Download ECE/CS 4984: Lecture 1

COM555 Mobile Technology
Lecture: Week 2
Wireless Environment and
Wireless LANs
Introduction and Overview
Wireless and Mobile Networks
Summary Prev Week:
● Currently there are more wireless (mobile)
phone subscribers than wired phone
subscribers!
● # wireless Internet-connected devices
exceeds # wireline Internet-connected
devices
■ laptops, Smartphones Tablets, Phablets promise
anytime untethered Internet access
● two important (but different) challenges
■ wireless: communication over wireless link
■ mobility: handling the mobile user who changes
point of attachment to network
Lecture Objectives
● Discuss the impact of the wireless environment on
networks
● Provide an overview of current fixed and mobile
wireless technologies
● Introduce the basic operation of IEEE 802.11 and
Bluetooth WLANs/WPANs
● Introduce and compare Wireless and Mobile Network
Characteristics
Wireless Environment and Wireless LANs
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Impact of Wireless Environment
on Networks
● The wireless spectrum
● Physical impairments
● Contention for the shared medium
● Effects of mobility
● Restrictions on terminal equipment
● Security
The Wireless Spectrum
The Wireless Spectrum
● Continuum of electromagnetic waves
■ Data, voice communication
■ Arranged by frequencies
○ Lowest to highest
■ Spans 9 KHz and 300 GHz
● Wireless services associated with one area
● FCC oversees United States frequencies
● ITU oversees international frequencies
■ Air signals propagate across borders
The Wireless Spectrum (cont’d.)
The wireless spectrum
Wireless Spectrum (1)
Broadcast TV
• VHF: 54 to 88 MHz, 174 to 216 MHz
• UHF: 470 to 806 MHz
30 MHz
300 MHz
3 GHz
30 GHz
FM Radio
• 88 to 108 MHz
Digital TV
• 54 to 88 MHz, 174 to 216 MHz, 470 to 806 MHz
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Wireless Spectrum (2)
3G Broadband Wireless
• 746-794 MHz, 1.7-1.85 GHz,
2.5-2.7 GHz
30 MHz
300 MHz
3 GHz
30 GHz
Cellular Phone
• 800-900 MHz
Personal Communication Service (PCS)
• 1.85-1.99 GHz
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Wireless Spectrum (3)
Wireless LAN
(IEEE 802.11b/g)
• 2.4 GHz
30 MHz
300 MHz
Bluetooth
• 2.45 GHz
Wireless Environment and Wireless LANs
Wireless LAN
(IEEE 802.11n)
• 2.4 and 5 GHz
3 GHz
30 GHz
Local Multipoint Distribution
Services (LMDS)
• 27.5-31.3 GHz
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Characteristics of Wireless
Transmission
Characteristics of Wireless Transmission
● Similarities with wired
■ Layer 3 and higher protocols
■ Signal origination
○ From electrical current, travel along conductor
● Differences from wired
■ Signal transmission
○ No fixed path, guidance
● Antenna
■ Signal transmission and reception
■ Same frequency required on each antenna
○ Share same channel
Characteristics of Wireless
Transmission (cont’d.)
Wireless transmission and reception
Antennas
● Radiation pattern
■ Relative strength over three-dimensional area
○ All electromagnetic energy antenna sends, receives
● Directional antenna
■ Issues wireless signals along single direction
● Omnidirectional antenna
■ Issues, receives wireless signals
○ Equal strength, clarity
○ All directions
● Range
■ Reachable geographical area
Signal Propagation
● LOS (line-of-sight)
■ Signal travels
○ In straight line, directly from transmitter to receiver
● Obstacles affect signal travel
■ Pass through them
■ Absorb into them
■ Subject signal to three phenomena
○ Reflection: bounce back to source
○ Diffraction: splits into secondary waves
○ Scattering: diffusion in multiple different directions
Signal Propagation (cont’d.)
● Multipath signals
■ Wireless signals follow different paths to destination
■ Caused by reflection, diffraction, scattering
■ Advantage
○ Better chance of reaching destination
■ Disadvantage
○ Signal delay
Signal Degradation
● Fading
■ Change in signal strength
○ Electromagnetic energy scattered, reflected,
diffracted
● Attenuation
■ Signal weakens
○ Moving away from transmission antenna
■ Correcting signal attenuation
○ Amplify (analog), repeat (digital)
● Noise
■ Usually the worst problem
○ No wireless conduit, shielding
Multipath signal propagation
Narrowband, Broadband, and
Spread Spectrum Signals
● Defines wireless spectrum use:
■ Narrowband
○ Transmitter concentrates signal energy at single
frequency, very small frequency range
■ Broadband
○ Relatively wide wireless spectrum band
○ Higher throughputs than narrowband
■ Spread-spectrum
○ Multiple frequencies used to transmit signal
○ Offers security
Narrowband, Broadband, and
Spread Spectrum Signals
● FHSS (frequency hopping spread spectrum)
■ Signal jumps between several different frequencies within
band
■ Synchronization pattern known only to channel’s receiver,
transmitter
● DSSS (direct-sequence spread spectrum)
■ Signal’s bits distributed over entire frequency band at
once
■ Each bit coded
○ Receiver reassembles original signal upon receiving bits
Fixed versus Mobile
● Fixed communications wireless systems
■ Transmitter, receiver locations do not move
■ Transmitting antenna focuses energy directly toward
receiving antenna
○ Point-to-point link results
■ Advantage
○ No wasted energy issuing signals
○ More energy used for signal itself
● Mobile communications wireless systems
■ Receiver located anywhere within transmitter’s range
○ Receiver can roam
Physical Impairments: Noise
● Unwanted signals added to the message signal
● May be due to signals generated by natural
phenomena such as lightning or man-made sources,
including transmitting and receiving equipment as
well as spark plugs in passing cars, wiring in
thermostats, etc.
