Wireless Networking Technologies and Standards

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Wireless Networking Technologies and Standards

• CS637 Spring 2007
• Department of Computer Science,
• CSU San Marcos

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Introduction Mobile computing and wireless
systems allow users to access and process
information without any spatial or temporal
constraints. The location of the user, whether
mobile or static, will not affect the ability of
the user to utilize the mobile platform. Access
to these information systems via a varied
platform of mobile computing and wireless systems
create the illusion that information and data
resources are available on the spot where in
reality they may be located far away. One of the
most exciting advents in the realm of mobile
computing and wireless systems following the
widespread utilization of mobile phones by
millions of subscribers is the ability to deliver
data through various mobile devices. Wireless
Internet or Wireless LAN, as it is commonly
called, is a culmination of various different
types of wireless networking technologies and
standards, primarily those that revolve around
providing service for Internet connectivity.
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Some historical milestones achieved in mobile
computing are as follows
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Introduction

• Discuss concepts in mobile wireless network
  technologies
• Wireless LAN infrastructure, organization and
  standards
• Focus on the IEEE 802.11 network architecture,
  implementation issues and security
• Briefly discuss emerging technologies

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I. Overview of mobile computing and wireless
systems

• Primary Types of Wireless communication
• Radio Frequencies (RF)
• Infrared Transmission
• RF 800MHz to 900MHz range
• RF media
• Microwave
• Infrared
• Spread Spectrum

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Frequencies and the Radio spectrum
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Different spectrum types Each type of
transmissions requires a specified environment in
order to obtain successful delivery of signal
from sender to receiver.
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Overview of mobile computing and wireless systems
(cont)

• Most radio frequency media require line-of-sight
• Each type of transmission requires a specified
  environment to obtain successful delivery of
  signal
• Modulation is used to achieve transmission
• We will concentrate on digital wireless systems

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Various types of wireless communications
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The wireless spectrum
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Personal communication systems (PCS) PCS started
off as an alternative to the conventional
cellular system and as a possible improvement to
it. In America, PCS started being used in the
mid 1990s and in 1994 the FCC started auctioning
off space in the newly designed PCS Band. Two PCS
Types exist, namely narrowband and broadband.
Commonly, narrowband PCS handles data and
wideband PCS handles voice. Most advanced paging
services use 900MHz frequencies PCS narrowband
while most broadband uses 2GHz frequencies for
voice, data and video services. Most broadband
PCS systems use higher frequencies, lower power
and smaller cells than conventional cellular
systems. The spectrums properties such as
higher frequencies waves that are shorter,
shorter distance travel than most low frequency
signals creates the need fore more base stations
spaced closely together. This means that PCS
systems require 50 to 100 more base stations
than standard 800MHz cellular systems.
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Cellular Cellular systems are made up of the
cellular layout itself, namely a carefully
engineered network of radio base stations and
antennas, base station controllers and mobile
switches. Base station controllers are units that
manage several base stations at a time and mobile
switches gathers traffic from dozens of cells and
pass it to a public switched telephone
network. The area a base station covers is called
a cell, and the area where the base station and
antennas are located is called a cell site. The
following diagram demonstrates how each base
station appears like a cell in a honeycomb, hence
the term cellular. Cell sizes for cellular
systems range from 6/10th of a mile to 30 miles
in radius.
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A simplified view of how wireless transmissions
work
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II. Technologies used in wireless networks

• Spread spectrum technology its use in wireless
  systems and in wireless LANs
• Characteristics low peak power, wide bandwidth
• Why is spread spectrum better than traditional
  narrow band?
• Difficult to intercept/detect (secure)
• High bandwidth

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II. Technologies used in wireless networks (cont)

• Application of Spread Spectrum
• WPANs
• WMANs
• WWANs
• Wireless LANs primarily use spread spectrum
  technology implemented on a combination of 802.11
  and 802.15 devices

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Different types of Spread Spectrum

• Frequency Hopping Spread Spectrum (FHSS)
• Data spread over 83MHz using frequency agility
• Direct Sequence Spread Spectrum (DSSS)
• Data spread over a 22MHz wide set of frequencies

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FHSS transmissions

• Carrier signal switches frequencies or hops
  according to a pseudorandom sequence
• Sequence is predetermined with specified time
  intervals
• Sequence is repeated after list of frequencies is
  exhausted

