Wireless LANs
(WLANs)
Chapter 5
Updated January 2007
Panko’s
Business Data Networks and Telecommunications, 6th edition
Copyright 2007 Prentice-Hall
May only be used by adopters of the book
Orientation
• LANs Are Governed by Layer 1 and 2 Standards
– So they are governed by OSI Standards
• Wired LAN Standards
– Chapter 3 (UTP and optical fiber transmission)
– Chapter 4 (Ethernet 802.3 Layer 1 and 2 standards)
• Chapter 5
–
–
–
–
Wireless LAN (WLAN) Standards
Physical layer wireless transmission
Wireless data link layer operation
Management
5-2
Figure 5-1: Local Wireless
Technologies, Continued
• 802.11
– The dominant WLAN technology today
– Standardized by the 802.11 Working Group
802.11
5-3
Figure 5-2: Wireless LAN (WLAN) Access Point
Large Wired Ethernet LAN
Wireless
Access
Point
Ethernet Switch
UTP
Router
Server
Internet
Radio
Transmission
Communication
Laptop
Mobile
Client
Wireless access point
(WAP) bridges wireless
stations to resources on
wired LAN—servers and
routers for Internet access
5-4
Figure 5-3: Access Router with Wireless Access
Point and Wireless NICs
PC Card
WNIC
for a Notebook
Computer
Access Router
with Access Point
USB WNIC
Internal
WNIC
For Desktop PC
5-5
Figure 5-1: Local Wireless
Technologies, Continued
• 802.11 Wireless LANs
– Today, mostly speeds of tens of megabits per second
with distances of 30 to 100 meters or more
• Can serve many users in a home or office
– Increasingly, 100 Mbps to 600 Mbps with 802.11n
– Organizations can provide coverage throughout a
building or a university campus by installing many
access points
5-6
Figure 5-1: Local Wireless Technologies,
Continued
• Other Local Wireless Technologies
– Mesh networking: multiple access points can route
frames to their destination (Figure 5-4) without using a
wired LAN
– Being standardized at 802.11s
Host
A
Host
B
C
A
E
Frame
Forw arding
802.11
Frame
F
D
5-7
Radio Propagation
Figure 5-5: Frequency Measurement
• Frequency
– Light waves are measured in wavelengths (Ch. 3)
– Radio waves are measured in terms of frequency
– Measured in hertz (Hz)—the number of complete
cycles per second
1 Second
Two cycles in 1 second, so frequency is two Hertz (Hz).
5-9
Figure 5-5: Frequency Measurement,
Continued
• Measuring Frequencies
– Frequency measures increases by factors of 1,000 (not
1,024)
– Kilohertz (kHz) [Note the lower-case k]
– Megahertz (MHz)
– Gigahertz (GHz)
5-10
Figure 5-6: Omnidirectional and Dish
Antennas
Dish Antenna
Omnidirectional Antenna
Focuses signals in a narrow range
Spread signals in all directions
Signals can be sent over long distances
Rapid signal attenuation
--------Must point at the sender
No need to point at receiver
Good for fixed subscribers
Good for mobile subscribers
5-11
Figure 5-7: Wireless Propagation Problems
1.
Electromagnetic
Interference
(EMI) from
Other stations,
2.
Microwave
Attenuation: signal gets
ovens, etc.
weaker with distance
Blocking
3.
Object Shadow
Zone
(Dead Spot)
5-12
Figure 5-7: Wireless Propagation Problems
Laptop
Direct Signal
Blocking
Object
4. Multipath
Interference
Reflected Signal
Direct and reflected signals may interfere
5-13
Inverse Square Law Attenuation
• Inverse square law attenuation
– To compare relative power at two distances
• Divide the longer distance by the shorter distance
• Square the result; this is the relative power ratio
– Examples
• 100 mW (milliwatts) at 10 meters
• At 20 meters, 100 / (20/10)2 = 100 mW / 4 = 25 mW
• At 30 meters, 100 / (30/10)2 = 100 mW / 9 = 11 mW
– Much faster attenuation than UTP or fiber
5-14
Frequently-Depended Propagation
Problem
• Some Problems are Frequency-Dependent
– Higher-frequency signals attenuate faster
• Absorbed more rapidly by water in the air
– Higher-frequency signals blocked more by obstacles
• At lower frequencies, signal refract (bend) around
obstacles like an ocean wave hitting a buoy
• At higher frequencies, signals do not refract; leave a
complete shadow behind obstacles
5-15
Figure 5-8: The Frequency Spectrum, Service
Bands, and Channels
1.
