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CWNA Guide to Wireless LANs,
Second Edition
Chapter One
It’s a Wireless World
A Day in the Life of a Wireless
User: Home


Hotspots: Locations where wireless
data services are available
Wireless local area network
(WLAN): Essentially identical to
standard local area network (LAN)
• Except devices not connected by wires
• Can increase productivity
2
A Day in the Life of a Wireless
User: Car

Bluetooth wireless standard:
Enables short range wireless
communication
• Used in many small devices
3
A Day in the Life of a Wireless
User: Office

Fixed broadband wireless:
Wireless transmissions between
immobile devices
• Typically between office buildings
• Utilizes small, customized antennas

Free space optics (FSO):
Alternative to high-speed fiber optic
transmissions
4
A Day in the Life of a Wireless
User: On Site

Radio frequency identification
(RFID) tags:
• “Electronic barcodes”
• Used to identify items
• Can be read if anywhere within range of
transmitted radio signal

Depending on device
5
Wireless Local Area Networks
(WLANs)


Wi-Fi (Wireless Fidelity): Based on
standard that transmits at up to 11 Mbps
Computers on WLAN must have wireless
network interface cards (wireless NIC or
Wireless adapter)
• Performs same basic functions as standard
NIC, plus more


Access point (AP): Transfers signals
between wireless NICs
Patch cable connects AP to wired LAN or
Internet
6
Bluetooth


Low-power wireless data and voice
transmission technology
Bluetooth devices communicate via radio
modules
• Link manager: Software that helps identify
other Bluetooth devices, creates links between
devices, and sends and receives data


Transmit data at up to 1 Mbps over 10
meters
Bluetooth devices within range of each
other automatically connect
• Master and slave
7
Telecommunications Links





Integrated Services Digital Networks
(ISDN): Transmits at 256 Kbps
T-1 lines: Transmit at 1.544 Mbps
Cable modems: Use television cable
connection
Digital subscriber lines (DSL): Use
telephone lines
WiMax: Signal transmitted between
antennas
• Up to 75 Mbps and over up to 35 miles
• Fixed Broadband
8
Telecommunications Links
(continued)

FSO: Transmit at speeds up to 1.25 Gbps
over up to 4 miles
• Line-of-site transmission
Figure 1-6: Free space optics transceiver
9
Cellular Telephony

Global Systems for Mobile (GSM) a
communications technology
• Coverage includes most of US and parts
of Europe and Japan
• Transmission speeds up to 9.6 Kbps
• Uses Wireless Application Protocol
(WAP)

Standard way to transmit, format, and
display data for devices like cell phones
and handheld devices
10
Cellular Telephony (continued)
Figure 1-8: Browsing the World Wide Web
11
Cellular Telephony (continued)


WAP cell phone runs a microbrowser
that uses Wireless Markup Language
(WML) instead of HTML
WAP gateway or proxy: Translates
between WML and HTML
Figure 1-9: WAP communications
12
Radio Frequency Identification
(RFID)

Like an electronic barcode:
• Can contain larger amounts of updatable information
• Information transmitted via radio waves
• Range typically about 1 foot at 5 Mbps
Figure 1-10: RFID tag
13
Wireless LAN Applications:
Business

Wireless LAN technologies have
significantly changed how business
conducted
• Meetings not confined to conference
rooms
• Easier to connect to network resources
and Internet
• Can create office in space where
traditional infrastructure does not exist
14
Wireless LAN Applications:
Healthcare (continued)
Figure 1-12: Video pill
15
Wireless Advantages and
Disadvantages: Advantages



Mobility is Primary advantage of
wireless technology
Easier and Less Expensive Installation:
Installing network cabling in older
buildings difficult and costly
Increased Reliability
• eliminates certain types of cable failures and
increases overall network reliability
16
Wireless Advantages and
Disadvantages: Advantages

Disaster Recovery:
• Hot site: Off-site facility that can run
business’s operations if primary site is not
available


Generally maintained by third party
Expensive
• Cold site: Customer provides and installs
equipment

Many businesses use cold sites and WLANs as major
piece of disaster recovery plan
• No consideration given to network cabling
17
Wireless Advantages and
Disadvantages: Disadvantages

Security: Wireless signals broadcast in
open air
• Security for wireless LANs is prime concern
Unauthorized users might access network
 Attackers might view transmitted data
 Employees could install rogue access points
 Attackers could easily crack existing wireless security
Radio Signal Interference: Signals from other devices can
disrupt wireless transmissions
Health Risks: Wireless devices emit RF energy



