics & Wireless
Networks
Dr. Kemal Akkaya
E-mail: kemal@cs.siu.edu
© Kemal Akkaya
Wireless & Network Security
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Radio Basics (RF)
 Already seen how a radio signal looks like
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Sinusoids
Carrier wave
Information Signal
Signal is modulated onto carrier wave
Carrier has more bandwidth than the info itself
 Radio Waves
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Frequency Range :3 KHz to 300 GHz
Easy to generate
Can travel long distances
Can penetrate buildings
They are both used for indoor and outdoor communication
They are omni-directional: can travel in all directions
They can be narrowly focused at high frequencies (greater than 100MHz)
using parabolic antennas (like satellite dishes)
 All signals converted to analog
 Unguided media allows analog transmission only
 Analog Signal usage: TV, Radio
 Digital Signal usage: Cell Phone, Wireless Network
 Can be transmitted through antennas
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 Electrical conductor
Antennas
 Transmits (radiates) electromagnetic waves into space
 Receives electromagnetic waves from space
 Same antenna can be used as both transmitter and receiver
 Radiation Pattern of an Antenna
 The graphical representation of radiation in all directions in the space
 What is the ideal radiation pattern?
 Radiate equally in all directions in the space
 Sun is the best example
 Omni directional radiation pattern
 Real antennas are not isotropic
 Dipoles
 Half-wave dipoles (Hertz)
 Quarter-wave dipoles (Marconi)
 Reflective parabolic
λ/2
Isotropic Radiator
Half-wave dipole
 In satellite applications
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Wireless & Network Security
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Directional Antennas
 Directional antennas are very common
 Radiation pattern in a certain direction
 Often used for base stations in a cellular system
Antenna
Omni-directional
 Beam width (half-power beam width)
 Measure of directivity of antenna
 Angle within which power radiated is at least half of
what it is in the most preferred direction
Directional
 Antenna gain
 Power output, in a particular direction, compared to that
produced in any direction by a perfect omni-directional
antenna (isotropic antenna)
 Measured in dBi :decibels relative to an isotropic
radiator
 A gain of 3dB means:
Beam Width
 Antennas improves the signal upon the isotropic antenna
in that direction by 3dB
y
y
x
side view (xy-plane)
z
z
side view (yz-plane)
x
directed
antenna
top view (xz-plane)
Courtesy www.superpass.com
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Wireless & Network Security
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Radio Propagation
 Signal Propagation Ranges
 Transmission range
Courtesy Dr. Y. Richard Yang
 Communication possible
 Low error rate
 Detection range
 Detection of the signal possible
 No communication possible
 Interference range
sender
transmission
distance
 Signal may not be detected
 Signal adds to the background noise
detection
interference
 Wireless Propagation Modes
 How a signal radiated from an antenna travels? There are three
different routes:
 Ground-wave propagation
 Sky-wave propagation
 Line-of-sight propagation (LOS)
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Propagation Modes
 Radio signal behaves like light in
free space
 Ground Wave
 Frequencies up to 2 MHz
 Follows contour of the earth
 Example: AM Radio
 Sky Wave
 Signal reflected from ionosphere and
earth’s surface
 Can travel thousands of kilometers
 Frequency: 2-30MHz
 Amateur Radio, Military Comm.
 Line of Sight
 Transmitting and receiving antennas
must be within line of sight
 Frequency: More then 30MHz
 TV, satellite, optical comm.
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Impairments in LOS Transmission
 In any system the signal received is different than the signal transmitted
 Impairments that degrade analog signal quality
 Errors in digital signal
 Most common impairments in general (wired, wireless)
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Attenuation and Attenuation distortion
Free Space Loss
Noise
Thermal noise
Atmospheric absorption
Multi-path propagation
Fading
Refraction
Reflection, Scattering, Shadowing, Diffraction
 Receiving power of the signal depends on these factors
 A signal may arrive at a receiver  Multi-path and fading!!
 many different times
 many different directions
 due to vector addition
 Reinforce
 Cancel
 signal strength differs from place to place
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What is Fading?
 The main problem in wireless transmission
 Definition:
 Time variation of received signal power caused by changes in the
transmission medium or path (s).