Frequency
Spectrum
(0 Hz to
Infinity)
4.
Signals in different channels do not
interfere with one another
Channel 5, Signal A
2.
Service
Band
(FM Radio,
Cellular
Telephony,
etc.)
Channel 4, Signal D
Channel 3, Signal B
Channel 2, No Signal
Channel 1, Signal E
3.
Multiple
Channels
within a
Service
Band; each
Channel
carries a
different
signal
0 Hz
5-16
Figure 5-9: Channel Bandwidth and
Transmission Speed (Study Figure)
• Signal Bandwidth
– Chapter 3 discussed a wave operating at a single
frequency
– However, most signals are spread over a range of
frequencies
– The higher the speed, the greater the spread of
frequencies
Amplitude
Signal
Frequency
5-17
Figure 5-9: Channel Bandwidth and
Transmission Speed (Study Figure)
• Channel Bandwidth
– Higher-speed signals need wider-bandwidth channels
– Channel bandwidth is the highest frequency in a channel
minus the lowest frequency
– An 88.0 MHz to 88.2 MHz channel has a bandwidth of
0.2 MHz (200 kHz)
Amplitude
88.0 MHz
88.2 MHz
Frequency
Bandwidth = 0.2 MHz = 200 kHz
5-18
Figure 5-9: Channel Bandwidth and
Transmission Speed (Study Figure)
• Shannon Equation
– Specifies the connection between channel bandwidth
and the channel’s maximum signal transmission speed
– C = B [ Log2(1+S/N) ]
• C = Maximum possible transmission
speed in the channel (bps)
• B = Bandwidth (Hz)
• S/N = Signal-to-Noise Ratio
– Measured as a ratio
– If given in dB, must convert to ratio
5-19
Figure 5-9: Channel Bandwidth and
Transmission Speed (Study Figure)
• Shannon Equation
– C = B [ Log2 (1+S/N) ]
• Note that doubling the bandwidth doubles the
maximum possible transmission speed
• Increasing the bandwidth by X increases the
maximum possible speed by X
– Wide bandwidth is the key to fast transmission
– Increasing S/N helps slightly but usually cannot be done
to any significant extent
5-20
Figure 5-9: Channel Bandwidth and
Transmission Speed (Study Figure)
• Broadband and Narrowband Channels
– Broadband means wide channel bandwidth and
therefore high speed
– Narrowband means narrow channel bandwidth and
therefore low speed
– Narrowband is below 200 kbps
– Broadband is above 200 kbps
5-21
Figure 5-9: Channel Bandwidth and
Transmission Speed (Study Figure)
• Channel Bandwidth and Spectrum Scarcity
– Why not make all channels broadband?
