Chapter 14:
Wireless Networks
Principles of Computer Networks
and Communications
M. Barry Dumas and Morris Schwartz
Objectives

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

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
Describe the role of wireless transmission in computer communications and
the physical phenomena that enable wireless communications
Identify characteristics that are common in all wireless networks
Differentiate between contemporary wireless network configurations and
provide practical examples of their application
Provide examples of alternative LAN protocol sets including their
capabilities and drawbacks
Identify the essential elements of a wireless personal area network including
various configurations, protocols, advantages, and disadvantages
associated with Bluetooth
Describe the essential elements of the IEEE802.15.1 WPAN and IEEE
802.16 wireless standards
Explain various aspects of cellular telephony including current
configurations
Provide characteristics of satellite communications including differentiating
between orbital differences
Chapter 14
Principles of Computer Networks and
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Overview
“Wireless networks employ electromagnetic waves,
primarily radio waves and microwaves,
to carry transmissions over the air
or through the vacuum of space
using antennas to transmit and receive signals.”

For transmission



The electromagnetic carrier is modulated to represent the data signal
Multiplexing allows many transmissions to take place simultaneously
without interfering with one another
Upon receipt

Chapter 14
At the receiver, the signal is demodulated to extract the data
Principles of Computer Networks and
Communications
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Wireless Local Area Networks

Wireless local area networks (WLANs)

Use radio wave carriers to transmit signals
among nodes
 Typically share the networking burden with
wired counterparts
 Provide flexibility and mobility over wired
LANs
Most WLANs operate in 2.4 GHz and 5 GHz bands
Chapter 14
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Wireless Local Area Networks

National information infrastructure (NII)

Collection of network types that includes




Radio and television
Public switched telecommunications network
Private communications networks
Includes the U-NII that defines the industrial, scientific, medical (ISM)
bands that are unlicensed in the United States and most countries
Band
Definition
Range
900 MHz
915 ± 13 MHz
9.02 to 9.28 MHz
2.4 GHz
2.45 ± 0.05 GHz
2.40 to 2.50 GHz
5 GHz
5.8 ± 0.075 GHz
5.725 to 5.875 GHz
Chapter 14
Principles of Computer Networks and
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Wireless Local Area Networks

Wireless local area networks (WLANs)
Advantages
 Easy creation; no cables; can connect to wired LANs
 Provide access in places where wiring is not feasible/too costly
 Simple connection (usually automatic) for spontaneous participation
 Mobility and unconstrained physical configuration (within range)
Disadvantages
 Possible interference from electromagnetic radiation in ISM bands
 Potential for eavesdropping/security breaches
 Limited data rates compared to wired networks
 Incompatibilities due to proprietary schemes in the market
Chapter 14
Principles of Computer Networks and
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Wireless Local Area Networks

WLANs—topology

Fundamental structure of a WLAN is a
Basic service set (BSS)

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Computers in a WLAN are called stations
Minimum BSS has two stations
Stations can be mobile or fixed computers
Can include an access point that connects



Mobile station vs. portable station
Mobile station
operates while moving
Portable station
able to be moved
within the LAN
Wirelessly to the BSS
By wire (through LAN/backbone) to the organization’s wired network
Independent basic service set (IBSS) (aka “ad hoc network”)


Chapter 14
An independent standalone LAN
LAN stations can communicate only within the LAN
Principles of Computer Networks and
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WLANs—Independent basic
service set (IBSS)
Devices within the IBSS
can communicate
with the server
or with one another
Fig 14.1
A WLAN IBSS
with server
Chapter 14
Principles of Computer Networks and
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WLANs—Basic service set (BSS)
wired
access
Fig. 14.2
A WLAN BSS
with access point
Chapter 14
Principles of Computer Networks and
Communications
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Wireless Local Area Networks

WLANs—Basic service set (BSS) (cont.)

Basic building blocks of extended WLANs

When two or more BSSs are connected by
their access points to the same wired LAN

Wired portion is called a distribution system (DS)
DS + BSSs  extended service set (ESS)
Chapter 14
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WLANs BSS and ESS
Wired access
Fig.
14.3
Chapter 14
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Wireless Local Area Networks

The distribution system (DS) (wired portion of an ESS) provides the
following participation services

Association



Disassociation


Within an ESS, a station in one BSS needs to communicate with a station in a
different BSS
Integration


Within an ESS, a station can move to another BSS (different access point)
Distribution


When a station leaves a BSS or shuts down
Re-association


Required for station participation
Stations can associate with only one access point at a time
The DS communicates between ESS stations and the other wired LANs in the
corporate network
Inter-ESS movement

Chapter 14
Stations can move from one ESS to another ESS
Principles of Computer Networks and
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Wireless Local Area Networks

The distribution system (DS) (wired portion of an ESS) provides
the following station specific services

