CIS 267 Wireless Networks Lecture Chapter 13

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CIS 267 Wireless NetworksLecture Chapter 13

• Vaughn L. Lucas

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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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Wireless Networks

• Terrestrial Cellular
• Analog and Digital Cellular, TDMA and CDMA and
  CDPD, PCS
• Terrestrial
• Packet radio networks
• Satellite
• GPS, narrowband, broadband Geostationary, LEO

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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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AMPS and GSM

• AMPS North and Latin America, China
• First generation analog, ATT, 1970s
• Second generation TDMA 31 channel gain
• Third generation various tricks may get 101
• GSM European standard spreading
• Started as digital TDMA, 1990
• Open standard, spreading, security features
  built-in (smart-card), including PCS bands, no
  modem data comm at 9.6kbps

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Advanced Mobile Phone Service
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AMPS Components

• Mobile Units
• contains a modem that can switch between many
  frequencies
• 3 identification numbers electronic serial
  number, system ID number, mobile ID number
• Base Transceiver
• full-duplex communication with the mobile
• Mobile Switching Center

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Global System for Mobile Communication

• Developed to provide common 2nd-generation
  technology for Europe
• 200 million customers worldwide, almost 5 million
  in the North America
• GSM transmission is encrypted
• Spectral allocation 25 MHz for base transmission
  (935960 MHz), 25 MHz for mobile transmission
  (890915 MHz)

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GSM Layout
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Multiple Access

• Four ways to divide the spectrum among active
  users
• frequency-division multiplexing (FDM)
• time-division multiplexing (TDM)
• code-division multiplexing (CDM)
• space-division multiplexing (SDM)

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Choice of Access Methods

• FDM, used in 1st generation systems, wastes
  spectrum
• Debate over TDMA vs CDMA for 2nd generation
• TDMA advocates argue there is more successful
  experience with TDMA.
• CDMA proponents argue that CDMA offers additional
  features as well, such as increased range.
• TDMA systems have achieved an early lead in
  actual implementations
• CDMA seems to be the access method of choice for
  third-generation systems

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Third Generation Systems

• Intended to provide provide high speed wireless
  communications for multimedia, data, and video
• Personal communications services (PCSs) and
  personal communication networks (PCNs) are
  objectives for third-generation 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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WAP Protocol Stack
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Wireless Telephony ApplicationsA Sample
Configuration
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Cell Phone Modem Use
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PCS

• Personal Communications Services
• Frequencies auctioned by FCC (early 1995)
• The biggest spenders were Wireless Co, L.P. (the
  alliance between Sprint and cable MSOs, including
  TCI, Cox, and Comcast), spending over 2.1B on 29
  licenses ATT Wireless PCS Inc., spending nearly
  1.7B on 21 licenses and PCS PRIMECO, L.P. (the
  alliance between RBOCsBell Atlantic, NYNEX, US
  West, and AirTouch), spending over 1.1B for 11
  licenses, total 7.1Billion
• Smaller, cheaper, smarter, much lower power

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Cellular and PCS Frequencies
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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
• will remain 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 35,000 km
• Polar regions and the far northern and southern
  hemispheres are poorly served
• Even at speed of light, about 300,000 km/sec, the
  delay in sending a signal from a point on the
  equator beneath the satellite 35,838 km to the
  satellite and 35,838 km 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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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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Satellites GPS

• GPS Global Positioning System
• US Government system of 24 satellites
• Receiver triangulates from 3 or 4 satellites
• Government degrades standard signal to 100 meter
  accuracy
• Accurate to within about 1 centimeter with
  differential GPS -- compares with nearby fixed
  ground station

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Satellites Iridium

• http//www.iridium.com
• Motorola formed, is technology supplier
• All 66 birds up in 1998
• Cellular system with mobile base stations

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Satellites Teledesic

• Broadband LEO targeted for 2003
• Players include Craig McCaw, Bill Gates, others
• Lowest speed 2Mbps, standard 64Mbps
• http//www.teledesic.com

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