– There is only a limited amount of spectrum at desirable
frequencies
– Making each channel broader than needed would mean
having fewer channels or widening the service band
– Service band design requires tradeoffs between speed
requirements, channel bandwidth, and service band size
5-22
Figure 5-9: Channel Bandwidth and
Transmission Speed (Study Figure)
• The Golden Zone
– Most organizational radio technologies operate in the
golden zone in the high megahertz to low gigahertz
range
– At higher frequencies,
propagation problems
are severe
– At lower frequencies,
there is not enough
total bandwidth
Higher Frequency
Golden Zone
Lower Frequency
5-23
Spread Spectrum
Transmission
Figure 5-11: Spread Spectrum
Transmission (Study Figure)
• Unlicensed Bands
– WLANs operate in unlicensed service bands
• You do not need a license to have or move your
stations
– Two unlicensed bands are widely used: the 2.4 GHz
band and the 5 GHz band
• 5 GHz has worse propagation characteristics
• 2.4 GHz has fewer available channels
5-25
Figure 5-11: Spread Spectrum
Transmission, Continued
• Spread Spectrum Transmission
– You are REQUIRED BY LAW to use spread spectrum
transmission in unlicensed bands
• Spread spectrum transmission uses much larger
channels than transmission speed requires
• Spread spectrum transmission is required to reduce
propagation problems at high frequencies
• Especially multipath interference
– Spread spectrum transmission is NOT used for security
in WLANs
• This surprises many people
5-26
Figure 5-11: Spread Spectrum
Transmission, Continued
• There are Several Spread Spectrum
Transmission Methods (Figure 5-13)
Not Used
in 802.11
– Older Techniques
• Frequency Hopping Spread Spectrum (FHSS) up
to 4 Mbps (The book says 2 Mbps)
• Direct Sequence Spread Spectrum (DSSS) up to
11 Mbps
– Orthogonal Frequency Division Multiplexing (OFDM)
is used at 54 Mbps
– MIMO for speeds of 100 Mbps to 600 Mbps
5-27
Figure 5-13: Spread Spectrum Transmission Methods
Frequency Hopping
Spread Spectrum
(FHSS)
Only used in
Old 802.11 systems
And Bluetooth
Signal only uses its normal bandwidth, but it jumps
around within a much wider channel
If there are propagation problems at specific frequencies,
most of the transmission will still get through
Limited to low speeds of about 4 Mbps; used by
Bluetooth (later)
5-28
Figure 5-13: Spread Spectrum Transmission Methods
Only used in old
802.11 networks
Direct Sequence
Spread Spectrum
(DSSS)
Wideband but
Low-Intensity Signal
Signal is spread over the entire bandwidth of the
wideband channel
The power per hertz at any frequency is very low
Interference will harm some of the signal, but most of the
signal will still get through and will be readable
Used in 802.11b (11 Mbps), which is discussed later
5-29
Figure 5-13: Spread Spectrum Transmission
Methods
Orthogonal
Frequency
Division
Multiplexing
(OFDM)
Subcarrier 1
Subcarrier 2
Subcarrier 3
OFDM divides the broadband channel into subcarriers
Sends part of the signal in each subcarrier
The subcarrier transmissions are redundant so that if
some carriers are lost, the entire signal still gets through
Used in 802.11a and 802.11g at 54 Mbps (later)
5-30
Figure 5-20: Multiple Input/Multiple Output (MIMO)
Transmission
Signal 2
Reflected
Signal 1
Reflected
Y
A
B
Signal 1
X
Signal 2
Two or more signals can be sent at the same time in the same channel.
The receiver uses multipath time differences to distinguish between
them. This is an example of smart radio technology.
5-31
802.11 WLAN Operation
Figure 5-14: Typical 802.11 WLAN
Operation
Ethernet
Switch
802.3 Frame
802.11 Frame
UTP
WAP
802.3 Frame
Client PC
Server
Large Wired LAN
Radio
Transmission
Laptop
Wireless access points
(WAPs) bridge the networks
(translate between the
802.11 wireless frame and
the Ethernet 802.3 frame
used within the LAN)
5-33
Figure 5-14: Typical 802.11 WLAN
Operation, Continued
Ethernet
Switch
UTP
802.3 Frame
Client PC
Server
Large Wired LAN
WAP
A
WAP
B
Laptop
802.11 Frame
Handoff or Roaming
(if mobile computer
moves to another
access point,
it switches service
to that access point)
5-34
Figure 5-15: Stations and Access Points Transmit
in a Single Channel
Sw itch
Laptop
Access
Point B
Client PC
Collision if 2
Devices send
Simultaneously
The access point and all the stations it serves transmit in a
single channel. If tw o devices transmit at the same time, their
signals w ill collide, becoming unreadable. Media access control
(MAC) methods govern w hen devices transmit so that only
one device transmits at a time.