Authentication


A station must identify itself before it can associate with a BSS
Two versions of authentication



Open system authentication
 Station access is never denied
 Station simply identifies itself during association
Shared key
 Controls station access
 Station must possess a secret key to be authenticated
 A secret key is distributed by Wired Equivalent Privacy (WEP)
De-authentication


Chapter 14
A station’s authentication is terminated
Occurs when a station leaves a BSS or is disassociated
Principles of Computer Networks and
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Wireless Local Area Networks

WLAN protocols

Contained in the 802.11 specifications
 Exist in lowest two architecture model layers


Physical—defines electrical/spectrum and bit transmission/receipt
Data link—responsible for



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Frame assembly
Node-to-node error control
Physical addressing
Inter-node synchronization
Medium (channel) access
Two protocol sets

Client/server (LAN paradigm)


Ad hoc (wireless personal area networks paradigm)


Chapter 14
Employs 802.x protocols used by wired LANs
Small coverage areas
Used in Bluetooth networks
Principles of Computer Networks and
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Wireless Local Area Networks

WLAN protocols—physical layer of 802.11

Physical—defines electrical/spectrum and bit transmission/receipt
 Defines four transmission methods (infrared or radio frequency)
1.
2.
3.
4.
(IR)
(RF)
(RF)
(RF)
Infrared (IR)
Frequency hopping spread spectrum (FHSS)
Direct sequence spread spectrum (including high rate) (HR/DSSS)
Orthogonal frequency division multiplexing (OFDM)
For nodes to communicate, each must use
the same transmission method.
Chapter 14
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Wireless Local Area Networks

WLAN protocols—physical layer of 802.11
1.
Except for Bluetooth
rarely used in WLANs
Infrared (IR)




Signals are carried by infrared light
Very short useful range [5 to 6 meters; 15 to 20 feet]
Commonly found in TV remote controls and wireless computer peripherals
(keyboard, mouse, etc.)
Standards developed by the infrared data association (IRDA)
Advantages

Works in electrically noisy environments without interference

Signals can reflect (off walls, ceilings, etc.) to reach target

Inexpensive
Disadvantages

Very limited span

Line-of-sight required

Unable to penetrate solid objects
Chapter 14
Could be an advantage
if security is an issue
(i.e., difficult to intercept)
Principles of Computer Networks and
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Wireless Local Area Networks

WLAN protocols—physical layer of 802.11
1.
Infrared (IR) irDA-defined physical layer protocols
Protocol
Descriptor
Data Rate
IrDA-FIR
fast infrared
up to 4 Mbps
IrDA-MIR
medium infrared
up to 1.15 Mbps
IrDA-SIR
serial infrared
“slow infrared”
up to 115 Kbps
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Wireless Local Area Networks

WLAN protocols—physical layer of 802.11
(RF) Frequency hopping spread spectrum (FHSS)
2.

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
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Narrow bandwidth, only a small portion of 2.4 GHz
spectrum
Entire spectrum is used by constantly shifting the signal
(hopping) across the spectrum
A master station establishes the hopping sequence that is
followed by participating stations
Transmissions appear to take place over a single (virtual)
communications channel
Particularly popular in Bluetooth and HomeRF networks
Chapter 14
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Wireless Local Area Networks

WLAN protocols—physical layer of 802.11
3.
(RF) Direct sequence spread spectrum (high rate) (HR/DSSS)




Spreads the signal over the entire 2.4 GHz spectrum
Entire spectrum is used by substituting a redundant sequence of
bits (chipping code) for each bit of the signal to be transmitted
Because the chipping code data rate is higher than the
original signal rate, there is no delay in signal transmission
Most often used in WiFi 802.11b (11 Mbps) or 802.11g
(below 20 Mbps)
DSSS and FHSS will interfere with each other!
These are not usually found in business environments.
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Wireless Local Area Networks

WLAN protocols—physical layer of 802.11
4.
(RF) Orthogonal frequency division multiplexing (OFDM)

Similar to frequency division multiplexing (FDM), except


Chapter 14
FDM transmits signals from multiple sources at the same time,
with each source assigned a separate sub-band frequency
OFDM assigns all of the sub-bands to a single source for a specified
time

Carrier frequencies are produced so that peak amplitudes of
each frequency coincide with minimum amplitudes of adjacent
frequencies

Modulators see frequencies in only a particular carrier sub-band
Principles of Computer Networks and
Communications
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Wireless Local Area Networks

WLAN protocols—802.11 variations


(2001) 802.11a
(1999) 802.11b




(54 Mbps, 2.4 GHz)
Backward compatible with 802.11b
Essentially eliminated need for 802.11a
(2006) 802.11n