• Not known if or to what extent low levels of RF might cause
adverse health effects
18
CWNA Guide to Wireless LANs, Second Edition
Chapter Two
Wireless LAN Devices and Standards
WLAN Devices: Access Point

Three major parts:
• Antenna and radio transmitter/receiver
• RJ-45 wired network interface
• Special bridging software


To interface wireless devices to other
devices
Two basic functions:
• Base station for wireless network
• Bridge between wireless and wired
networks
20
WLAN Devices: Access Point

Range depends on several factors:
• Type of wireless network, walls, doors, and
other solid objects (think refrigerator)

Number of wireless clients that single AP
can support varies:
• Theoretically over 100 clients
• No more than 50 for light network use
• No more than 20 for heavy network use

Power over Ethernet (PoE): Power
delivered to AP through unused wires in
standard unshielded twisted pair (UTP)
Ethernet cable
21
WLAN Devices: Remote
Wireless Bridge

Bridge: Connects two network segments together
• Even if they use different types of physical media

Remote wireless bridge: Connects two or more
wired or wireless networks together
• Transmit at higher power than WLAN APs
• Use directional antennas to focus transmission in single
direction
• Delay spread: Minimize spread of signal so that it can reach
farther distances
• Have software enabling selection of clearest transmission
channel and avoidance of noise and interference
22
WLAN Devices: Remote
Wireless Bridge

Four modes:
• Access point mode: Functions as standard AP
• Root mode: Root bridge can only communicate
with other bridges not in root mode
• Non-root mode: Can only transmit to another
bridge in root mode
• Repeater mode: Extend distance between LAN
segments

Placed between two other bridges
23
Advantages and Disadvantages
of Standards
Table 2-1: Advantages and disadvantages of standards
24
3 Types of Standards


De Facto, De jure and Consortia
De facto standards: Common practices
that the industry follows for various
reasons
• Ranging from ease of use to tradition to what
majority of users do
• Usually established by success in marketplace

De jure standards: Official standards
• Controlled by organization or body that has
been entrusted with that task
• Process for creating these standards can be
very involved
25
Types of Standards (continued)


One complaint against de jure
standards is amount of time it takes
for a standard to be completed
Consortia: Usually industrysponsored organizations that want to
promote a specific technology
• Goal is to develop a standard that
promotes organization’s specific
technology in little time
26
Enforcing Standards

Marketplace itself enforces some standards
• Standards created by consortia often regulated
by marketplace

De jure standards often enforced by outside
regulatory agency
• Ensure that participants adhere to prescribed
standards
• Must have power to enforce standards and
effectively punish those who refuse to abide by
them
27
Wireless Standards Organizations
and Regulatory Agencies

Three primary standard-setting and
regulatory bodies that play major
role in wireless LAN technology
• Institute of Electrical and Electronics
Engineers (IEEE)
• Wi-Fi Alliance
• U.S. Federal Communications
Commission (FCC)
28
Institute of Electrical and
Electronics Engineers (IEEE)

Establishes standards for telecommunications
• Also covers wide range of IT standards

World’s largest technical professional society
• 37 Societies and Councils
• Publish technically focused journals, magazines, and
proceedings
• Work on over 800 standards

Best known for its work in establishing standards
for computer networks
• Project 802

February 1980….hence 80+2 = 802
29
Institute of Electrical and
Electronics Engineers
Table 2-2: Current IEEE 802 committees
30
Wi-Fi Alliance

Wireless Ethernet Compatibility
Alliance (WECA): Consortium of wireless
equipment manufacturers and software
providers formed to promote wireless
network technology
• Three goals:



Encourage wireless manufacturers to use IEEE
802.11 technologies
Promote and market these technologies to consumers
at home, and in small and large organizations
Test and certify that wireless products adhere to the
IEEE 802.11 standards
31
Wi-Fi Alliance (continued)

WECA changed to Wi-Fi Alliance in 2002
• Reflected name of certification that it uses (Wi-Fi)
to verify that products follow IEEE standards
• Only products that pass Wi-Fi Alliance tests may
be referred to as Wi-Fi Certified

Wi-Fi Alliance now allows businesses to
apply to be registered as a Wi-Fi ZONE
• Qualifies them to be placed in online database of
wireless hotspot locations

Can be accessed through Alliance’s Web site
32
FCC: Regulating the Radio
Frequency Spectrum

Two unregulated bands used for WLANs
• Industrial, Scientific, and Medical (ISM) band
• Unlicensed National Information Infrastructure
(U-NII) band


