Chapter 14: Wireless WANs

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Chapter 14Wireless WANs

• Business Data Communications, 5e

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Reasons for Wireless Networks

• Mobile communication is needed.
• Communication must take place in a terrain that
  makes wired communication difficult or
  impossible.
• A communication system must be deployed quickly.
• Communication facilities must be installed at low
  initial cost.
• The same information must be broadcast to many
  locations.

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Problems with Wireless Networks

• Operates in a less controlled environment, so is
  more susceptible to interference, signal loss,
  noise, and eavesdropping.
• Generally, wireless facilities have lower data
  rates than guided facilities.
• Frequencies can be more easily reused with guided
  media than with wireless media.

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Cellular Wireless Networks

• One of the most revolutionary developments in
  telecommunications
• Supports users in locations that are not easily
  served by wired networks
• Used for mobile telephones, personal
  communications systems, wireless Internet and
  wireless Web applications, and more

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Cellular Network Organization

• Uses multiple low-power transmitters (100W)
• Areas divided into cells, each one served by its
  own antenna.
• Each cell allocated a band of frequencies, and is
  served by a base station
• Adjacent cells are assigned different frequencies
  to avoid interference or crosstalk
• Cells sufficiently distant from each other can
  use the same frequency band

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Cellular Geometries
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Frequency Reuse Patterns
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Increasing Capacity

• Adding new channels
• Frequency borrowing Frequencies are taken from
  adjacent cells by congested cells
• Cell splitting Cells in areas of high usage can
  be split into smaller cells.
• Cell sectoring Cell divided into wedge-shaped
  sectors. Each sector is assigned a separate
  subset of the cell's channels, and directional
  antennas at the base station are used to focus on
  each sector.
• Microcells Useful in city streets in congested
  areas, along highways, and inside large public
  buildings

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Cellular System Overview
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Mobile to Base Channels

• Control channels are used to exchange information
  having to do with setting up and maintaining
  calls and with establishing a relationship
  between a mobile unit and the nearest BS
• Traffic channels carry a voice or data connection
  between users

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Steps in a Mobile Call

• Monitor for strongest signal
• Request for connection
• Paging
• Call accepted
• Ongoing call
• Handoff

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Mobile Telephony

• First Generation
• analog voice communication using frequency
  modulation.
• Second Generation
• digital techniques and time-division multiple
  access (TDMA) or code-division multiple access
  (CDMA)
• Third Generation
• evolving from second-generation wireless systems
• will integrate services into one set of standards.

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Multiple Access

• Four ways to divide the spectrum among active
  users
• frequency-division multiple access (FDMA)
• time-division multiple access (TDMA)
• code-division multiple access (CDMA)
• space-division multiple access (SDMA)
• FDMA and TDMA discussed in Chapter 17
• CDMA and SDMA discussed here

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CDMA

• Based on direct sequence spread spectrum (DSSS)
• Provides immunity from various kinds of noise and
  multipath distortion. (The earliest applications
  of spread spectrum were military, where it was
  used for its immunity to jamming.)
• Can be used for hiding and encrypting signals.
• Several users can independently use the same
  (higher) bandwidth with very little interference

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Cellular Multiple Access Schemes
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Third Generation Systems

• Intended to provide provide high speed wireless
  communications for multimedia, data, and video
• Reflects trend toward universal personal
  telecommunications and communications access
• Personal communications services (PCSs) and
  personal communication networks (PCNs) are
  objectives for 3G wireless.
• Planned technology is digital using TDMA or CDMA
  to provide efficient spectrum use and high
  capacity

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Wireless Application Protocol (WAP)

• Programming model based on the WWW Programming
  Model
• Wireless Markup Language, adhering to XML
• Specification of a small browser suitable for a
  mobile, wireless terminal
• A lightweight communications protocol stack
• A framework for wireless telephony applications
  (WTAs)

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WAP Programming Model
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Wireless Markup Language

• Does not assume a standard keyboard or a mouse
  designed to work with telephone keypads,
  styluses, and other input devices common to
  mobile, wireless communication
• Documents are subdivided into small, well-defined
  units of user interaction called cards users
  navigate by moving back and forth between cards.
• Uses a small set of markup tags appropriate to
  telephony-based systems

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Microbrowser

• Based on a user interface model appropriate for
  mobile, wireless devices.
• Traditional 12-key phone keypad is used to enter
  alphanumeric characters
• Users navigate among the WML cards using up and
  down scroll keys rather than a mouse.
• Navigation features familiar from the Web (e.g.,
  Back, Home, and Bookmark) are provided as well.

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Wireless Telephony ApplicationsA Sample
Configuration
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Satellite Communications

• Two or more stations on or near the earth
  communicate via one or more satellites that serve
  as relay stations in space
• The antenna systems on or near the earth are
  referred to as earth stations
• Transmission from an earth station to the
  satellite is an uplink, from the satellite to the
  earth station is downlink
• The transponder in the satellite takes an uplink
  signal and converts it to a downlink signal

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Geostationary Satellites

• Circular orbit 35,838 km above the earths
  surface
• Rotates in the equatorial plane of the earth at
  exactly the same angular speed as the earth
• Remains above the same spot on the equator as the
  earth rotates

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Advantages of Geostationary Orbits

• Satellite is stationary relative to the earth,
  so no frequency changes due to the relative
  motion of the satellite and antennas on earth
  (Doppler effect).
• Tracking of the satellite by its earth stations
  is simplified.
• One satellite can communicate with roughly a
  fourth of the earth three satellites separated
  by 120 cover most of the inhabited portions of
  the entire earth excluding only the areas near
  the north and south poles

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Problems withGeostationary Orbits

• Signal can weaken after traveling that distance
• Polar regions and the far northern and southern
  hemispheres are poorly served
• Even at speed of light, the delay in sending a
  signal 35,838 km each way to the satellite and
  back is substantial

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LEO and MEO Orbits

• Alternatives to geostationary orbits
• LEO Low earth orbiting
• MEO Medium earth orbiting

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Satellite Orbits
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LEO Characteristics

• Circular or slightly elliptical orbit lt 2000 km
• Orbit period is in the range of 1.5 to 2 hours
• Diameter of coverage is about 8000 km
• Round-trip signal propagation delay is lt 20 ms
• Maximum time that the satellite is visible from a
  fixed point on earth (above the radio horizon) is
  up to 20 minutes
• System must be able to cope with large Doppler
  shifts, which change the frequency of the signal
• Significant atmospheric drag on a LEO satellite
  results in gradual orbital deterioration.

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LEO Advantages

• Reduced propagation delay
• Received LEO signal is much stronger than that of
  GEO signals for the same transmission power
• LEO coverage can be better localized so that
  spectrum can be better conserved.
• On the other hand, to provide broad coverage over
  24 hours, many satellites are needed.

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Types of LEOs

• Little LEOs Intended to work at communication
  frequencies below1 GHz using no more than 5 MHz
  of bandwidth and supporting data rates up to 10
  kbps
• Big LEOs Work at frequencies above 1 GHz and
  supporting data rates up to a few megabits per
  second

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MEO Characteristics

• Circular orbit at an altitude of 5000 to 12,000
  km
• Orbit period is about 6 hours
• Diameter of coverage is 10,000 to 15,000 km
• Round trip signal propagation delay lt 50 ms
• Maximum time that the satellite is visible from a
  fixed point on earth (above the radio horizon) is
  a few hours

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Satellite Network Configurations
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Satellite Network Applications

• Television distribution
• Long-distance telephone transmission
• Private business networks