1. Wireless & Network Security
Department of Computer Science
Southern Illinois University Carbondale
Wireless & Network Security
Lecture 3: Radio Basics & Wireless Networks
Dr. Kemal Akkaya
Wireless & Network Security

2. Radio Basics (RF) Already seen how a radio signal looks like
Sinusoids
Carrier wave
Information Signal
Signal is modulated onto carrier wave
Carrier has more bandwidth than the info itself
Radio Waves
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
Wireless & Network Security

3. Antennas Electrical conductor Radiation Pattern of an Antenna
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
In satellite applications
Isotropic Radiator
λ/2
Half-wave dipole
Wireless & Network Security

4. Directional Antennas Directional antennas are very common
Omni-directional
Directional
Antenna
Directional antennas are very common
Radiation pattern in a certain direction
Often used for base stations in a cellular system
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
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:
Antennas improves the signal upon the isotropic antenna in that direction by 3dB
Beam Width
side view (xy-plane) x y
side view (yz-plane) z
top view (xz-plane)
directed
antenna
Courtesy
Wireless & Network Security

5. Radio Propagation Signal Propagation Ranges Wireless Propagation Modes
Transmission range
Communication possible
Low error rate
Detection range
Detection of the signal possible
No communication possible
Interference range
Signal may not be detected
Signal adds to the background noise
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)
Courtesy Dr. Y. Richard Yang
distance
sender
transmission
detection
interference
Wireless & Network Security

6. 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.
Wireless & Network Security

7. 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)
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
Wireless & Network Security

8. 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
signal at sender
signal at receiver
LOS pulses
Multipath pulses
Courtesy Dr. Y. Richard Yang
Causes of fading:
Free space loss
Multi-path propagation
Reflection, scattering, diffraction, refraction
Interference with other transmitters
Atmospheric absorption
Mobility
Fast fading, small fading
Wireless & Network Security

9. 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
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 d4 for different environments
Environment
Path Loss Exponent, n
Free space 2
Urban area cellular radio
2.7 to 3.5
Shadowed urban cellular radio
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
Wireless & Network Security

10. 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
Refraction:
Bending of radio waves as they propagate through the atmosphere
Reflection, scattering, diffraction
Courtesy Dr. Y. Richard Yang
reflection
scattering
diffraction
refraction
Wireless & Network Security

11. 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
Wireless & Network Security

12. Slow and Fast Fading in an Urban Mobile Environment
-70
-60
-50
-40
-30
Distance between transmitter and receiver (m)
Received Power (dBm)
Fast Fading
Slow Fading
Wireless & Network Security

13. Spread Spectrum Motivation: To solve this problem: Initial motivation:
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
Wireless & Network Security

14. Frequency Hopping Spread Spectrum
Simplest approach: FHSS
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
Wireless & Network Security

15. Types of FHSS Two versions of FHSS
Fast Hopping: several frequencies per user bit
Slow Hopping: several user bits per frequency
user data
slow
hopping
(3 bits/hop)
fast
(3 hops/bit) 1 tb t f f1 f2 f3 td
tb: bit period td: dwell time
Wireless & Network Security

16. 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 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
Wireless & Network Security

17. 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)
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
Wireless & Network Security

18. Example for DSSS
Wireless & Network Security

19. 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
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
Wireless & Network Security

20. Buzzwords Mobile Computing Nomadic 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.
Wireless & Network Security

21. 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 LANs
IEEE : SIU’s LAWN,
Wireless PANs
e.g. Infrared: Bluetooth
Mobile Ad-hoc networks
e.g. Emergency relief, military
Sensor networks
e.g. Environmental sensing-MICA motes
Wireless WAN Generations:
1G (Past)
AMPS, TACS: No data
2G (Past/Present)
IS-136, GSM: <10Kbps circuit switched data
2.5G (Past/Present)
GSM-GPRS, GPRS-136: <100Kbps packet switched
3G (Present/Immediate Future)
IMT-2000: <2Mbps packet switched
4G (Future)
20-40 Mbps!!
Wireless & Network Security

22. Examples 802.11 / WiFi Wireless LAN PicoNet Bluetooth
Wireless & Network Security

23. Applications: Home Networking
Courtesy Dr. Richard Yang, Yale
Wireless & Network Security

24. Applications: Outdoor Networking
ad hoc
UMTS, WLAN,
DAB, GSM,
cdma2000, TETRA, ...
Personal Travel Assistant,
PDA, laptop,
GSM, UMTS, WLAN,
Bluetooth, ...
Courtesy Dr. Richard Yang, Yale
Wireless & Network Security

25. Application: Environmental Monitoring
Wireless Sensor Nodes monitor an area of interest
Wireless & Network Security

26. 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
Wireless & Network Security

27. 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 b
9.6 Kbps for wide-area wireless communication
Wired networks
Mbps for Ethernet
100 Mbps for FDDI
155 Mbps for ATM
1 Gbps for Gigabit Ethernet
Disconnections
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
Same characteristics
Outdoor: Radio
Indoor: Infrared
Rural Areas: Satellite
Wireless & Network Security

28. Heterogeneous Devices
Laptop
fully functional
standard applications
battery;
Mobile phones
voice, data
simple graphical displays
GSM
Desktop
fully functional
standard applications
unlimited power supp
Gbps Ethernet
Sensors,
embedded
controllers
PDA
data
simpler graphical displays
802.11
Performance/Weight/Power Consumption
Wireless & Network Security

29. 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
Wireless & Network Security

30. 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
Wireless & Network Security