Intended for devices that provide short-range, highspeed wireless digital communications
Negative features of unregulated bands:
• Devices from different vendors may attempt to
use same frequency

Can cause interference and unpredictability
33
FCC: Regulating the Radio
Frequency Spectrum
Table 2-4: Unlicensed bands
34
Types of Wireless LANs

Since late 1990s, IEEE has approved
four standards for wireless LANs:
• IEEE
• IEEE
• IEEE
• IEEE

802.11
802.11b
802.11a
802.11g
IEEE 802.11n expected to be
approved by 2006
35
IEEE 802.11


Specified that wireless transmission could
take place via infrared (IR) or radio
signals
Infrared Transmissions:
• Can send data by the intensity of the infrared
light wave
• Light spectrum: All types of light
• Infrared light: Can be used for wireless
transmissions

Invisible
• Emitter: Device that transmits a signal
• Detector: Device that receives a signal
36
IEEE 802.11 (continued)

Infrared Transmissions (continued):
• Advantages:



Does not interfere with other communications signals
Not affected by other signals
Does not penetrate walls
• Disadvantages:




Lack of mobility
Limited range
Confined to indoor use
Slow transmission speed
37
IEEE 802.11 (continued)

Radio Wave Transmissions:
• Radio waves can penetrate through objects

Provides mobility
• Radio waves travel longer distances
• Can be used indoors and outdoors
• Radio waves can travel at much higher speeds
than infrared transmissions
• IEEE 802.11 standard outlining radio wave
transmissions has become preferred method
for wireless LANs
38
IEEE 802.11b


802.11 standard’s 2 Mbps bandwidth not
sufficient for most network applications
802.11b amendment added two higher
speeds (5.5 Mbps and 11 Mbps) to original
802.11 standard
• Uses ISM band

Supports wireless devices up to 115 meters
(375 feet) apart
• Radio waves decrease in power over distance
• 802.11b standard specifies that, when devices
out of range to transmit at 11 Mbps, devices
drop transmission speed to 5.5 Mbps
39
IEEE 802.11a

IEEE 802.11a standard specifies maximum
rated speed of 54 Mbps
• Also supports 48, 36, 24, 18, 12, 9,and 6 Mbps
transmissions using U-NII band

802.11a and 802.11b published at same
time
• 802.11a came to market later due to technical
issues and high production cost

Range of 802.11a is less than that of
802.11b
40
IEEE 802.11g

Effort to combine best features of
802.11a and 802.11b
• Data transfer rates to 54 Mbps
• Support devices up to 115 meters apart

802.11g standard specifies that
devices operate entirely in ISM
frequency
41
Projected IEEE 802.11n



Currently in evaluation stage
Top speed of 802.11n standard will
be anywhere from 100 Mbps to 500
Mbps
Ratification may not occur until 2006
• Devices based on standard may appear
prior to that

802.11 pre-N
42
CWNA Guide to Wireless LANs,
Second Edition
Chapter Three
How Wireless Works
Frequency (continued)


Frequency: Rate at which an event
occurs
Cycle: Changing event that creates
different radio frequencies
• When wave completes trip and returns back to
starting point it has finished one cycle

Hertz (Hz): Cycles per second
• Kilohertz (KHz) = thousand hertz
• Megahertz (MHz) = million hertz
• Gigahertz (GHz) = billion hertz
44
Frequency (continued)


Frequency of radio wave can be
changed by modifying voltage
Radio transmissions send a carrier
signal
• Increasing voltage will change
frequency of carrier signal
45
Analog Modulation



Amplitude: Height of carrier wave
Amplitude modulation (AM): Changes
amplitude so that highest peaks of carrier
wave represent 1 bit while lower waves
represent 0 bit
Frequency modulation (FM): Changes
number of waves representing one cycle
• Number of waves to represent 1 bit more than
number of waves to represent 0 bit

Phase modulation (PM): Changes
starting point of cycle
• When bits change from 1 to 0 bit or vice versa
46
Antenna Concepts

Radio waves transmitted/received using antennas
Figure 3-24: Antennas are required for sending and receiving
radio signals
47
Characteristics of RF Antenna
Transmissions

Polarization: Orientation of radio waves as
they leave the antenna
Figure 3-25: Vertical polarization
48
Characteristics of RF Antenna
Transmissions (continued)

Wave propagation: Pattern of wave dispersal
Figure 3-26: Sky wave propagation
49
Characteristics of RF Antenna
Transmissions (continued)
Figure 3-27: RF Line of Sight (LOS) propagation
50
Antenna Types and Their
Installations

