Business data communications 4e chapter 13

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 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