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Example of a single FHSS transmission
Hop of 5 frequencies over a 5MHz band
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DSSS

• More popular than FHSS due to ease of
  implementation and high data rates
• Combines data signal with a higher data rate bit
  sequence (called a chipping code)
• Chips determine how wide spreading occurs, the
  number of chips per bit and number of chips per
  second determines data rate

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DSSS channel allocation and spectral relationship
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III. Organizations and standards

• Two primary organizations
• Federal Communications Commission (FCC)
• Institute of Electrical and Electronics Engineers
  (IEEE)
• FCC laws
• IEEE standards that exist within the laws

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FCC

• Makes laws that govern the frequency, power rate
  and transmission technologies that wireless LANs
  can operate under
• Also stipulates laws for wireless LAN hardware
• Two primary frequency bands for use
• Industrial Scientific and Medical band (ISM)
• Unlicensed National Information Infrastructure
  band (UNII)

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ISM and UNII Spectra
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ISM bands

• Three license-free bands 900MHz, 2.4 GHz and
  5.8GHz bands
• 900MHz Uses 902MHz to 928MHz and additionally
  defined as 915MHz /- 13MHz
• 2.4GHz Used by 802.11, 802.11b, 802.11g, 2.4000
  GHz and 2.5000 GHz
• 5.8GHz Uses 5.725GHz and 5.875Ghz yielding a
  150MHz bandwidth

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UNII Bands

• Three 100MHz wide bands
• Lower Band 5.15 GHz and 5.25 GHz with a max
  power output of 50mW (indoor use)
• Middle Band 5.25 GHz and 5.35 GHz with a max
  power output of 200mW (indoor/outdoor use)
• Upper Band 5.725 GHz and 5.825 GHz with a max
  power output of 1 Watt (outdoor use)
• Four non-overlapping channels separated by 5MHz

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IEEE

• Works under constraints of FCC laws
• Four main IEEE standards for wireless LANs
• 802.11
• 802.11b
• 802.11a
• 802.11g

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IEEE 802.11

• First standard developed
• Uses all available transmission technologies
  DSSS, FHSS, infrared
• DSSS at 1Mbps, 2 Mbps, 11 Mbps
• FHSS at 1Mbps and 2 Mbps
• Some proprietary modes operate at 3-10Mbps
• Operate strictly under the 2.5 GHz ISM

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IEEE 802.11a

• Operates under the 5 GHz UNII band
• Systems using 5 GHz frequencies cannot
  communicate with other systems
• Not backwards compatible
• Data rates at 6, 9, 12, 18, 24, 36. 48 and 54
  Mbps
• Proprietary modes can reach 108 Mbps using rate
  doubling

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IEEE 802.11b

• Most popular referred to as High Rate. Also
  sometimes called WiFi
• DSSS system with data rates of 1, 2, 5.5 and
  11Mbps
• Does not describe any FHSS systems
• Backwards compatible with 802.11 devices at 1Mbps
  and 2Mbps
• Uses 2.4GHz ISM band

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IEEE 802.11g

• Functions as the same maximum data rate as
  802.11a
• Provides compatibility for 802.11b devices
• Operates under the 2.5 GHz ISM band using
  Orthogonal Frequency Division Multiplexing (OFDM)
  for higher data rate
• New to market, getting popular

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Other Technologies

• HomeRF
• Operates under the 2.5GHz band.
• Uses FHSS on a 5MHz carrier frequency and a max
  power output of 125mW
• 50 hops per second and is 5-20 times faster than
  most 802.11 devices
• Bluetooth
• Operates under the 2.5GHz band.
• Hop rate of 1600 hops per second, a lot of
  overhead
• Usually used for low power, short range WPAN
  devices

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IV. 802.11 network architecture
802.11 and corresponding OSI layers
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IV. 802.11 network architecture (cont)

• 802.11 focuses on the Datalink and Physical layer
• Two modes exist within the 802.11 network
  architecture definitions
• Infrastructure Mode
• Ad Hoc Mode

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Infrastructure Mode

• Must contain at least one Access Point connected
  to a wired network
• A set of wireless stations
• BSS (Basic Service Set) 1 AP, n Stations
• ESS (Extended Service Set) Two or more BSS