Laptop
5-35
Media Access Control
Box
• Only one station or the access point can transmit at
a time
• To control access (transmission), two methods can
be used
– CSMA/CA+ACK (mandatory)
– RTS/CTS (optional unless 802.11b and g stations share
an 802.11g access point)
5-36
Figure 5-16: CSMA/CA+ACK in 802.11
Wireless LANs
Box
• CSMA/CA (Carrier Sense Multiple Access with
Collision Avoidance)
• CSMA
– Sender Always Listens for Traffic
• Carrier is the signal; sense is to listen
– If there is traffic, the sender waits
– If there is no traffic …
• If the time since the last transmission is more
than a critical value, the station may send
immediately
5-37
Figure 5-16: CSMA/CA+ACK in 802.11
Wireless LANs
Box
• CSMA/CA (Carrier Sense Multiple Access with
Collision Avoidance)
– If there is no traffic
• If the time since the last transmission is less than
a critical value, the station sets a random timer
and waits
– If there is no traffic at the end of the waiting
time, the station sends
– If there is traffic, CSMA starts over again
5-38
Figure 5-16: CSMA/CA+ACK in 802.11
Wireless LANs
Box
• ACK (Acknowledgment)
– Receiver immediately sends back an
acknowledgment when it receives a frame
• Does not wait to send an ACK
• This avoids interference with other stations, which
must wait
– If sender does not receive the acknowledgment, it
retransmits the frame using CSMA/CA
– 802.11 with CSMA/CA+ACK is a reliable protocol!
5-39
Figure 5-17: Request to Send/Clear to Send
(RTS/CTS)
Box
Switch
RTS
Client PC
Access
Point B
Radio
Link
Laptop
Server
Large Wired LAN
1. Device that wishes
to transmit may send a
Request-to-Send message
5-40
Figure 5-17: Request to Send/Clear to Send
(RTS/CTS)
Box
Must Wait
Switch
CTS
WAP
Client PC
Server
Large Wired LAN
Radio
Link
May Send
Frames
2. Wireless access point broadcasts
a Clear-to-Send message.
Station that sent the RTS
may transmit unimpeded.
Other stations hearing the CTS must wait
5-41
Recap
Box
• CSMA/CA+ACK is mandatory
• RTS/CTS is optional
– However, it is mandatory if 802.11b and 802.11g NICs
share the same 802.11g access point
5-42
802.11 WLAN Standards
Figure 5-18: Specific 802.11 Wireless LAN
Standards
802.11a
802.11b
802.11g
if 802.11g
access
802.11g
point
serves an
802.11b
station
Unlicensed Band
5 GHz
2.4 GHz
2.4 GHz
2.4 GHz
Crowded Band?
No
Yes
Yes
Yes
Attenuation
Higher
Lower
Lower
Lower
Price
Higher
Lower
Lower
Lower
Very Low
High
Higher
Lower
Market Acceptance
5-44
Figure 5-18: Specific 802.11 Wireless LAN
Standards
Source for throughput data: Broadband.com
802.11a, operating at
a higher frequency,
has more attenuation
Than 802.11b
802.11a
802.11g
if 802.11g
access
802.11b 802.11g
point
serves an
802.11b
station
Not
11 Mbps 54 Mbps
Specified
Rated Speed*
54 Mbps
Throughput, 3 m
25 Mbps
6 Mbps 25 Mbps
12 Mbps
Throughput, 30 m
12 Mbps
6 Mbps 20 Mbps
11 Mbps
*Maximum rated speed. There are slower modes if propagation is poor.
