wireless networks
TRANSCRIPT
Wireless LANs (WLANs)
Local Wireless Technologies
• Physical-Layer Transmission
– Uses radio transmission
– Gives mobility
Wireless availability
• 43,850 locations in 2003.
• Estimated in 2004 to grow to over 200,000 locations in 2008. (actually grew to 101,000 commercial WiFi by end of 2005)
• 25 million WiFi routers shipped in 2005
• January 2006, 186 US cities have Wireless municipal networks, but we’re still 14th in the World for wireless penetration.
Wireless LAN (WLAN) Access Point
Server
Internet
Router
Ethernet Switch
LaptopMobileClient
WirelessAccessPoint
Large Wired Ethernet LAN
UTP RadioTransmission
Wireless access point (WAP) bridges wireless stations to resources on wired LAN—servers and routers for Internet access
Communication
Access Router with Wireless Access Point and Wireless NICs
PC CardWNIC
for a NotebookComputer
InternalWNIC
For Desktop PC
USB WNIC
Access Routerwith Access Point
Local Wireless Technologies, Continued
• 802.11
– The dominant WLAN technology today
– Standardized by the 802.11 Working Group
802.11
Local Wireless Technologies, Continued
• 802.11 Wireless LANs
– Speeds up to tens of megabits per second with distances of 30 to 100 meters or more
• Can serve many users in a home or office
– Soon to be 100 Mbps to 600 Mbps with 802.11n
– Organizations can provide coverage throughout a building or a university campus by installing many access points
802.11 WLAN Standards
Specific 802.11 Wireless LAN Standards
802.11b 802.11g
802.11gif 802.11g
accesspoint
serves an802.11bstation
2.4 GHz 2.4 GHz 2.4 GHzUnlicensed Band
Lower Lower LowerAttenuation
Yes
802.11a
5 GHz
Higher
No Yes YesCrowded Band?
Lower Lower LowerPrice Higher
Higher LowerMarket Acceptance Very Low High
Specific 802.11 Wireless LAN Standards
802.11b 802.11g
802.11gif 802.11g
accesspoint
serves an802.11bstation
11 Mbps 54 MbpsNot
SpecifiedRated Speed*
6 Mbps 25 Mbps 12 MbpsThroughput, 3 m
6 Mbps
802.11a
54 Mbps
25 Mbps
12 Mbps 20 Mbps 11 MbpsThroughput, 30 m
Source for throughput data: Broadband.com
802.11a, operating ata higher frequency,
has more attenuationThan 802.11b
*Maximum rated speed. There are slower modes if propagation is poor.
Specific 802.11 Wireless LAN Standards
802.11g
802.11gif 802.11g
accesspoint
serves an802.11bstation
Aggregate throughputs;Individual throughputs are lower
Are These AggregateOr IndividualThroughputs?
20 Mbps 11 MbpsThroughput, 30 m
802.11a
12 Mbps
11 Mbps 54 MbpsNot
SpecifiedRated Speed
802.11b
6 Mbps
54 Mbps
Specific 802.11 Wireless LAN Standards
802.11b 802.11a 802.11g
802.11gif 802.11g
accesspoint
serves an802.11bstation
3 Up to 24 3 3Number of Non-Overlapping Channels
2.4 GHz 5 GHz 2.4 GHz 2.4 GHzUnlicensed Band
2.4 GHz non-overlapping channels are 1, 6, and 11
A new Wireless LAN Standard
• A separate standard, 802.16 (or WiMAX), transmits at 70 Mbps and has a range of up to 30 miles.
• It can operate in licensed or unlicensed bands of the spectrum from 2-6 GHz. WiMAX typically links multiple 802.11 networks or sends Internet data over long distances.
Wireless LANs (WLANs) cont.
Local Wireless Technologies, Continued
• Bluetooth
– For personal area networks (PANs)
• Multiple devices carried by a person, or
• Multiple devices around a desk
• Limited to about 10 meters
• Limited to 3 Mbps with a slower reverse channel
– Cable replacement technology
USBBluetoothAdapter
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
Local Wireless Technologies, Continued
• Other Local Wireless Technologies
– Mesh networking: multiple access points can route frames to their destination without using a wired LAN
– Being standardized at 802.11s
FrameForwardingA
D
C E
F
HostA
HostB
802.11Frame
Radio Propagation orHow wireless data gets there!
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).
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)
Omnidirectional and Dish Antennas
Omnidirectional Antenna
Spread signals in all directionsRapid signal attenuation
-----No need to point at receiverGood for mobile subscribers
Dish Antenna
Focuses signals in a narrow rangeSignals can be sent over long distances
-----Must point at the sender
Good for fixed subscribers
Wireless Propagation Problems
2.Attenuation: signal getsweaker with distance
3.Shadow
Zone(Dead Spot)
1.Electromagnetic
Interference(EMI) from
Other stations,Microwaveovens, etc.
BlockingObject
Wireless Propagation Problems
Reflected Signal
LaptopDirect Signal
4. MultipathInterference
Direct and reflected signals may interfere
BlockingObject
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
Frequency-Dependent 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
The Frequency Spectrum, Service Bands, and Channels
Channel 5, Signal A
Channel 1, Signal E
Channel 2, No Signal
Channel 3, Signal B
Channel 4, Signal D
0 Hz
2.ServiceBand
(FM Radio,Cellular
Telephony,etc.)