Omni-directional antenna: Radiates signal
in all directions equally
• Most common type of antenna

Semi-directional antenna: Focuses energy
in one direction
• Primarily used for short and medium range
remote wireless bridge networks

Highly-directional antennas: Send
narrowly focused signal beam
• Generally concave dish-shaped devices
• Used for long distance, point-to-point wireless
links
51
CWNA Guide to Wireless LANs,
Second Edition
Chapter Four
IEEE 802.11 Physical Layer
Standards
Introduction
Figure 4-2: OSI data flow
53
Introduction (continued)
Table 4-1: OSI layers and functions
54
Wireless Modulation Schemes

Four primary wireless modulation
schemes:
•
•
•
•


Narrowband transmission
Frequency hopping spread spectrum (FHSS)
Direct sequence spread spectrum (DSSS)
Orthogonal frequency division multiplexing
(OFDM)
Narrowband transmission used
primarily by radio stations
Other three used in IEEE 802.11 WLANs
55
Narrowband Transmission
Figure 4-3: Narrowband transmission
56
Spread Spectrum Transmission
Figure 4-4: Spread spectrum transmission
57
Spread Spectrum Transmission

Advantages over narrowband:
• Resistance to narrowband interference
• Lower power requirements
• Less interference on other systems
• More information transmitted
• Increased security
• Resistance to multipath distortion (e.g.
reflections off of buildings and
structures)
58
IEEE 802.11 Physical
Layer Standards


IEEE wireless standards follow OSI
model, with some modifications
Data Link layer divided into two
sublayers:
• Logical Link Control (LLC) sublayer:
Provides common interface, reliability,
and flow control
• Media Access Control (MAC) sublayer:
Appends physical addresses to frames
59
IEEE 802.11 Physical Layer
Standards (continued)

Physical layer divided into two sublayers:
• Physical Medium Dependent (PMD) sublayer:
Makes up standards for characteristics of
wireless medium (such as DSSS or FHSS) and
defines method for transmitting and receiving
data
• Physical Layer Convergence Procedure (PLCP)
sublayer: Performs two basic functions


Reformats data received from MAC layer into frame
that PMD sublayer can transmit
“Listens” to determine when data can be sent
60
IEEE 802.11 Physical Layer
Standards (continued)
Figure 4-10: Data Link sublayers
61
IEEE 802.11 Physical Layer
Standards (continued)
Figure 4-11: PHY sublayers
62
IEEE 802.11b Physical Layer
Standards (continued)

PLCP frame made up of three parts:
• Preamble: prepares receiving device for rest
of frame
• Header: Provides information about frame
• Data: Info being transmitted







Synchronization field
Start frame delimiter field
Signal data rate field
Service field
Length field
Header error check field
Data field
63
IEEE 802.11b Physical Layer
Standards (continued)

Physical Medium Dependent
Standards: PMD translates binary 1’s and
0’s of frame into radio signals for
transmission
• Can transmit at 11, 5.5, 2, or 1 Mbps
• 802.11b uses ISM band

14 frequencies can be used
• Two types of modulation can be used


Differential binary phase shift keying (DBPSK):
For transmissions at 1 Mbps
Differential quadrature phase shift keying
(DQPSK): For transmissions at 2, 5.5, and 11 Mbps
64
IEEE 802.11b Physical Layer
Standards (continued)
Table 4-2: 802.11b ISM channels
65
IEEE 802.11a Physical
Layer Standards

IEEE 802.11a achieves increase in
speed and flexibility over 802.11b
primarily through OFDM
• Use higher frequency
• Accesses more transmission channels
• More efficient error-correction scheme
66
U-NII Frequency Band


Total bandwidth available for IEEE 802.11a
WLANs using U-NII is almost four times
that available for 802.11b networks using
ISM band
Disadvantages:
• In some countries outside U.S., 5 GHz bands
allocated to users and technologies other than
WLANs
• Interference from other devices is growing

Interference from other devices one of
primary sources of problems for 802.11b
and 802.11a WLANs
67
IEEE 802.11g Physical Layer
Standards



802.11g combines best features of
802.11a and 802.11b
Operates entirely in 2.4 GHz ISM
frequency
Two mandatory modes and one optional
mode
• CCK mode used at 11 and 5.5 Mbps
(mandatory)
• OFDM used at 54 Mbps (mandatory)
• PBCC-22 (Packet Binary Convolution
Coding): Optional mode

Can transmit between 6 and 54 Mbps
68
IEEE 802.11g Physical Layer
Standards (continued)
Table 4-8: IEEE 802.11g Physical layer standards
69
IEEE 802.11g Physical Layer
Standards (continued)