5-45
Figure 5-18: Specific 802.11 Wireless LAN
Standards, Continued
• Transmission Speed and Distance
– As a station moves away from an access point,
transmission speed falls
• There are several modes of operation specified in
each standard
• The fastest mode only works with a very strong signal
• As the user moves away, the signal strength becomes
too low
• That station and the access point switch to a slower
mode
• This slows things down for all users
5-46
Figure 5-18: Specific 802.11 Wireless LAN
Standards
Unlicensed Band
Number of NonOverlapping Channels
802.11b
802.11a
802.11g
if 802.11g
access
802.11g
point
serves an
802.11b
station
2.4 GHz
5 GHz
2.4 GHz
2.4 GHz
3
Up to 24
3
3
2.4 GHz non-overlapping channels are 1, 6, and 11
5-47
Figure 5-19: Interference Between Nearby Access
Points Operating on the Same Channel
OK
Access Point A
Channel 1
OK
Access Point D
Channel 6
In 802.11b and
802.11g
nonoverlapping
channels are
1, 6, and 11
Access Point B
Channel 6
Interference
Access Point E
Channel 6
Interference
Access Point C
Channel 6
OK
Interference
OK
Access Point F
Channel 11
Access Point Channels Should be Selected to
Minimize Mutual Interference
5-48
802.11n
• Under Development
– Rated speeds of 100 Mbps to 600 Mbps
– Will operate in both the 2.4 GHz and 5 GHz bands
– May use twice current bandwidth per channels (~20
MHz) to roughly double speed
– Will use MIMO
– Currently a draft standard
5-49
WLAN Security
Figure 5-21: WLAN Security Threats
(Study Figure)
• Drive-By Hackers
– Sit outside the corporate premises and read network
traffic
– Can send malicious traffic into the network
– Easily done with readily available downloadable software
• War Drivers
– Merely discover unprotected access points–become
drive-by hackers only if they break in
5-51
Figure 5-21: WLAN Security Threats,
Continued
• Rogue Access Points
– Unauthorized access points set up by department or
individual
– Often have very poor security, making drive-by hacking
easier
– Often operate at high power, attracting many clients
5-52
Figure 5-21: WLAN Security Threats,
Continued
• Evil Twin Access Points
– Create a fake access point outside walls of firm using a
PC
– Legitimate internal client associates with the evil twin
access point, which operates at high power
Legitimate
Client
Legitimate
AP
Evil Twin AP
Duped Association
5-53
Figure 5-21: WLAN Security Threats,
Continued
• Evil Twin Access Points
– Evil twin then associates with a legitimate internal
access point masquerading as the internal clients
– This connects the evil twin to the firm’s internal network
Legitimate
Client
Legitimate
AP
2.
Associates
As Legitimate
Client
Evil Twin AP
1. Associates
5-54
Figure 5-21: WLAN Security Threats,
Continued
• Evil Twin Access Points
– Evil twin can then read all traffic, even if the sender and
receive encrypt their messages because the evil twin
steals authentication credentials passed between the
clients and the legitimate access point
– Also can insert traffic
– Classic man-in-the-middle attack
Legitimate
Client
Legitimate
AP
Evil Twin AP
5-55
Figure 5-22: 802.11 Security
Standards (Study Figure)
• Wired Equivalent Privacy (WEP)
– Initial security provided with 802.11 in 1997
– Everyone shared the same secret key
– Other weaknesses
– Readily available programs can crack WEP keys in less
than 10 minutes
– WEP should never be used in corporations
– By 2001, WLAN security was in crisis
5-56
Figure 5-22: 802.11 Security
Standards, Continued
• Wireless Protected Access (WPA)
– The Wi-Fi Alliance normally certifies interoperability of
802.11 equipment
– Created WPA as a stop-gap security standard in 2002
until the IEEE 802.11i standard discussed next was
finished
– WPA lightened 802.11i security so that older access
points and wireless NICs could be upgraded to WPA
5-57
Figure 5-22: 802.11 Security
Standards, Continued
• 802.11i
– Created by the IEEE
– Uses powerful AES-CCMP encryption with 128-bit keys
for confidentiality and key management
– Wi-Fi Alliance calls 802.11i “WPA2”
– Should be used if equipment supports it.
– Vendor support has been slow in coming.