 In fixed environment is caused by atmospheric conditions
 In mobile environments creates more complex effects
LOS pulses
Multipath pulses
Courtesy Dr. Y. Richard Yang
signal at sender
 Causes of fading:
signal at receiver
 Free space loss
 Multi-path propagation
 Interference with other transmitters
 Atmospheric absorption
 Reflection, scattering, diffraction,  Mobility
refraction
© Kemal Akkaya
 Fast fading, small fading
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Free Space Loss
 Main source of attenuation in Wireless Transmission
 Any type of signal disperses with distance as signal is being spread
over larger and larger area
 Can be expressed in terms of decibels
Pr
2
c2
1
L


K* 2 2
2
2
Pt
(4d )
(4fd )
f d
 Here Pr is the mean received
signal power
 Pt is the transmitted signal power
 f is the frequency of the signal
 d is the distance between
transmitter and receiver
 It is inversely proportional to d2
for free space
 Can be up to
environments
© Kemal Akkaya
d4
Environment
Free space
Urban area cellular radio
Shadowed urban cellular radio
Path Loss Exponent, n
2
2.7 to 3.5
3 to 5
In building line-of-sight
1.6 to 1.8
Obstructed in building
4 to 6
Obstructed in factories
2 to 3
for different
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Multi-path Propagation
 Reflection:
 When signal
encounters a surface
that is large relative to
the wavelength of the
signal
 Scattering:
 Occurs when incoming
signal hits an object
whose size in the order
of the wavelength of
the signal or less
 Diffraction:
 Occurs at the edge of
an impenetrable body
that is large compared
to wavelength of radio
wave
Reflection, scattering, diffraction
Courtesy Dr. Y. Richard Yang
 Refraction:
reflection
refraction
© Kemal Akkaya
scattering
diffraction
Wireless & Network Security
 Bending of radio waves
as they propagate
through the
atmosphere
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Mobility effects
 As the user moves, signal paths may change
 Distance to sender will change
 Obstacles will further away
 Fast Fading
 As the mobile moves over small distances, the instantaneous received
signal will fluctuate rapidly giving rise to small-scale fading
 The reason is that the signal is the sum of many contributors coming
from different directions and since the phases of these signals are
random, the sum behave like a noise
 Occurs when receiver moves only about one half of the wavelength
 Slow Fading
 As the mobile moves away from the transmitter over larger distances,
the local average received signal will gradually decrease. This is called
large-scale fading
 As the receiver covers distances larger than the wavelength
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Slow and Fast Fading in an Urban
Mobile Environment
Received Power (dBm)
Slow Fading
-30
-40
-50
-60
Fast Fading
-70
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Distance between transmitter and receiver (m)
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 Motivation:
Spread Spectrum
 In radio transmission, sometimes narrow band signals can be wiped out
 Reasons:
 Fading due to multi path propagation
 Interference from other devices
 To solve this problem:
 We can transmit at many different frequencies during transmission
 Spread transmission over a wider bandwidth
 Initial motivation:
 To prevent jamming and interference in military applications
 What is jamming?
 To send noise like signals to distort information signals
 Need to know the frequency of the information signal
 Used in every wireless networks today
 Eliminates interference and multi-path effects
 Multiple transmitters can transmit at same frequency range
 Actually FCC requires it for signals in ISM band having a certain
transmission power
© Kemal Akkaya
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Frequency Hopping Spread Spectrum
 Simplest approach: FHSS
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Discrete changes of carrier frequency
There are multiple base frequencies (or channels)
Transmitter randomly hops to one of those frequencies
Receiver should to the same thing
There is a shared spreading code between transmitter and receiver
 Spreading code = Hopping Sequence
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Types of FHSS
 Two versions of FHSS
 Fast Hopping: several frequencies per user bit
 Slow Hopping: several user bits per frequency
tb
user data
0
1
f
0
1
1
t
td
f3
slow
hopping
(3 bits/hop)
f2
f1
t
td
f
f3
fast
hopping
(3 hops/bit)
f2
f1
t
tb: bit period
© Kemal Akkaya
td: dwell time
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Spread Code
 How to share the spread code?
 Use predefined sequences
 Sequence 1 <1, 12, 23, 3, 5…>
 Receiver listens a fixed frequency channel specifically for the
sequence
 IEEE 802.11 uses 96 1 MHz Channels
 Dwell time is around 390ms
 History of Spread Spectrum
 First invented by Hollywood Actress, Heidy Lamarr and George
Antheil, an avant gard composer in 1940s
 Advantages
 Frequency selective fading and interference limited to short period
 Say you have 80 channels and 1 is knocked by interference
 The other 79 are still free and ready to be used
 Jammer must jumped each frequency
 Simple implementation
 Uses only small portion of spectrum at any time
© Kemal Akkaya
Wireless & Network Security
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Direct Sequence Spread Spectrum (DSSS)
 Each bit in original signal is represented by multiple bits
in the transmitted signal
 Transmitted signal is code CHIP
 Spreading code spreads signal across a wider frequency
band
 Spread is in direct proportion to number of bits used
 Uses a pseudorandom bit sequence
 One technique combines digital information stream with
the spreading code bit stream using exclusive-OR (XOR)
 802.11 DS PHY uses Barker Sequence
 11-to-1 Spreading Ratio: 1 bit  11 bits
 DSSS is very resilient to interference
 Many chips can be corrupted before the bits are lost
 Multiple users can share the bandwidth
 By using different chipping sequences
 No need to allocate different frequencies
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Wireless & Network Security
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Example for DSSS
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Mobile and Wireless Data Networks
 Experiencing a tremendous growth over the last decade
 Wide deployment of access infrastructure
 In-door, out-door, MAN, WAN
 Growth of Wireless Data
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Miniaturization of computing machinery : laptop  PDA  embedded sensors
Increasing mobile work force
Luxury of tether less computing
Information on demand anywhere/anyplace
 Some Facts:
 In 2005, more than 1/3rd of internet users had internet connectivity through a
wireless enabled device (750 million users)!!! (Source: Intermarket group)
 In the year 2004 revenue from wireless data was $34B, and by the year 2010
the number of wireless data subscribers will hit 1B!!