Chapter 14
Original WiFi standard
(2003) 802.11g

(54 Mbps, 5 GHz)
(11 Mbps, 2.4 GHz)
(100 to 600 Mbps, 5 GHz)
Uses multiple input/multiple output (MIMO) signaling with
many data streams traveling over the same frequencies,
and each data stream carrying different information
Principles of Computer Networks and
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Wireless Local Area Networks

WLAN protocols—802.11 variations summarized
Susceptible to microwave/portable phone interference!
Standard
Speed
802.11a
54 Mbps
60 ft
5 GHz
OFDM
802.11b
11 Mbps
300 ft
2.4 GHz
DSSS
802.11g
54 Mbps
300 ft
2.4 GHz
OFDM
802.11n
100 Mbps to
600 Mbps
60 ft
5 GHz
MIMO
Chapter 14
Range Frequency
Principles of Computer Networks and
Communications
Method
22
Wireless Local Area Networks

WLAN protocols—data link layer of 802.11

As with all 802 LANs, the data link layer is subdivided:



When an ESS (collection of BSSs) is created,
component BSSs appear to the LLC as a single IBSS



Chapter 14
Logical link control (LLC)
Media access control (MAC)
Stations can communicate with other stations on the ESS
Stations can move to any BSS on the ESS
A station’s physical address is one of the 48-bit MAC
addresses of the (wireless) NIC
Principles of Computer Networks and
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Wireless Personal Area Networks

Wireless personal area network (WPAN)




Accommodates data sharing and connectivity
Small, often impromptu groups
Limited span (e.g., same room)
Originally designed to replace desktop cable
connections
Predominantly Bluetooth!
Chapter 14
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Wireless Personal Area Networks

WPAN—Bluetooth






802.15.1 establishes Bluetooth
as a de jure standard
Based on the 802.11 standard
However, does not use 802.x LAN protocols
Not designed for LAN communications, large-scale data
Operates in the 2.4 GHz band
Operates in a piconet (supporting 2 to 8 devices)
Uses FHSS to hop from channel to channel within the 79
(1 MHz) sub-bands (channels) of the 2.4 GHz band
FHSS avoids interference from other 2.4 GHz devices
(e.g., portable phones, baby monitors, microwaves, etc.)
Chapter 14
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Wireless Personal Area Networks

WPAN—piconet



Supports 2 to 8 devices (needs at least two active members)
Is established automatically (on the fly)
Devices entering a piconet

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


[with less than 8 devices] are assigned an address
[with 8 or more devices] can be on standby
First member assumes the role of master; others act as slaves
Members can be mobile or stationary
Mobile members can move within a piconet
as long as they stay within range of the master
A collection of piconets is a scatternet
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WPAN—piconet
1
M
1
M
2
3
5
7
4
6
Fig 14.4A and B
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WPAN—piconet
1
M
2
M3
M1
1
M2
1
2
3
1
2
Fig 14.4C
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Wireless Metropolitan Area
Networks

Wireless metropolitan area network (WMAN)
[aka WiMAX]

802.16
 Operates in 2 to 11 GHz band [as of 802.16a]
 High data-data-rate broadband system (to 70 Mbps)
 Can operate over substantial distances (> 30 miles)
 Uses same logical link control (LLC) as other 802
networks, which means:
WiMAX and WiFi networks
can interconnect!
Chapter 14
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Wireless Metropolitan Area
Networks

WMAN (WiMAX)

Provides four key wireless functionalities

High-speed connectivity


Last-mile broadband


Connects mobile devices to access points
Backhaul alternative

Chapter 14
High speed without need for telco last-mile local loops
Hot spot (hot zone) coverage


Alternative to contracting for wired services
Provides wireless access from remote sites to the core network
Principles of Computer Networks and
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Wireless Metropolitan Area
Networks

WMAN (WiMAX)—standards in other countries

European Telecommunications Standard Institute
(ETSI) wireless standards
802.11—WiFi
 802.15—PAN
 802.16—WiMAX
Compatible!


HiperLAN—high performance radio LAN
HiperPAN—high performance radio PAN
HiperMAN—high performance radio MAN
Korean Telecommunications Technology Association
(KTTA)

Chapter 14
802.16—WiMAX
WiBro—wireless broadband
Principles of Computer Networks and
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Cellular Telephony

Terms

Base stations



Cell



Logical way of thinking about a coverage region (usually hexagonal)
Base station coverage areas
Cell phone



Stationary, ground-based sites linked to neighboring sites
Are connected to and controlled by MSCs
Low-power transmitter/receiver for voice and data
Communicating wirelessly through a collection of base stations
Mobile switching centers (MSCs)
(aka mobile telephone switching offices—MTSOs)




Chapter 14
Establish call connections
Coordinate all base stations
Provide links to the wired telephone network and the Internet
Keep calling and billing records
Principles of Computer Networks and
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Cellular Telephony

