Chapter 14:Wireless WANs

Business Data Communications, 6e

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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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Frequency Reuse Patterns

• Each cell has a base transceiver• Generally 10 to 50 frequencies assigned to

each cell• Each cell can have K/N frequencies –

where K = total number of frequencies and N = number of cell within the pattern

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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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Typical Macro/Micro Cell ParametersMacrocell Microcell

Cell Radius 1 to 20 km 0.1 to 1 kmTransmission Power

1 to 10 W 0.1 to 1 W

Average Delay Speed

0.1 to 10 ns 10 to 100 ns

Maximum bit rate

0.3 Mbps 1 Mbps

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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

• Mobile unit initialization• Mobile-originated call• Paging• Call accepted• Ongoing call• Handoff

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Other Mobile Functions

• Call blocking• Call termination• Call drop• Calls to/from fixed and remote mobile

subscriber

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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)

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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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IMT-2000 3rd Generation Capabilities

• Voice quality comparable to PSTN

• 144 kbps data rate for motor vehicles

• 384 kbps for pedestrians• Support for 2.048 Mbps

for office use• Support for packet and

circuit switched data services

• Adaptive Internet interface

• More efficient spectrum use

• Support for a wide variety of mobile equipment

• Flexibility

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Wireless Application Protocol (WAP)

• Designed to work with all wireless technologies• 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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WAP Network Schematic

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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 Earth Orbit

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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

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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

• Require fewer hand-offs than LEOSs

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Satellite Network Configurations

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Satellite Network Applications

• Television distribution• Long-distance telephone transmission• Private business networks

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Typical VSAT Configuration