● Sometimes modeled in the aggregate as a random
signal in which power is distributed uniformly across
all frequencies (white noise)
● Signal-to-noise ratio (SNR) often used as a metric in
the assessment of channel quality
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Physical Impairments: Interference
● Signals generated by communications devices
operating at roughly the same frequencies may
interfere with one another
■ Example: IEEE 802.11b and Bluetooth devices, microwave
ovens, some cordless phones
■ CDMA systems (many of today’s mobile wireless systems)
are typically interference-constrained
● Signal to interference and noise ratio (SINR) is
another metric used in assessment of channel quality
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Physical impairments: Fading (1)
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Physical impairments: Fading (2)
● Strength of the signal decreases with distance
between transmitter and receiver: path loss
■ Usually assumed inversely proportional to distance to the
power of 2.5 to 5
● Slow fading (shadowing) is caused by large
obstructions between transmitter and receiver
● Fast fading is caused by scatterers in the vicinity of
the transmitter
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Diversity
● A diversity scheme extracts information from
multiple signals transmitted over different fading
paths
● Appropriate combining of these signals will reduce
severity of fading and improve reliability of
transmission
● In space diversity, antennas are separated by at least
half a wavelength
■ Other forms of diversity also possible
■ Polarization, frequency, time diversity
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Contention for the Medium
C
packets
A
B
● If A and B simultaneously transmit to C over the
same channel, C will not be able to correctly decode
received information: a collision will occur
● Need for medium access control mechanisms to
establish what to do in this case (also, to maximize
aggregate utilization of available capacity)
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Effects of Mobility
home network
wide area
network
visited network
2
Figure from
Kurose & Ross
foreign agent contacts home
agent home: “this mobile is
resident in my network”
1
mobile contacts
foreign agent on
entering visited
network
● Destination address not equal to destination location
● Addressing and routing must be taken care of to
enable mobility
● Can be done automatically through handoff or may
require explicit registration by the mobile in the
visited network
● Resource management and QoS are directly affected
by route changes
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Form Factors
● Form factors (size, power dissipation, ergonomics,
etc.) play an important part in mobility and
nomadicity
■ Mobile computing: implies the possibility of seamless
mobility
■ Nomadic computing: connections are torn down and reestablished at new location
● Battery life imposes additional restrictions on the
complexity of processing required of the mobiles
units
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Security
● Safeguards for physical
security must be even
greater in wireless
communications
● Encryption: intercepted
communications must not
be easily interpreted
● Authentication: is the node
who it claims to be?
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Wireless Networks
● Mobile wireless WANs
● Fixed wireless WANs
● WLANs: the 802.11 family
● WLANs/WPANs: Bluetooth
Generations in Mobile Wireless Service
● First Generation (1G)
■ Mobile voice services
● Second Generation (2G)
■ Primarily voice, some low-speed data (circuit switched)
● Generation 2½ (2.5G)
■ A bridge (for GSM) to 3G
● Third Generation (3G)
■ Seamless integration of voice and data
■ High data rates, full support for packet switched data
● Fourth Generation (4G): Long-Term Evolution(LTE)
■ All IP
■ IPv6
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WLANs: IEEE 802.11 Family
● 802.11 working group
■ Specify an open-air interface between a wireless client and a
base station or access point, as well as among wireless
clients
● IEEE 802.11a
■ Up to 54 Mbps in the 5 GHz band
■ Uses orthogonal frequency division multiplexing (OFDM)
● IEEE 802.11b (Wi-Fi)
■ 11 Mbps (with fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz
band
■ Uses Direct Sequence Spread Spectrum (DSSS)
● IEEE 802.11g
■ 20+ Mbps in the 2.4 GHz band
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WLANs: IEEE 802.11 Family
● IEEE 802.11n
■ 2.4 GHz
■ MIMO Antennas
■ 54 Mbps to 600 Mbps and 5GHz
● IEEE 802.11ac (Jan 2014)
■ 80 – 160 MHz Channels in the 5 GHz band.