Characteristics of 802.11g standard:
•
•
•
•
•
Greater throughput than 802.11b networks
Covers broader area than 802.11a networks
Backward compatible
Only three channels
If 802.11b and 802.11g devices transmitting in
same environment, 802.11g devices drop to
11 Mbps speeds
• Vendors can implement proprietary higher
speed

Channel bonding and Dynamic turbo
70
CWNA Guide to Wireless LANs,
Second Edition
Chapter Five
IEEE 802.11 Media Access Control and
Network Layer Standards
IEEE Wireless LAN Configurations:
Basic Service Set

Basic Service Set (BSS): Group of
wireless devices served by single AP
• infrastructure mode

BSS must be assigned unique identifier
• Service Set Identifier (SSID)


Serves as “network name” for BSS
Basic Service Area (BSA): Geographical
area of a BSS
• Max BSA for a WLAN depends on many factors

Dynamic rate shifting: As mobile
devices move away from AP, transmission
speed decreases
72
IEEE Wireless LAN Configurations:
Extended Service Set


Extended Service Set (ESS): Comprised
of two or more BSS networks connected
via a common distribution system
APs can be positioned so that cells overlap
to facilitate roaming
• Wireless devices choose AP based on signal
strength
• Handoff
73
IEEE Wireless LAN Configurations:
Independent Basic Service Set

Independent Basic Service Set (IBSS):
Wireless network that does not use an AP
• Wireless devices communicate between themselves
• Peer-to-peer or ad hoc mode


BSS more flexible than IBSS in being able to
connect to other wired or wireless networks
IBSS useful for quickly and easily setting up
wireless network
• When no connection to Internet or external network
needed
74
IEEE 802.11 Media Access Control
(MAC) Layer Standards

Media Access Control (MAC) layer
performs several vital functions in a WLAN
•
•
•
•

Discovering WLAN signal
Joining WLAN
Transmitting on WLAN
Remaining connected to WLAN
Mechanics of how functions performed
center around frames sent and received in
WLANs
75
Discovering the WLAN:
Beaconing

At regular intervals, AP (infrastructure
network) or wireless device (ad hoc
network) sends beacon frame
• Announce presence
• Provide info for other devices to join network

Beacon frame format follows standard
structure of a management frame
• Destination address always set to all ones
76
Discovering the WLAN: Beaconing

Beacon frame body contains following
fields:
•
•
•
•
•
•


Beacon interval
Timestamp
Service Set Identifier (SSID)
Supported rates
Parameter sets
Capability information
In ad hoc networks, each wireless device
assumes responsibility for beaconing
In infrastructure networks beacon interval
normally 100 ms, but can be modified
77
Discovering the WLAN: Scanning


Receiving wireless device must be looking
for beacon frames
Passive scanning: Wireless device
simply listens for beacon frame
• Typically, on each available channel for set
period

Active scanning: Wireless device first
sends out a management probe request
frame on each available channel
• Then waits for probe response frame from all
available APs
78
Discovering the WLAN: Scanning
Figure 5-8: Active scanning
79
Joining the WLAN: Authentication

Unlike standard wired LANS, authentication
performed before user connected to network
• Authentication of the wireless device, not the user



IEEE 802.11 authentication: Process in which
AP accepts or rejects a wireless device
Open system authentication: Most basic, and
default, authentication method
Shared key authentication: Optional
authentication method
• Utilizes challenge text
80
Joining the WLAN: Authentication
Figure 5-9: Open system authentication
81
Joining the WLAN:
Authentication (continued)
Figure 5-10: Shared key authentication
82
Joining the WLAN:
Authentication

Open system and Shared key
authentication techniques are weak
• Open System: Only need SSID to connect
• Shared Key: Key installed manually on devices


Can be discovered by examining the devices
Digital certificates: Digital documents
that associate an individual with key value
• Digitally “signed” by trusted third party
• Cannot change any part of digital certificate
without being detected
83
Joining the WLAN: Association

Association: Accepting a wireless device
into a wireless network
• Final step to join WLAN

After authentication, AP responds with
association response frame
• Contains acceptance or rejection notice


If AP accepts wireless device, reserves
memory space in AP and establishes
association ID
Association response frame includes
association ID and supported data rates
84
Transmitting on the WLAN:
Distributed Coordination Function
(DCF)


MAC layer responsible for controlling
access to wireless medium
Channel access methods: Rules for
cooperation among wireless devices
• Contention: Computers compete to use
medium