Basic functionality (simple)

When a call is initiated, a connection is established
between the caller’s cell phone and the base station
of the cell the caller is in
 As the caller begins to move out of range for that cell,
the base station senses the drop in signal power and
relays that information to the MSC
 The MSC automatically “hands off” the call to the
base station of the cell the caller is moving into
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Cellular Telephony

Where are cells located?
Some viewpoints:
Base station 1 coverage
1
Fig 14.5
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Cellular Telephony

Generations and systems

First Generation (1G—early 1980s)



Analog based, multiplexed by FDMA
Advanced mobile phone system (AMPS)
Used 850 MHz band (824–894 MHz)


824–849 MHz
869–894 MHz
mobile unit to base station
base station to mobile unit
Problems
 Noise and poor quality
 Coverage was limited
 Cells had limited capacity
 Easy to tap airborne signals (steal phone codes)
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Cellular Telephony

Generations and systems

European and U.S. GSMs
are not compatible
Second Generation (2G—late 1980s to 1990s)



Introduced digital service
Employ powerful authentication techniques
Three schemes

Chapter 14
Digital AMPS [D-AMPS]
 Digital version of AMPS, based on TDMA
 Uses 850 MHz band [824–894 MHz; same as AMPS]
 Phone voice coders (vocoders) converted analog voice
to digital
Principles of Computer Networks and
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Cellular Telephony

Personal communication system (PCS)
 Uses code division multiple access (CDMA)
 Digital system combines DSSS with chipping codes
 Uses 1,900 MHz band (1,850–1,910 MHz)

Global system for mobile communications (GSM)—developed
in Europe
 Uses combination of FDMA to divide bands into channels
and TDMA to create time slots within the channels
 Uses 850 MHz and 1,900 MHz band in United States
 Uses 900 MHz and 1,800 MHz band in Europe and Asia
Sprint
Verizon
AT&T
Cingular
Nextel
T-Mobile
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Cellular Telephony

Generations and systems

Third Generation (3G)


Addressed speed shortcomings of 2G cell phones (144 Kbps to 2+ Mbps)
[With the speed] Enabled access to more services





Web browsing
Web-based applications
Multimedia
E-mail (with or without attachments)
Works with smart phones (i.e., cell phones, PDAs with cell phone features)
Problems
 Memory
 Online costs
Chapter 14
Although 3G mobile devices can
access broadband services,
connection cost (at cell phone rates)
is still a limiting factor.
Principles of Computer Networks and
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Cellular Telephony

Generations and systems

Evolving Third Generation (3G+)

Three schemes




Universal mobile telephone service (UMTS)
(GSM-type, wide-band code division)
 Designed to run over existing GSM networks
 Will probably replace GSM
CDMA20000 (enhanced 2G code division multiple access)
TD-SCDMA (time division + synchronous code division)
Data rates as high as 14 Mbps
Fourth generation (4G) technology holds the prospect of
data rates between 100 Mbps–1 Gbps
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Satellites
“…line of sight still is required
from the earth transmitter to the satellite,
from the satellite to the earth receiver, and indeed from
one satellite to another.”

There cannot be successful communication


If the (transmit/receive) earth stations cannot “see”
the satellite
If the satellites cannot “see” each other
Echo I (994 mile altitude) orbited the earth every 90 minutes
A spot on earth could “see” Echo I for only 10 minutes each orbit!
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Satellites

Transmission signals



Uplink—from earth location to satellite
Downlink—from satellite to an earthbound station
Orbits




Geosynchronous earth orbits (GEOs)
Medium earth orbits (MEOs)
None of these
Low earth orbits (LEOs)
are synchronous
Highly elliptical orbits (HEOs)
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Satellites

Geosynchronous earth orbits (GEOs)





Appear stationary to an observer on earth
Match the rotation of the earth
22,240 miles (35,786 km) above the earth
Typically centered around the equator
Can see 35 to 40% of the earth within latitude bands
If a GEO satellite is in line of sight,
it will always be in line of sight.
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Satellites
For non-synchronous orbits

Satellites do not appear to be stationary
 Constellations (parades of satellites) are used for coverage
 Transmissions from a “departing” satellite (moving out of line of sight)
are handed off to an incoming satellite

Medium earth orbits (MEOs)


Low earth orbits (LEOs)


Range from 5,000–15,000 km
(3,100–9,300 miles)
Range from 100–2,000 km
(100–1,240 miles)
Highly elliptical orbits (HEOs)

Ranges in altitude from 500–50,000 km
(less than 311–more than 31,000 miles)
Chapter 14
Principles of Computer Networks and
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Only orbit used
for polar regions
43
Satellites

Communications satellites use
microwave signals between 1.5 and 30 GHz
Table 14.1
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