Wireless Networks
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Wireless Networking:
Intangible Media
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Depend on transmission at some kind of
electromagnetic frequency, typically radio
frequency, through the atmosphere to carry
data transmissions from one networked
device to another
Appear most frequently in conjunction with
wired networks
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Capabilities of
the Wireless World
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Create temporary connections to existing
wired networks
Establish back-up or contingency
connectivity for existing wired networks
Extend a network’s span beyond the reach of
wire- or fiber-optic-based cabling
Permit certain users to roam with their
machines, within certain limits
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Commercial Applications for
Wireless Networking
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Ready access to data for mobile professionals
Delivery of network access into isolated facilities
Access in environments in which layout and
settings change constantly
Improved customer services in busy areas, e.g,
70% of Starbuck’s 4,000 stores were equipped
with wireless LAN by 2003
Network connectivity in facilities where in-wall
wiring would be impossible to install or
prohibitively expensive
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Cities with Wireless Internet
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Seoul
Taipei
Singapore
Hong Kong (being planned)
Mexico City (being planned)
Some cities in USA are given free wireless
internet access (see http://ilovefreewifi.com/)
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Questions

What are the major challenges to wireless
networking when compared to wired
networking?
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Types of Wireless Networks
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
Local area networks (LANs)
Extended LANs
Mobile computing
Worldwide wireless LAN shipments is
expected to grow from 1.7 million in 1999 to
almost 12 million by 2004
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Wireless LAN Applications
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Still necessary to attach a network interface
to a computer, but the interface attaches to
an antenna and an emitter rather than to a
cable
Requires an access point device (connected
to a wired network) to bridge wireless
components and the wired network
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Wireless Access Point Device
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Wireless Access Point Devices
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How Do WLANs Work?
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Sending signals through the air between
devices
Signal can be transmitted sometimes up to
1,000 feet
Dependent on the signal nature, a signal
may be able to pass through nonmetal wall
and barriers
WLAN cards are needed
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Modes of communication
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Device-to-device interaction
Access point (AP) connections to a wired LAN
 APs
are stationary devices usually installed in
a ceiling (which is analogous to a tower in a
cellular phone network)
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Wireless LAN Transmission

Most common frequencies used
 Radio:
10 KHz to 1 GHz
 Microwave: 1 GHz to 500 GHz
 Infrared: 500 GHz to 1 THz

Primary technologies used
 Infrared
 Laser
 Narrowband,
single-frequency radio
 Spread-spectrum radio (most common)
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Broadcast Medium Principles
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Inverse relationship between frequency and
distance (high frequency  short distance)
(when transmission power is ignored)
Direct relationship between frequency and
data transfer rate and bandwidth
Higher-frequency technologies often use
tight-beam broadcasts and require a clear
line of sight between sender and receiver to
ensure correct delivery
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Infrared LAN Technologies
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Use infrared light beams to send signals
between pairs of devices
Have high frequency and bandwidth; work
well for LAN applications
Require a line of sight between sender and
receiver
Less reliable than the radio technologies
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Kinds of Infrared LANs
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Line-of-sight networks
Reflective wireless networks
 Via

a central hub (or transceiver)
Scatter infrared networks
 Bounce
transmission off walls and ceilings
 Distance limited to ~30m
 Tend to have a slow transmission

Broadband optical telepoint networks
 Fastest
of its kind
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Laser-based LAN Technologies
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Require a clear line of sight between sender
and receiver
Devices are subject to many of the same
limitations as infrared but are not as subject
to interference from visible light sources;
however, laser is much more sensitive to
outside disturbances than infrared
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Narrow-band, Single-frequency
Radio LAN Technologies
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Use low-powered, two-way radio
communications
Require no line-of-sight between sender
and receiver
Broadcast range is 5000 square meters and
cannot go through steel or load-bearing
walls
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Narrow-band, Single-frequency
Wireless LAN Characteristics
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High-powered, Singlefrequency LAN Characteristics
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Spread-spectrum LAN
Technologies

Address weaknesses of single-frequency
communications
 Use
multiple frequencies simultaneously;
improve reliability and reduce susceptibility to
interference
 Make eavesdropping more difficult

Two main kinds
 Frequency-hopping
 Direct-sequence
modulation
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Spread-spectrum LAN
Characteristics
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Frequency-hopping Spreadspectrum
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Frequency is divided into hops
Blocks of data are transmitted on different
frequencies known to the intended receiver.
The receiver gets the signals and puts the
block in the right order.
Provides built-in security (but not error
detection) because the frequencies hop
around at undetermined times and
frequencies and cannot be easily determined
by potential hackers
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Direct-sequence Modulation
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Breaks a message into bits and modulates
each bit by a redundant bit pattern (known
as chip or chipping code) before each
information bit is transmitted
Tends to use bandwidth inefficiently but
transmissions are more reliable and secured
 Chip
may be used for error detection and
correction purpose
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Direct-sequence Modulation
Example
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Information bits to be transmitted
 1011
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Chipping code
 010
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Transmitted bits (modulo-2 addition)
 101010101101
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Wireless Extended LAN
Technologies

Wireless bridge
A
pair of devices, typically narrow-band and
tight beam, that relay network traffic from
one location to another
 Available using spread-spectrum radio,
infrared, and laser technologies
 Can span distances from hundreds of meters
up to 25 miles
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Wireless Extended LAN
Characteristics
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Microwave Networking
Technologies
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Can deliver higher transmission rates than
radio-based systems
Transmitters and receivers must share a
common, clear line of sight
Usually require FCC approval and licensing
More expensive than radio systems
Two types
 Terrestrial
 Satellite
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Terrestrial Microwave
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Uses line-of-sight communication between
pairs of Earth-based transmitters and
receivers to relay information
Expensive; usually positioned well above
ground level
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Terrestrial Microwave
LAN/WAN Characteristics
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Satellite Microwave
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Uses geosynchronous satellites to send and
relay signals between sender and receiver
Usually leased for an exorbitant fee
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Satellite Microwave WAN
Characteristics
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Security issues

(1/4)
Adopt an encryption mechanism
 If
possible, use Wi-Fi Protected Access (WPA)
and WPA2 instead of wired Equivalent Privacy
(WEP)
 All clients and access points are to be
configured with the same key for encryption
and decryption (manually as there is no key
management protocol)
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Security issues

(2/4)
Use a Service Set Identifier (SSID)
a
case-sensitive string attached to all packets
on a wireless network to identify each packet
as part of that network
 all wireless devices attempting to
communicate with each other must share the
same SSID
 never use the default SSID given by the
manufacturer
 turn off SSID broadcasting
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Security issues

(3/4)
MAC addressing filtering
A
list of client computers’ MAC addresses can
be inputted into an Access Point so that only
those computers will be allowed access.
 This is a good method of security but only
recommended for smaller networks as there is
a high rate of work involved in entering each
MAC address into every Access Point.
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Security issues

(4/4)
Restrict unnecessary traffic
 If
your router is equipped with a firewall, turn
it on.
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Change the default administrator password
Protect your PC with antivirus software and
apply any new system patches timely
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Summary

(1/2)
Wireless networking
 Provides
cable-free LAN access
 Extends span of LANs
 Provides WAN links
 Supports mobile computing needs
 Uses a variety of electromagnetic frequency
ranges
• Narrow-band and spread-spectrum radio
• Microwave
• Infrared
• Laser transmission
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Summary

(2/2)
Common security approaches
 Wired
equivalent privacy
 Service set identifier
 MAC address filtering
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