Presentation on theme: "Chapter 14: Wireless WANs"— Presentation transcript:
1Chapter 14: Wireless WANs Business Data Communications, 6e
2Reasons 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.
3Problems 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.
4Cellular Wireless Networks One of the most revolutionary developments in telecommunicationsSupports users in locations that are not easily served by wired networksUsed for mobile telephones, personal communications systems, wireless Internet and wireless Web applications, and more
5Cellular 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 stationAdjacent cells are assigned different frequencies to avoid interference or crosstalkCells sufficiently distant from each other can use the same frequency band
8Frequency Reuse Patterns Each cell has a base transceiverGenerally 10 to 50 frequencies assigned to each cellEach cell can have K/N frequencies – where K = total number of frequencies and N = number of cell within the pattern
9Increasing Capacity Adding new channels Frequency borrowing: Frequencies are taken from adjacent cells by congested cellsCell 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
10Typical Macro/Micro Cell Parameters MacrocellMicrocellCell Radius1 to 20 km0.1 to 1 kmTransmission Power1 to 10 W0.1 to 1 WAverage Delay Speed0.1 to 10 ns10 to 100 nsMaximum bit rate0.3 Mbps1 Mbps
12Mobile to Base Channels Control channels are used to exchange information having to do with setting up and maintaining calls and with establishing a relationship between a mobile unit and the nearest BSTraffic channels carry a voice or data connection between users
13Steps in a Mobile Call Mobile unit initialization Mobile-originated callPagingCall acceptedOngoing callHandoff
14Other Mobile Functions Call blockingCall terminationCall dropCalls to/from fixed and remote mobile subscriber
15Mobile Telephony First Generation Second Generation Third Generation analog voice communication using frequency modulation.Second Generationdigital techniques and time-division multiple access (TDMA) or code-division multiple access (CDMA)Third Generationevolving from second-generation wireless systemswill integrate services into one set of standards.
16Multiple Access Four ways to divide the spectrum among active users frequency-division multiple access (FDMA)time-division multiple access (TDMA)code-division multiple access (CDMA)space-division multiple access (SDMA)
17CDMA Based on direct sequence spread spectrum (DSSS) Provides immunity from various kinds of noise and multipath distortion. (The earliest applications of spread spectrum were military, where it was used for its immunity to jamming.)Can be used for hiding and encrypting signals.Several users can independently use the same (higher) bandwidth with very little interference
19Third Generation Systems Intended to provide provide high speed wireless communications for multimedia, data, and videoReflects trend toward universal personal telecommunications and communications accessPersonal communications services (PCSs) and personal communication networks (PCNs) are objectives for 3G wireless.Planned technology is digital using TDMA or CDMA to provide efficient spectrum use and high capacity
20IMT-2000 3rd Generation Capabilities Voice quality comparable to PSTN144 kbps data rate for motor vehicles384 kbps for pedestriansSupport for Mbps for office useSupport for packet and circuit switched data servicesAdaptive Internet interfaceMore efficient spectrum useSupport for a wide variety of mobile equipmentFlexibility
21Wireless Application Protocol (WAP) Designed to work with all wireless technologiesProgramming model based on the WWW Programming ModelWireless Markup Language, adhering to XMLSpecification of a small browser suitable for a mobile, wireless terminalA lightweight communications protocol stackA framework for wireless telephony applications (WTAs)
23Wireless 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 communicationDocuments are subdivided into small, well-defined 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
24MicrobrowserBased on a user interface model appropriate for mobile, wireless devices.Traditional 12-key phone keypad is used to enter alphanumeric charactersUsers 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.
26Satellite Communications Two or more stations on or near the earth communicate via one or more satellites that serve as relay stations in spaceThe antenna systems on or near the earth are referred to as earth stationsTransmission from an earth station to the satellite is an uplink, from the satellite to the earth station is downlinkThe transponder in the satellite takes an uplink signal and converts it to a downlink signal
28Geostationary Satellites Circular orbit 35,838 km above the earth’s surfaceRotates in the equatorial plane of the earth at exactly the same angular speed as the earthRemains above the same spot on the equator as the earth rotates
29Advantages 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
30Problems with Geostationary Orbits Signal can weaken after traveling that distancePolar regions and the far northern and southern hemispheres are poorly servedEven at speed of light, the delay in sending a signal 35,838 km each way to the satellite and back is substantial
32LEO Characteristics Circular or slightly elliptical orbit < 2000 km Orbit period is in the range of 1.5 to 2 hoursDiameter of coverage is about 8000 kmRound-trip signal propagation delay is < 20 msMaximum time that the satellite is visible from a fixed point on earth (above the radio horizon) is up to 20 minutesSystem must be able to cope with large Doppler shifts, which change the frequency of the signalSignificant atmospheric drag on a LEO satellite results in gradual orbital deterioration.
33LEO Advantages Reduced propagation delay Received LEO signal is much stronger than that of GEO signals for the same transmission powerLEO 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.
34Types of LEOsLittle LEOs: Intended to work at communication frequencies below1 GHz using no more than 5 MHz of bandwidth and supporting data rates up to 10 kbpsBig LEOs: Work at frequencies above 1 GHz and supporting data rates up to a few megabits per second
35MEO Characteristics Circular orbit at an altitude of 5000 to 12,000 km Orbit period is about 6 hoursDiameter of coverage is 10,000 to 15,000 kmRound trip signal propagation delay < 50 msMaximum time that the satellite is visible from a fixed point on earth (above the radio horizon) is a few hoursRequire fewer hand-offs than LEOSs