Business data communications 5e by stallings chapter 14

Business Data Communications, 5e3 Problems with Wireless Networks • Operates in a less controlled environment, so is more susceptible to interference, signal loss, noise, and eavesdro

Trang 1

Chapter 14:

Wireless WANs

Business Data Communications, 5e

Trang 2

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.

Trang 3

3

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.

Trang 4

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

Trang 5

5

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

Trang 6

Cellular Geometries

Trang 7

7

Frequency Reuse Patterns

Trang 8

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

Trang 9

9

Cellular System Overview

Trang 10

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

Trang 11

11

Steps in a Mobile Call

• Monitor for strongest signal

• Request for connection

• Paging

• Call accepted

• Ongoing call

• Handoff

Trang 12

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

Trang 13

13

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

Trang 14

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

Trang 15

15

Cellular Multiple Access Schemes

Trang 16

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

Trang 17

• 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)

Trang 18

WAP Programming Model

Trang 19

19

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

Trang 21

21

Wireless Telephony Applications:

A Sample Configuration

Trang 22

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

Trang 23

23

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

Trang 24

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

Trang 25

25

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

Trang 26

LEO and MEO Orbits

• Alternatives to geostationary

orbits

• LEO: Low earth orbiting

• MEO: Medium earth orbiting

Trang 27

Business Data Commun

ications, 5e

27

Satellite Orbits

Trang 28

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

Trang 29

29

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

Trang 30

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

Trang 31

31

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

Trang 32

Satellite Network Configurations

Trang 33

33

Satellite Network Applications

• Television distribution

• Long-distance telephone transmission

• Private business networks

Định dạng
Số trang	33
Dung lượng	533,5 KB