If two devices send frames simultaneously, collision
results and frames become unintelligible
Must take steps to avoid collisions
85
Transmitting on the WLAN:
Distributed Coordination Function

Carrier Sense Multiple Access with Collision
Detection (CSMA/CD): Before networked
device sends a frame, listens to see if another
device currently transmitting
• If traffic exists, wait; otherwise send
• Devices continue listening while sending frame


If collision occurs, stops and broadcasts a “jam” signal
CSMA/CD cannot be used on wireless networks:
• Difficult to detect collisions
• Hidden node problem
86
Transmitting on the WLAN:
Distributed Coordination Function

Distributed Coordination Function
(DCF): Specifies modified version of
CSMA/CD
• Carrier Sense Multiple Access with
Collision Avoidance (CSMA/CA)
• Attempts to avoid collisions altogether
• Time when most collisions occur is
immediately after a station completes
transmission
• All stations must wait random amount of time
after medium clear

Slot time
87
Transmitting on the WLAN:
Distributed Coordination Function

CSMA/CA also reduces collisions via
explicit frame acknowledgment
• Acknowledgment frame (ACK): Sent by
receiving device to sending device to confirm
data frame arrived intact
• If ACK not returned, transmission error
assumed

CSMA/CA does not eliminate collisions
• Does not solve hidden node problem
88
Transmitting on the WLAN: Point
Coordination Function (PCF)

Polling: Channel access method in which
each device asked in sequence if it wants
to transmit
• Effectively prevents collisions


Point Coordination Function (PCF): AP
serves as polling device or “point
coordinator”
Point coordinator has to wait only through
point coordination function IFS (PIFS)
time gap
• Shorter than DFIS time gap
89
Transmitting on the WLAN: Point
Coordination Function (continued)

If point coordinator hears no traffic after
PIFS time gap, sends out beacon frame
• Field to indicate length of time that PCF
(polling) will be used instead of DCF
(contention)

Receiving stations must stop transmission for that
amount of time
• Point coordinator then sends frame to specific
station, granting permission to transmit one
frame

802.11 standard allows WLAN to alternate
between PCF (polling) and DCF
(contention)
90
Transmitting on the WLAN: Quality
of Service (QoS) and 802.11e




DCF does not work well for real-time,
time-dependent traffic
Quality of Service (QoS): Capability to
prioritize different types of frames
Wi-Fi Multimedia (WMM): Modeled after
wired network QoS prioritization scheme
802.11e draft: defines superset of
features intended to provide QoS over
WLANs
• Proposes two new mode of operation for
802.11 MAC Layer
91
Transmitting on the WLAN:
Quality of Service and 802.11e
Table 5-1: Wi-Fi Multimedia (WMM)
92
Transmitting on the WLAN: Quality
of Service and 802.11e

802.11e draft (continued):
• Enhanced Distributed Channel Access
(EDCA): Contention-based but supports
different types of traffic


Four access categories (AC)
Provides “relative” QoS but cannot guarantee service
• Hybrid Coordination Function Controlled
Channel Access (HCCA): New form of PCF
based upon polling

Serves as a centralized scheduling mechanism
93
Remaining Connected to the
WLAN: Reassociation

Reassociation: Device drops connection
with one AP and establish connection with
another
• Several reason why reassociation may occur:


Roaming
Weakened signal
• When device determines link to current AP is
poor, begins scanning to find another AP

Can use information from previous scans
94
Remaining Connected to the
WLAN: Power Management

At set times AP send out beacon to all
stations
• Contains traffic indication map (TIM)
• At same time, all sleeping stations switch into
active listening mode

Power management in ad hoc mode:
• Ad hoc traffic indication message (ATIM)
window: Time at which all stations must be
awake

Wireless device sends beacon to all other devices
• Devices that previously attempted to send a frame to a
sleeping device will send ATIM frame indicating that
receiving device has data to receive and must remain
awake
95
WLAN Network Layer Standards:
WLAN IP Addressing

In standard networking, IP protocol
responsible for moving frames between
computers
• Network layer protocol

TCP/IP works on principle that each
network host has unique IP address
• Used to locate path to specific host
• Routers use IP address to forward packets
• Prohibits mobile users from switching to
another network and using same IP number

Users who want to roam need new IP address on
every network
96
WLAN Network Layer Standards:
Mobile IP

Provides mechanism within TCP/IP
protocol to support mobile computing
• Computers given home address,

Static IP number on home network
• Home agent: Forwarding mechanism that
keeps track of where mobile computer located
• When computer moves to foreign network, a
foreign agent provides routing services


