Chapter 14: Wireless WANs Business Data Communications, 5e 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 Business Data Commun ications, 5e 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 Business Data Commun ications, 5e Cellular Wireless Networks • One of the most revolutionary developments in telecommunications • Supports users in locations that are not easily served by wired networks • Used for mobile telephones, personal communications systems, wireless Internet and wireless Web applications, and more Business Data Commun ications, 5e Cellular Network Organization • Uses multiple low-power transmitters (≤100W) • Areas divided into cells, each one served by its own antenna • Each cell allocated a band of frequencies, and is served by a base station • Adjacent cells are assigned different frequencies to avoid interference or crosstalk • Cells sufficiently distant from each other can use the same frequency band Business Data Commun ications, 5e Cellular Geometries Business Data Commun ications, 5e Frequency Reuse Patterns Business Data Commun ications, 5e Increasing Capacity • Adding new channels • Frequency borrowing: Frequencies are taken from adjacent cells by congested cells • Cell splitting: Cells in areas of high usage can be split into smaller cells • Cell sectoring: Cell divided into wedge-shaped sectors Each sector is assigned a separate subset of the cell's channels, and directional antennas at the base station are used to focus on each sector • Microcells: Useful in city streets in congested areas, along highways, and inside large public buildings Business Data Commun ications, 5e Cellular System Overview Business Data Commun ications, 5e Mobile to Base Channels • Control channels are used to exchange information having to with setting up and maintaining calls and with establishing a relationship between a mobile unit and the nearest BS • Traffic channels carry a voice or data connection between users Business Data Commun ications, 5e 10 Wireless Markup Language • Does not assume a standard keyboard or a mouse; designed to work with telephone keypads, styluses, and other input devices common to mobile, wireless communication • Documents are subdivided into small, welldefined units of user interaction called cards; users navigate by moving back and forth between cards • Uses a small set of markup tags appropriate to telephony-based systems Business Data Commun ications, 5e 19 Microbrowser • Based on a user interface model appropriate for mobile, wireless devices • Traditional 12-key phone keypad is used to enter alphanumeric characters • Users navigate among the WML cards using up and down scroll keys rather than a mouse • Navigation features familiar from the Web (e.g., Back, Home, and Bookmark) are provided as well Business Data Commun ications, 5e 20 Wireless Telephony Applications: A Sample Configuration Business Data Commun ications, 5e 21 Satellite Communications • Two or more stations on or near the earth communicate via one or more satellites that serve as relay stations in space • The antenna systems on or near the earth are referred to as earth stations • Transmission from an earth station to the satellite is an uplink, from the satellite to the earth station is downlink • The transponder in the satellite takes an uplink signal and converts it to a downlink signal Business Data Commun ications, 5e 22 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 • Remains above the same spot on the equator as the earth rotates Business Data Commun ications, 5e 23 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 Business Data Commun ications, 5e 24 Problems with Geostationary Orbits • Signal can weaken after traveling that distance • Polar regions and the far northern and southern hemispheres are poorly served • Even at speed of light, the delay in sending a signal 35,838 km each way to the satellite and back is substantial Business Data Commun ications, 5e 25 LEO and MEO Orbits • Alternatives to geostationary orbits • LEO: Low earth orbiting • MEO: Medium earth orbiting Business Data Commun ications, 5e 26 Satellite Orbits Business Data Commun ications, 5e 27 LEO Characteristics • • • • • Circular or slightly elliptical orbit < 2000 km Orbit period is in the range of 1.5 to hours Diameter of coverage is about 8000 km Round-trip signal propagation delay is < 20 ms Maximum time that the satellite is visible from a fixed point on earth (above the radio horizon) is up to 20 minutes • System must be able to cope with large Doppler shifts, which change the frequency of the signal • Significant atmospheric drag on a LEO satellite results in gradual orbital deterioration Business Data Commun ications, 5e 28 LEO Advantages • Reduced propagation delay • Received LEO signal is much stronger than that of GEO signals for the same transmission power • LEO coverage can be better localized so that spectrum can be better conserved • On the other hand, to provide broad coverage over 24 hours, many satellites are needed Business Data Commun ications, 5e 29 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 Business Data Commun ications, 5e 30 MEO Characteristics • • • • • Circular orbit at an altitude of 5000 to 12,000 km Orbit period is about hours Diameter of coverage is 10,000 to 15,000 km Round trip signal propagation delay < 50 ms Maximum time that the satellite is visible from a fixed point on earth (above the radio horizon) is a few hours Business Data Commun ications, 5e 31 Satellite Network Configurations Business Data Commun ications, 5e 32 Satellite Network Applications • Television distribution • Long-distance telephone transmission • Private business networks Business Data Commun ications, 5e 33 ... same frequency band Business Data Commun ications, 5e Cellular Geometries Business Data Commun ications, 5e Frequency Reuse Patterns Business Data Commun ications, 5e Increasing Capacity • Adding... interference Business Data Commun ications, 5e 14 Cellular Multiple Access Schemes Business Data Commun ications, 5e 15 Third Generation Systems • Intended to provide provide high speed wireless communications. .. lightweight communications protocol stack • A framework for wireless telephony applications (WTAs) Business Data Commun ications, 5e 17 WAP Programming Model Business Data Commun ications, 5e 18 Wireless