Business data communications 4e chapter 13

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  Communication facilities must be installed at low initial cost  The same information must be broadcast to many locations 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 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 Advanced Mobile Phone Service AMPS Components  Mobile Units  contains a modem that can switch between many frequencies  identification numbers: electronic serial number, system ID number, mobile ID number  Base Transceiver  full-duplex communication with the mobile  Mobile Switching Center Global System for Mobile Communication  Developed to provide common 2nd-generation technology for Europe  200 million customers worldwide, almost 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) GSM Layout 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) 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 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 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) WAP Programming Model WAP Protocol Stack Wireless Telephony Applications: A Sample Configuration 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 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 Problems with Geostationary 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 LEO and MEO Orbits  Alternatives to geostationary orbits  LEO: Low earth orbiting  MEO: Medium earth orbiting Satellite Orbits Types of LEOs  Little LEOs: Intended to work at communication frequencies below1 GHz using no more than MHz of bandwidth and supporting data rates up to 10 kbps  Big LEOs: Work at frequencies above GHz and supporting data rates up to a few megabits per second ... Generation Systems  Intended to provide provide high speed wireless communications for multimedia, data, and video  Personal communications services (PCSs) and personal communication networks... GHz using no more than MHz of bandwidth and supporting data rates up to 10 kbps  Big LEOs: Work at frequencies above GHz and supporting data rates up to a few megabits per second ... XML  Specification of a small browser suitable for a mobile, wireless terminal  A lightweight communications protocol stack  A framework for wireless telephony applications (WTAs) WAP Programming