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Infrastructure Mode
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Ad Hoc Mode

• Generally called peer-to-peer mode
• Also called Independent Basic Service Set (IBSS)
• A collection of 802.11 wireless stations that
  communicate directly with one another without the
  use of an access point

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Ad Hoc Mode
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Scanning and Connecting

• Process of listening for signals from an access
  point is called scanning
• Two types of scanning
• Active scanning
• Passive scanning
• Service Set Identifiers (SSID) and beacons are
  used by access points to announce their
  availability

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Service Set Identifier (SSID)

• 2 to 32 characters, alphanumeric, case sensitive,
  unique
• Essentially a network name
• SSID sent in beacons, probe requests, probe
  responses and other types of frames
• Client stations need SSID to register on a
  wireless network

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Beacons

• Beacon management frames are short frames that
  establish transmission and synchronize
  communication
• Provide the following functions
• Time synchronization
• FH or DS parameters
• SSID information
• Traffic Indication Map (TIM)

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Passive Scanning
Client stations listen for beacons From nearby
Access points
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Active Scanning
Client stations send probe request Frames to seek
out active access points
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Frame Structure and Types

• Maximum frame size of 1518 bytes
• Maximum payload of 1500 bytes
• Carry protocols and data from higher layers
  within its frame body
• 3 types of frames
• Management frames
• Control frames
• Data frames

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Management Frame Structure

• Types
• Association Request Frame
• Association Response Frame
• Re-association Request Frame
• Re-association Response Frame
• Probe Request Frame
• Probe Response Frame
• Beacon Frame
• ATIM Frame (Announcement Time Interval Frame)
• Disassociation Frame
• Authentication Frame
• De-authentication Frame

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Management Frame Structure
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Control Frames

• Types
• Request to Send (RTS)
• Clear to Send (CTS)
• Acknowledgement (ACK)

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Control Frames
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Data Frames

• Data frames carry packets from higher layers
  within the body of its frame to transport data
  from one station to another
• More details of all the different frame types
  covered in the paper

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Type and subtype identifiers
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Implementation Issues

• Collision Detection
• Utilize CSMA/CA
• Dynamic Rate Shifting
• Automated Rate Selection (ARS) and Dynamic Rate
  Shifting (DRS) are speed adjustment methods
• Take care of drops in data rate when distance
  increases between stations or when interference
  occurs

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Dynamic Rate Shifting
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Implementation Issues (cont)

• Distributed Coordination Function (DCF)
• Method used to contend for access
• Uses CSMA/CA and DCF is the mode at access points
• Point Coordination Function (PCF)
• Method used to contend for access via polling
• Access points performs polling to guarantee a
  certain amount of latency for applications
  requiring QoS

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Request to Send/Clear to Send (RTS/CTS) Two types
of carrier sense mechanisms are used on wireless
networks, namely, physical carrier sense and
virtual carrier sense. Physical carrier sense
functions by checking the signal strength called
a Received Signal Strength Indicator (RSSI) on
the carrier signal to see if a station is
transmitting. Virtual carrier sense functions by
checking a field called Network Allocation Vector
(NAV) which acts as a timer on a station.
Virtual carrier sense is implemented with the
RTS/CTS protocol.
Figure 21 RTS/CTS Handshaking As noted earlier
in this paper, RTS/CTS protocol is an extension
of the CSMA/CA protocol and can cause significant
network overhead. However, if collisions are
unusually high on a network, RTS/CTS can increase
traffic flow by decreasing the number of
collisions.
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Security

• IEEE 802.11 specifies Wired Equivalent Privacy or
  WEP (covered more in detail in my paper)
• Not as secure as it should be
• Other options for security
• Advanced Encryption Standard (AEP)
• Filtering

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Emerging Technologies

• HiperLAN2
• Mobility Management
• Multimedia and Adaptive Wireless Networking
• Ultra Wideband Technology
• 4G Applications and Services

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• HiperLAN2
• HiperLAN2 is a new 5GHz spectrum wireless LAN
  technology that attempts to improve upon current
  802.11b by providing capabilities to scale
  easily, ability to manage bandwidth according to
  performance predictions from each user and
  application, protocols that optimize throughput
  of available radio resource, quality of service
  capabilities and a higher level of security.
  HiperLAN2 2003

Figure 22 Example HiperLAN2 network