Assigns computer a care-of address
Computer registers care-of address with home agent
97
CWNA Guide to Wireless
LANs, Second Edition
Chapter Six
Planning and Deploying a Wireless LAN
Planning for a Wireless Network


“If you fail to plan, then you plan to fail”
Some steps involved in planning wireless
networks similar to planning wired network
• Many steps significantly different

Basic planning steps:
• Assessing needs
• Weighing benefits
• Calculating costs
99
Assessing Needs: The Need for
Mobility

Two significant changes in business world
over last 15 years:
• Workers have electronic tools to access
information and accomplish significantly more
• Restructuring of organizational hierarchies


Organizations are “flatter”
Teamwork is essential
• Together, can result in decreased productivity


Hinders ability to collaborate and make timely
decisions
“Mobile office” needed
100
Assessing Needs: The Need for
Mobility (continued)

A solution to need for mobility is WLANs
• Expand productivity zone of knowledge workers
• Improve quality and productivity of meetings
• Work can be performed in more locations at more times

WLANs have been shown to add one to two hours
a day of productive time per worker
• Enabling worker to respond to customers, partners, and
colleagues more quickly

WLANs too often viewed as optional add-on to
computer networks
101
Assessing Needs: Examining
the Business Entity

Determine if business case exists for bringing
wireless networking into corporate environment
•
•
•
•

What is the purpose or mission of the organization?
Is the current mission expected to change in the future?
What is the size of the organization?
How much growth is anticipated in the organization?
Obtaining firm conceptual grip on organization as
a whole and its current status will reveal if an
investment in wireless technology is wise
102
Assessing Needs: Reviewing
the Current Network

Question to ask when examining how
organization uses current network:
• How does current network support the
organization’s mission?
• What applications run on the network?
• How many users does network support?
• Strengths and weaknesses of the current
network?
• Anticipated growth in network technology?

Examining current network status reveals
much of this information
• Especially applications and number of users
103
Assessing Needs: Reviewing
the Current Network (continued)

Good time to document network in detail:
•
•
•
•
•
•
•
Number of clients
Types of clients
Number of servers
The topology of the network
What media is being used
Performance of the network
Types of devices connected to the network
104
Assessing Needs: Reviewing
the Current Network (continued)
Table 6-1: Current network table
105
Assessing Needs: Reviewing the
Current Network
Figure 6-1: Network diagram
106
Determining Benefits: Hard
Benefits

Benefits that can be easily measured
or quantified
• For WLANs, easily measured in
decreased cost of installation


e.g., elimination of cabling costs
Using wireless technology for MAN or
WAN can result in even higher
savings
107
Determining Benefits: Soft Benefits

Benefits that are difficult, if not
impossible, to quantify accurately
• Improved productivity
• Enhanced collaboration and faster
responsiveness
• Flexible mobility
• Adherence to standards
• Improved employee satisfaction
108
Calculating Return on Investment
(ROI)

Return on investment (ROI): Standard
measure of profitability of a project
• Total cost of project

Hardware, software, implementation costs, training,
operations staff, maintenance staff and services, and
connectivity fees
• Less tangible costs


Workload management and customer satisfaction
Several models for calculating ROI
109
Calculating Return on Investment
(continued)

Intel Corporation’s wireless LAN model:
•
•
•
•
Implement a pilot
Develop a report
Assemble data
Calculate ROI
Table 6-2: Three-year WLAN costs and benefits
110
Calculating Return on Investment
Figure 6-2: Intel’s ROI model for WLANs
111
Designing the Wireless LAN

Involves determining:
• Which deployment scenario is best
• Which IEEE wireless network standard
should be used
• Type of AP management to implemented
• Where wireless devices should be
located
112
Determining the Deployment
Scenario

First step in designing a WLAN is to decide
on correct deployment scenario:
• Ad hoc: Not connected to wired infrastructure

Useful where wireless infrastructure does not exist or
services to remote networks not required
• Infrastructure: WLAN devices connect to wired
corporate network via AP

Most corporate wireless LANs
• Hotspot: Provides wireless LAN service, for
free or for a fee, from variety of public areas
• Point-to-point remote wireless bridge: Typically
interconnects two LAN segments
113
Determining the Deployment
Scenario

Deployment scenarios (continued):
• Point-to-multipoint remote wireless
bridge: Connects multiple LAN segments
• Ethernet to wireless bridge: Connects
single device that has an Ethernet port
but not an 802.11 NIC
• Wireless gateway: Provide single
mechanism for managing and
monitoring the wireless network
114
Selecting the IEEE Wireless
Network Type


IEEE 802.11b, 802.11a, or 802.11g
Decision may depend on many factors
• Do other devices in area use same frequency
range as one of the network types?
• What kind of coverage is needed?
• What types of applications will be used?

