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

Multiple Access

Four ways to divide the spectrum among active users frequency-division multiplexing (FDM) time-division multiplexing (TDM) code-division multiplexing (CDM) space-division multiplexing (SDM)

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 withGeostationary 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