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Business data communications 5e by stallings chapter 14

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.

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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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ications, 5e

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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, 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
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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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ications, 5e

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Wireless Telephony Applications:
A Sample Configuration

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ications, 5e

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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 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
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