Understanding Wireless Networks: Challenges, Technologies, and Applications

1. 42.444 Telecommunications: A Management Perspective Wireless Networks Lecture 12 (Chapters 13) Dr Gerald Grant

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. Dr Gerald Grant

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. Dr Gerald Grant

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. Dr Gerald Grant

5. Advanced Mobile Phone Service Dr Gerald Grant

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 Dr Gerald Grant

7. Cellular architecture • One base station at the centre • A few kilometers in radius (10)

8. Spectrum Utilization Upstream Downstream < 869 MHz 894 MHz > < 824 MHz 849 MHz > 25 MHz 25 MHz First operator Second operator First operator Second operator • 416 channels • 30 kHz

9. 2 7 3 1 6 4 5 Frequency Reuse Reuse factor = 7 416 channels 395 voice 21 control Customer density

10. 2 7 3 1 6 4 5 Frequency Reuse Reuse factor = 7 Assign frequency blocks to each cell 416 channels 395 voice 21 control Customer density

11. 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) Dr Gerald Grant

12. GSM Layout Dr Gerald Grant

13. 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) Dr Gerald Grant

14. 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 Dr Gerald Grant

15. 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 Dr Gerald Grant

16. 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) Dr Gerald Grant

17. WAP Programming Model Dr Gerald Grant

18. WAP Protocol Stack Dr Gerald Grant

19. Wireless Telephony Applications:A Sample Configuration Dr Gerald Grant

20. 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. Dr Gerald Grant

21. 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 Dr Gerald Grant

22. 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. Dr Gerald Grant

23. LEO and MEO Orbits • Alternatives to geostationary orbits • LEO: Low earth orbiting • MEO: Medium earth orbiting Dr Gerald Grant

24. Satellite Orbits Dr Gerald Grant

25. 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 Dr Gerald Grant