5-58
Modes of Operation
• Both 802.11i and WPA (as a subset of 802.11i)
operate in two modes
– 802.1X mode and
– Preshared Key (PSK) Mode
WPA
802.11i
(WPA2)
Can use 802.1X
Mode?
Yes
Yes
Can use PSK
Mode?
Yes
Yes
5-59
Figure 5-22: 802.11 Security
Standards, Continued
• Pre-Shared Key (PSK) Mode
– Only for firms with a single access point
– Access point does all authentication and key
management
– All users must know an initial pre-shared key (PSK)
• Each, however, is later given a unique key
– If the pre-shared key is weak, it is easily cracked
• Pass phrases are used to generate keys; must be at
least 20 characters long
– Wi-Fi Alliance calls this “personal mode”
5-60
Figure 5-23: 802.11 Security in 802.1X
(Enterprise Mode)
• Operation
– Clients send authentication credentials to access point
– Access point sends these to an authentication server
– Central authentication server sends back OK or Reject
Accept
OK
Client
Credentials
Access Points
Credentials
Central Authentication
Server
Client
5-61
Figure 5-23: 802.11 Security in 802.1X
(Enterprise Mode)
• Central Authentication Server
– Provides consistency in authentication
– Same decision no matter what access point a client
connects to
– Attackers cannot search for a misconfigured access
point
Accept
OK
Client
Credentials
Access Points
Credentials
Central Authentication
Server
Client
5-62
Figure 5-23: 802.11 Security in 802.1X
(Enterprise Mode)
• Extensible Authentication Protocols (EAPs)
– Messages are standardized by an extensible
authentication protocol (EAP)
– There are several EAPs. The most popular is PEAP,
which Microsoft favors
Accept
OK
Credentials
Central Authentication
Server
Client
Access
Points
Credentials
Client
5-63
Figure 5-23: 802.11 Security in 802.1X
(Enterprise Mode)
• Keys
– Central authentication also provides keys to clients
– Changes the keys frequently
Key
Client
Key
Access Points
Central Authentication
Server
Client
5-64
Perspective
• WEP operates in only one mode: shared key
• Both WPA and 802.11i operate in both 802.1X
(enterprise) or pre-shared key (personal) mode
• 802.11i offers stronger security than WPA
• The Wi-Fi Alliance calls 802.11i “WPA2”
5-65
802.11 WLAN Management
Figure 5-24: Wireless LAN
Management (Study Figure)
• Access Points Placement in a Building
– Must be done carefully for good coverage and to
minimize interference between access points
– Lay out 30-meter to 50-meter radius circles on
blueprints
– Adjust for obvious potential problems such as brick
walls
– In multistory buildings, must consider interference in
three dimensions
5-67
Figure 5-24: Wireless LAN
Management (Study Figure)
• Access Points Placement in a Building
– Install access points and do site surveys to determine
signal quality
– Adjust placement and signal strength accordingly
– This is quite expensive
5-68
Figure 5-25: Wireless Access Point Management
Alternatives
Ethernet Sw itch
UTP
Manageable
WLAN
Sw itch
Central Management
Station
Management intelligence can be placed
in the access point or the WLAN switch
Manageable Smart
Access Point
Dumb
Access Point
Dumb
Access Point
5-69
Figure 5-24: Wireless LAN
Management (Study Figure)
• Remote Access Point Management
– Desired functionality
• Continuous transmission quality monitoring
• Immediate notification of failures
• Remote AP adjustment (power, channel, etc.)
• Ability to push software updates out to all APs or
WLAN switches
• Take appropriate actions automatically whenever
possible
5-70
Bluetooth
For Personal Area Networks (PANs)
Figure 5-26: Bluetooth Personal Area
Networks (PANs) (Study Figure)
• For Personal Area Networks (PANs)
– Devices around a desk (computer, mouse, keyboard,
printer)
– Devices on a person’s body and nearby (cellphone,
PDA, notebook computer, etc.)