If broader area of coverage needed,
802.11g standard should be considered
first
• Good balance of coverage area with speed
115
Selecting the IEEE Wireless
Network Type


If interference is an issue, then
802.11a standard should be
considered
Only consider 802.11b in areas
where low bandwidth is acceptable or
ad hoc wireless network will be used
• Slow speed and susceptibility to
interference
116
Deciding upon Access Point
Management


If using infrastructure wireless network,
must decide type of AP management
Fat access point: AP serves as
management point
• Configuration must be done through via AP

Thin access point: Lacks management
functions
• Management functions moved to Ethernet
network switch
• Management simplified, centralized
• Handoff time reduced
• Thin access points are proprietary
117
Deciding upon Access Point
Management


Thin AP approach does not provide overall
solution for managing entire network
(wired and wireless)
Several vendors working on
comprehensive network management
solutions
• Integrate wireless networks into same
deployment, operations, and management as
wired network
• e.g., Cisco’s Structured Wireless-Aware
Network (SWAN)
118
Determining the Location of the
Wireless Devices
Table 6-3: Interference by objects
119
Ad Hoc Mode


Wireless devices communicate
directly without an AP
Three main considerations:
• Stations must be arranged so that they
are all within proper distance limits
• All stations must send and receive
signals on same frequency
• Hidden node problem must be avoided
120
Ad Hoc Mode
Figure 6-3: Ad hoc hidden node problem
121
Infrastructure Mode

Positioning APs correctly for an
infrastructure WLAN is critical for ensuring
that coverage area is sufficient
• Interference by objects must be taken into
consideration
• Signal should not extend beyond building’s
exterior walls for security reasons

In an ESS infrastructure network with
multiple APs, important that each AP’s
channel set correctly
• Adjacent APs using same channel can cause
interference and lost frames
122
Infrastructure Mode (continued)
Figure 6-4: Interference from using same channel
123
Infrastructure Mode (continued)

IEEE 802.11b and 802.11g networks
divide frequency spectrum into 14
overlapping and staggered channels
• Only channels 1, 6,and 11 do not overlap



Channel reuse: Adjacent APs use
nonoverlapping channels (1, 6, and 11)
IEEE 802.11a networks have eight
nonoverlapping channels
Must ensure APs properly overlap
• No gaps, but not too close together
124
Infrastructure Mode (continued)
Figure 6-5: Channel reuse
125
Infrastructure Mode (continued)
Figure 6-6: Flip flop between access points
126
Infrastructure Mode

Must consider number of users who
will be associated with APs
• Consider not only how many users will
be associated with each AP but also
what they will be doing
127
Deploying a Wireless Network


If planning/designing done correctly,
deploying can be easiest step
Must consider actual placement of APs
• Place APs exactly where they were designed to
go
• To avoid interference, better to place APs
higher



Be careful if placing APs in plenums
If needed, can use PoE
Good idea to configure WLAN on own
network segment
128
Providing User Support:
Training


Planning, designing, and deploying WLAN
pointless if users don’t receive required
support
Training is vital to use of a WLAN
• Users must know how to use new hardware
and software
• Support staff must know how to manage
network and diagnose problems
• Increases effectiveness of new wireless
network

Minimizes drop in productivity normally associated
with installation of a new system
129
Providing User Support: Training

Group training session often most
effective training setting
• Preferably done at same time users
receive wireless-enabled laptops

Important to set appropriate user
expectations for support and how
they should request it
130
Providing User Support:
Support


Involves continuing follow-up in answering
questions and assisting users
User support functions can be organized in
variety of ways:
• Establishing informal peer-to-peer support
groups
• Creating formal user support groups
• Maintaining a help desk
• Assigning support to the information
technology department
• Outsourcing support to a third party
131
Providing User Support:
Support

Establishing and staffing internal help
desk is one of most effective means of
support
• Central point of contact for users who need
assistance using network
• Suggestions regarding a help desk:






One telephone number for help desk
Plan for increased call volume after network installed
Problem tracking
Use surveys to determine user satisfaction
Periodically rotate network personnel into help desk
Use info from help desk to organize follow-up training
132
Providing User Support: Support

User feedback essential when
installing new WLAN
• Possibly more essential than technical
feedback
• May have IT personnel contact users for
feedback
• May schedule meetings with users to
gather feedback
133