 What is Mobile and Wireless Computing?
 Distributed systems with portable computers and wireless communications
 User can access data anytime, anywhere
© Kemal Akkaya
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Buzzwords
 Mobile Computing
 Distributed systems with mobile users
 In-door/Out-door
 Vehicle/human speed
 Nomadic Computing
 Similar to Mobile computing
 Focuses more on in-door communications
 Pervasive Computing : Ubiquitous Computing
 May add some user interface integration
 Some says : AI + Mobile Computing stuff
 Applications:
 Military
 Border control, target tracking, intrusion detection etc.
 Civil
 Habitat monitoring, search and rescue, meeting rooms etc.
© Kemal Akkaya
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Wireless Network Types
 Satellite networks
 Iridium (66 satellites)
 Qualcomm’s Globalstar (48 satellites)
 Wireless WANs/MANs
 CDPD (Cellular Digital Packet Data )
 GPRS (General Packet Radio Service)
Wireless WAN Generations:
 1G (Past)
 AMPS, TACS: No data
 2G (Past/Present)
 IS-136, GSM: <10Kbps
circuit switched data
 2.5G (Past/Present)
 Wireless LANs
 GSM-GPRS, GPRS-136:
<100Kbps packet
switched
 IEEE 802.11 : SIU’s LAWN,
 Wireless PANs
 e.g. Infrared: Bluetooth
 Mobile Ad-hoc networks
 3G (Present/Immediate
Future)
 IMT-2000: <2Mbps packet
switched
 e.g. Emergency relief, military
 Sensor networks
 4G (Future)
 e.g. Environmental sensing-MICA
motes
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Wireless & Network Security
 20-40 Mbps!!
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Examples
802.11 / WiFi
Wireless LAN
PicoNet
Bluetooth
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Applications: Home Networking
Courtesy Dr. Richard Yang, Yale
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Applications: Outdoor Networking
UMTS, WLAN,
DAB, GSM,
cdma2000, TETRA, ...
Personal Travel Assistant,
PDA, laptop,
GSM, UMTS, WLAN,
Bluetooth, ...
Courtesy Dr. Richard Yang, Yale
© Kemal Akkaya
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Application: Environmental Monitoring
Wireless
Sensor
Nodes
monitor an
area of
interest
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Challenges of Wireless Computing
 1) Wireless Communication
Implications of using wireless communication for
mobile computing
The differences between wireless and wired
media
 2) Mobility
Consequences of mobility on mobile application
and system design
 3) Poor Resources due to Portability
Pressures that portability places in the design of
mobile end-systems
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1) Wireless Communication
 Limited Transmission Range
 10m-500m
 Limited Bandwidth
 Wireless networks deliver
lower bandwidth than wired
networks
 1 Mbps Infrared
communication
 11 Mbps wireless local radio
communications (shared),
IEEE 802.11b
 9.6 Kbps for wide-area
wireless communication
10-100 Mbps for Ethernet
100 Mbps for FDDI
155 Mbps for ATM
1 Gbps for Gigabit Ethernet
© Kemal Akkaya
 Network partitions
 Stall all applications
 Uncertainty of Performance
 Variance of bit errors
 Variance of delays
 Variance of bandwidth
 Security
 Easy to intrude in the wireless
network
 Heterogeneous devices and
network connections
 Wired links
 Wired networks
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 Disconnections
 Same characteristics
 Outdoor: Radio
 Indoor: Infrared
 Rural Areas: Satellite
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Heterogeneous Devices
Mobile phones
• voice, data
• simple graphical displays
• GSM
Sensors,
embedded
controllers
Laptop
• fully functional
• standard applications
• battery; 802.11
PDA
• data
• simpler graphical displays
• 802.11
Desktop
• fully functional
• standard applications
• unlimited power supp
• Gbps Ethernet
Performance/Weight/Power Consumption
© Kemal Akkaya
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2) Mobility
 Ability to change locations while connected to the
network
 A mobile computer can change its server
 DNS server, print server, etc.
 Dynamic Environment
 Network Access Point Changes
 Address changes: IP address
 Network Performance Changes
 Bandwidth, delay, error rate etc.
 Available resources change
 Depends on the network it connected to
 Data consistency changes
 Writing/Reading to/from mobile databases
 Security changes
 Endpoint authentication harder
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3) Poor Resources
 Mobile devices are fundamentally different from
stationary machines such as desktop computers
 Must be designed with variety of constraints in mind, such as size and
power consumption – properties much like a wristwatch
 They should also be portable
 Portability Constraints Include
 Low power consumption
 You would not want to carry a battery that is bigger than your computer!
 Increased risk of data loss
 Physical damage
 Unauthorized access
 Loss and Theft
 Small user-interfaces
 Requires a different windowing scheme
 Buttons versus Recognition
 Limited on-board storage, memory, CPU etc
 Physical restrictions, power constraints
© Kemal Akkaya
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