– Cable replacement technology
5-72
Figure 5-26: Bluetooth Personal Area
Networks (PANs), Continued
• Disadvantages Compared to 802.11
– Short distance (10 meters)
– Low speed (3 Mbps, with a slower reverse channel)
– Insufficient for WLAN in a building
5-73
Figure 5-26: Bluetooth Personal Area
Networks (PANs), Continued
• Advantages Compared to 802.11
– Low battery power drain so long battery life between
recharges
– Application profiles
• Define how devices will work together with little or no
human intervention
• Sending print jobs to printers
• File synchronization
• Etc.
• Somewhat rudimentary
• Devices typically only automate a few access profiles
5-74
Figure 5-26: Bluetooth Personal Area
Networks (PANs), Continued
• Bluetooth Trends
– Bluetooth Alliance is enhancing Bluetooth
– The next version of Bluetooth is likely to grow to use
ultrawideband transmission
• This should raise speed to 100 Mbps (or more)
• Transmission distance will remain limited to 10 meters
• Good for distributing television within a house
5-75
Figure 5-1: Local Wireless Technologies,
Continued
• Other Local Wireless Technologies
– Ultrawideband: Up to 250 Mbps (fast) over a distance of
10 meters (short)
– Ideal for video networking in homes
– ZigBee for almost-always-off sensor networks at low
speeds
– Allows battery lives of months or years
– Radio Frequency ID (RFID) tags: like UPC product tags
but readable from a small distance
– RFID reader sends probe signal that powers the RFID tag,
which then responds with its information
5-76
Topics Covered
Local Wireless Technologies
• 802.11 for Corporate WLANs
• Bluetooth for PANs
• Ultrawideband (UWB)
• RFIDs
• ZigBee
• Mesh Networks
5-78
Radio Propagation
• Frequencies and Channels
• Antennas
• Propagation Problems
– Inverse square law attenuation
– Dead spots / shadow zones
– Electromagnetic interference
– Multipath interference
– Attenuation and shadow zone problems increase with
frequency
5-79
Radio Propagation
• Shannon’s Equation and the Importance of
Channel Bandwidth
– C = B Log2(1+S/N)
• WLANs use unlicensed Radio Bands
• Spread Spectrum Transmission to Reduce
Propagation Problems
– FHSS (up to 4 Mbps)
– DSSS (up to 11 Mbps)
– OFDM (up to 54 Mbps)
– MIMO (100 Mbps to 600 Mbps)
5-80
802.11 Operation
• Wireless Access Point Bridge to the Main Wired
Ethernet LAN
– To reach servers and Internet access routers
– Transfers packet between 802.11 and 802.3 frames
• Need for Media Access Control (Box)
– CSMA/CA and RTS/CTS
– Throughput is aggregate throughput
5-81
802.11 Operation
• Bands
– 2.4 GHz band: Only 3 channels, lower attenuation
– 5 GHz band: Around 24 channels, higher attenuation
– More channels means less interference between nearby
access points
• Standards
– 802.11b: 11 Mbps, DSSS, 2.4 GHz band
– 802.11a: 54 Mbps, OFDM, 2.4 GHz band
– 802.11g: 54 Mbps, OFDM, 5 GHz band
– 802.11n: 100 Mbps – 600 Mbps, MIMO, Dual-Band
5-82
802.11 WLAN Security
• Wardrivers and Drive-By Hackers
• Core Security
– WEP (Unacceptably Weak)
– WPA (Lightened form of 802.11i)
– 802.11i (The gold standard today)
– 802.1X and PSK modes for WPA and 802.11i
• Rogue Access Points and Evil Twin Access Points
5-83
WLAN Management
• Surprisingly Expensive
• Access Point Placement
– Approximate layout
– Site survey for more precise layout and power
• Remote Access Point Management
– Smart access points or WLAN switches and dumb
access points
5-84
Bluetooth
• PANs
• Cable Replacement Technology
• Limited Speeds and Distance
• Application Profiles
• UWB in the Future?
5-85