Chapter 14: Wireless WANs Business Data Communications, 5e

Chapter 14:Wireless WANs

Business Data Communications, 5e


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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.


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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.


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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


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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


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Cellular Geometries


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Frequency Reuse Patterns


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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


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Cellular System Overview


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Mobile 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 BS

• Traffic channels carry a voice or data connection between users


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Steps in a Mobile Call

• Monitor for strongest signal

• Request for connection

• Paging

• Call accepted

• Ongoing call

• Handoff


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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.


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Multiple 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)

• FDMA and TDMA discussed in Chapter 17• CDMA and SDMA discussed here


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CDMA

• 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


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Cellular Multiple Access Schemes


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Third Generation Systems

• Intended to provide provide high speed wireless communications for multimedia, data, and video

• Reflects trend toward universal personal telecommunications and communications access

• Personal 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


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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)


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WAP Programming Model


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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, 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


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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.


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Wireless Telephony Applications:A Sample Configuration


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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


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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


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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


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Problems withGeostationary 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


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LEO and MEO Orbits

• Alternatives to geostationary orbits

• LEO: Low earth orbiting• MEO: Medium earth orbiting


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Satellite Orbits


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LEO Characteristics

• Circular or slightly elliptical orbit < 2000 km• Orbit period is in the range of 1.5 to 2 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.


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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.


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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


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MEO Characteristics

• Circular orbit at an altitude of 5000 to 12,000 km• Orbit period is about 6 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


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Satellite Network Configurations


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Satellite Network Applications

• Television distribution

• Long-distance telephone transmission

• Private business networks