Chapter 13: Wireless Networks Business Data Communications, 4e

Reasons for Wireless Networks 8Mobile communication is needed. 8Communication must take place in a terrain that makes wired communication difficult or impossible. 8A communication system must be deployed quickly. 8Communication facilities must be installed at low initial cost. 8The same information must be broadcast to many locations.

Problems with Wireless Networks 8Operates in a less controlled environment, so is more susceptible to interference, signal loss, noise, and eavesdropping. 8Generally, wireless facilities have lower data rates than guided facilities. 8Frequencies can be more easily reused with guided media than with wireless media.

Mobile Telephony 8First Generation 8analog voice communication using frequency modulation. 8Second Generation 8digital techniques and time-division multiple access (TDMA) or code-division multiple access (CDMA) 8Third Generation 8evolving from second-generation wireless systems 8will integrate services into one set of standards.

Advanced Mobile Phone Service

AMPS Components 8Mobile Units 8contains a modem that can switch between many frequencies 83 identification numbers: electronic serial number, system ID number, mobile ID number 8Base Transceiver 8full-duplex communication with the mobile 8Mobile Switching Center

Global System for Mobile Communication 8Developed to provide common 2nd- generation technology for Europe 8200 million customers worldwide, almost 5 million in the North America 8GSM transmission is encrypted 8Spectral allocation: 25 MHz for base transmission (935–960 MHz), 25 MHz for mobile transmission (890–915 MHz)

GSM Layout

Multiple Access 8Four ways to divide the spectrum among active users 8frequency-division multiplexing (FDM) 8time-division multiplexing (TDM) 8code-division multiplexing (CDM) 8space-division multiplexing (SDM)

Choice of Access Methods 8FDM, used in 1st generation systems, wastes spectrum 8Debate over TDMA vs CDMA for 2nd generation 8TDMA advocates argue there is more successful experience with TDMA. 8CDMA proponents argue that CDMA offers additional features as well, such as increased range. 8TDMA systems have achieved an early lead in actual implementations 8CDMA seems to be the access method of choice for third- generation systems

Third Generation Systems 8Intended to provide provide high speed wireless communications for multimedia, data, and video 8Personal communications services (PCSs) and personal communication networks (PCNs) are objectives for third-generation wireless. 8Planned technology is digital using TDMA or CDMA to provide efficient spectrum use and high capacity

Wireless Application Protocol (WAP) 8Programming model based on the WWW Programming Model 8Wireless Markup Language, adhering to XML 8Specification of a small browser suitable for a mobile, wireless terminal 8A lightweight communications protocol stack 8A framework for wireless telephony applications (WTAs)

WAP Programming Model

WAP Protocol Stack

Wireless Telephony Applications: A Sample Configuration

Geostationary Satellites 8 Circular orbit 35,838 km above the earth’s surface 8 rotates in the equatorial plane of the earth at exactly the same angular speed as the earth 8 will remain above the same spot on the equator as the earth rotates.

Advantages of Geostationary Orbits 8 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). 8 Tracking of the satellite by its earth stations is simplified. 8 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 8 Signal can weaken after traveling > 35,000 km 8 Polar regions and the far northern and southern hemispheres are poorly served 8 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 8 Alternatives to geostationary orbits 8 LEO: Low earth orbiting 8 MEO: Medium earth orbiting

Satellite Orbits

Types of LEOs 8Little 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 8Big LEOs: Work at frequencies above 1 GHz and supporting data rates up to a few megabits per second