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.
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
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 2ndgeneration 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-
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
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
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
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
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