Business data communications 4e chapter 13

Reasons for Wireless Networks Mobile communication is needed..  Generally, wireless facilities have lower data rates than guided facilities.. Mobile Telephony First Generation  anal

Chapter 13:

Wireless Networks

Business Data Communications, 4e

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

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.

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.

Advanced Mobile Phone Service

AMPS Components

 Mobile Units

 contains a modem that can switch between many frequencies

 3 identification numbers: electronic serial number, system ID number, mobile ID number

 Base Transceiver

 full-duplex communication with the mobile

 Mobile Switching Center

Global System for Mobile Communication

 Developed to provide common 2nd-generation

technology for Europe

 200 million customers worldwide, almost 5 million in the North America

 GSM transmission is encrypted

 Spectral allocation: 25 MHz for base transmission (935–

960 MHz), 25 MHz for mobile transmission (890–915 MHz)

GSM Layout

Choice of Access Methods

 FDM, used in 1st generation systems, wastes spectrum

 Debate over TDMA vs CDMA for 2nd generation

 TDMA advocates argue there is more successful experience with

TDMA

 CDMA proponents argue that CDMA offers additional features as

well, such as increased range.

 TDMA systems have achieved an early lead in actual implementations

 CDMA seems to be the access method of choice for third-generation systems

Third Generation Systems

 Intended to provide provide high speed wireless

communications for multimedia, data, and video

 Personal communications services (PCSs) and

personal communication networks (PCNs) are

objectives for third-generation wireless.

 Planned technology is digital using TDMA or CDMA

to provide efficient spectrum use and high capacity

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)

WAP Programming Model

WAP Protocol Stack

Wireless Telephony Applications:

A Sample Configuration

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

 will remain above the same spot on the equator as the earth rotates

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

Problems with Geostationary Orbits

 Signal can weaken after traveling > 35,000 km

 Polar regions and the far northern and

southern hemispheres are poorly served

 Even at speed of light, about 300,000 km/sec, the delay in sending a signal from a point on

the equator beneath the satellite 35,838 km to the satellite and 35,838 km back is substantial

LEO and MEO Orbits

 Alternatives to geostationary orbits

 LEO: Low earth orbiting

 MEO: Medium earth orbiting

Satellite Orbits

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