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Chapter 13: Wireless Networks

Chapter 13: Wireless Networks. Business Data Communications, 4e. 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.

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Chapter 13: Wireless Networks

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  1. Chapter 13:Wireless Networks Business Data Communications, 4e

  2. 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.

  3. 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.

  4. 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.

  5. Advanced Mobile Phone Service

  6. 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

  7. 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 (935–960 MHz), 25 MHz for mobile transmission (890–915 MHz)

  8. GSM Layout

  9. 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)

  10. 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

  11. 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

  12. 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)

  13. WAP Programming Model

  14. WAP Protocol Stack

  15. Wireless Telephony Applications:A Sample Configuration

  16. Geostationary Satellites • Circular orbit 35,838 km above the earth’s 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.

  17. 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

  18. 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.

  19. LEO and MEO Orbits • Alternatives to geostationary orbits • LEO: Low earth orbiting • MEO: Medium earth orbiting

  20. Satellite Orbits

  21. 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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