■ High-order implementations using 3 spatial streams and
higher-order multiplexing QAM-256 can implement 43.3
Mbps per stream or 1300 Mbps total
■ That’s 1300000000 bits every second…
■ That’s 1.3 Gbps in your local coffee-shop…!!
● This capacity on Wi Fi changes the ground rules for
mobile operators:
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WLANs: IEEE 802.11 Family
● Coming to a Transmitter near you soon…
● IEEE 802.11ad
● IEEE 802.11af
● IEEE 802.11ah
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WLAN: Frequency Ranges
● 2.4-GHz band (older)
■ Frequency range: 2.4–2.4835 GHz
■ 11 unlicensed communications channels
■ Susceptible to interference
● Unlicensed
■ No FCC registration required
● 5-GHz band (newer)
■ Frequency bands
○ 5.1 GHz, 5.3 GHz, 5.4 GHz, 5.8 GHz
■ 24 unlicensed bands, each 20 MHz wide
■ Used by weather, military radar communications
IEEE 802.11
● Characteristics
● Modes of operation
● Association, authentication and privacy
IEEE 802.11 Standard
● Final draft approved in 1997
● Operates in the 2.4 GHz industrial, scientific and
medical (ISM) band
● Standard defines the physical (PHY) and medium
access control (MAC) layers
■ Note that the 802.11 MAC layer also performs functions that
we usually associated with higher layers (e.g.,
fragmentation, error recovery, mobility management)
● Initially defined for operation at 1 and 2 Mbps
● Standard have evolved considerably.
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Reference Model (1)
Data Link Medium Access Control MAC sublayer
Layer
(MAC) sublayer
management
Physical
Layer
Physical Layer
station
convergence procedure
management
(PLCP) sublayer
PHY sublayer
management
Physical medium
Dependent (PMD)
sublayer
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Infrastructure Mode (1)
Wired LAN
Access Point
Mobile
Stations
● Basic Service Set (BSS)
● Access point serves as a local bridge
● Stations communicate through the access point,
which relays frames to/from mobile stations
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Infrastructure Mode (2)
Wired LAN
Access
Points
Mobile
Stations
● Extended Service Set (ESS)
● A set of infrastructure BSSs
● Access points communicate among themselves to
forward frames between BSSs and to facilitate
movement of stations between BSSs
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IEEE 802.11 Architecture
BSS: Stations
executing the same
MAC protocol
Access Point:
Acts as a bridge
and a relay point
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Association
● To deliver a message within the Distribution System,
each station must know which AP to access for a
given mobile station
● Before a station is allowed to send a message
through an AP, it must associate itself with that AP
■ At any given time, a station must be associated with no more
than one AP
■ An AP may be associated with multiple stations
● As it moves between BSSs, a mobile station may
reassociate itself with a different AP
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Authentication
● 802.11 provides link-level authentication between
stations
● 802.11 also supports shared key authentication
■ Requires that Wi Fi protected access (WPA) be enabled
● Typically, authentication is performed at association
with an AP
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Privacy
● Default state is “in the clear” – messages are not
encrypted
● Essential to use 802.11i encryption (WPA2)
● Also known as Advanced Encryption Standard (AES)
● WPA2 implemented in two versions,
■ Personal and Enterprise.
■ WPA2 Personal protects unauthorized network access by
utilizing a set-up password.
■ WPA2 Enterprise verifies network users through a server
mechanism,
● Goal is to achieve a level of security at least as good
as in a wired LAN
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Bluetooth
● Characteristics
● Comparison with IEEE 802.11
Introduction
● Motivation: cable
replacement in peripherals
and embedded devices
● Named after Harald
Blaatand “Bluetooth” II,
king of Denmark 940-981
A.D.
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Characteristics
● Operates in the ISM band (like 802.11b)
● Frequency hopping spread spectrum
● Up to 720 kbps data transfer with a range of 10 m
■ Transmission rate decreases if interference from other
devices is present
● Master/slave architecture
■ A collection of master + slaves is called a piconet
■ Up to 7 slave devices may communicate with a master
■ Piconets can be linked together to form a scatternet
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Comparison with 802.11
Characteristic
Bluetooth
IEEE 802.11b
IEEE 802.11a
Spectrum
2.4 GHz
2.4 GHz
5 GHz
Max Data Rate
725 kbps
11 Mbps
54 Mbps
Connections
Point-toMultipoint
Point-to-Point
Point-to-Point
FHSS
DSSS
OFDM
Frequency
Selection
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802.11ac (2013)
● 5th Generation Wi Fi
● Competition for Cellular Operators
● 1.3 Gbps Download (Ideal Conditions)
● Already available in hardware Products (Apple
MacBook etc)
● Read the Tutorial:
● IEEE 802.11 ac Tutorial
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