1.FrequencySpectrum(0 Hz toInfinity)
3.MultipleChannelswithin aServiceBand; eachChannelcarries adifferentsignal
4.Signals in different channels do not
interfere with one another
Channel Bandwidth and Transmission Speed
• 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
Channel Bandwidth and Transmission Speed
• 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
Channel Bandwidth and Transmission Speed
• 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
Channel Bandwidth and Transmission Speed
• 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
Channel Bandwidth and Transmission Speed
• The Golden Zone
– Most organizational radio technologies operate in the golden zone in the high megahertz to low gigahertz range
– At higher frequencies,propagation problemsare severe
– At lower frequencies,there is not enoughtotal bandwidth
Golden Zone
Higher Frequency
Lower Frequency
Spread Spectrum Transmission
• Unlicensed Bands
– WLANs operate in unlicensed service bands
• You do not need a license to have or move your stations
• You must tolerate interference from other users
• You must not cause unreasonable interference
– 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
Spread Spectrum Transmission, Cont.
• Spread Spectrum Transmission
– You are REQUIRED BY LAW to use spread spectrum transmission in unlicensed bands
• 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
Normal Radio Transmission and Spread Spectrum Transmission
Channel BandwidthRequired for Signal
Speed
Normal Radio:Bandwidth Is No
Wider thanRequired
Note: Height of Box Indicates Bandwidth of Channel
To conserve spectrum channel, bandwidths usually are set to be only as wide as signals in the service band need based on their speed
Normal transmission: Uses only the channel bandwidthrequired by your signaling speed
Normal Radio Transmission and Spread Spectrum Transmission
Channel BandwidthRequired for Signal
SpeedNote: Height of Box Indicates Bandwidth of Channel
Spread SpectrumTransmission:
Channel BandwidthIs Much Wider
than Needed
However, spread spectrum transmission uses much wider channels than are needed, which seems wasteful but improves propagation
Spread spectrum transmission:Uses channels much wider than signaling speed requires
802.11 WLAN Operation
Typical 802.11 WLAN Operation
Server
EthernetSwitch
LaptopWAP
Large Wired LAN
Client PC
UTP RadioTransmission
802.11 Frame802.3 Frame
802.3 Frame
Wireless access points (WAPs) bridge the networks (translates between the 802.11 wireless frame and the Ethernet 802.3 frame used within the LAN)
Typical 802.11 WLAN Operation, Continued
Server
EthernetSwitch
LaptopWAP
A
Large Wired LAN
Client PCWAP
B
UTP
Handoff or Roaming(if mobile computermoves to another
access point,it switches service
to that access point)
802.11 Frame802.3 Frame
Stations and Access Points Transmit in a Single Channel
Laptop
AccessPoint B
Switch
Client PC
Laptop
The access point and all the stations it serves transmit in asingle channel. If two devices transmit at the same time, theirsignals will collide, becoming unreadable. Media access control(MAC) methods govern when devices transmit so that onlyone device transmits at a time.
Collision if 2Devices send
Simultaneously
Media Access Control
• 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)
CSMA/CA in 802.11 Wireless LANs
• 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
CSMA/CA in 802.11 Wireless LANs
• 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
CSMA/CA in 802.11 Wireless LANs
• ACK (Acknowledgement)
– 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 acknowledgement, it retransmits the frame using CSMA/CA
– 802.11 with CSMA/CA+ACK is a reliable protocol!
Request to Send/Clear to Send (RTS/CTS)
Server
Switch
LaptopAccessPoint B
Large Wired LAN
RadioLink
Client PC
RTS
1. Device that wishesto transmit may send a
Request-to-Send message
Box
Request to Send/Clear to Send (RTS/CTS)
Server
Switch
May SendFrames
WAP
Large Wired LAN
RadioLink
Client PC
2. Wireless access point broadcastsa Clear-to-Send message.Station that sent the RTSmay transmit unimpeded.
Other stations hearing the CTS must wait
CTS
Box
Must Wait
Recap
• 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
Specific 802.11 Wireless LAN Standards
• 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
Specific 802.11 Wireless LAN Standards, Continued
• Transmission Speed and Distance
– When stations transmit more slowly, they take longer to transmit their frames
• This reduces the time available for other stations to transmit
• Consequently, throughput falls for everyone
– Even a few very distant stations can slow throughput for everyone substantially
Figure 5-19: Interference Between Nearby Access Points Operating on the Same Channel
Access Point AChannel 1
Access Point BChannel 6
Access Point CChannel 6
Access Point DChannel 6
Access Point EChannel 6
Access Point FChannel 11
OK
OK
OK
OK
Interference
Interference
Interference
In 802.11b and802.11g
nonoverlappingchannels are1, 6, and 11
Access Point Channels Should be Selected to
Minimize Mutual Interference
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
802.11e
• Standard for Quality of Service (QoS)
– Needed for voice and video transmission
– Wi-Fi Alliance calls 802.11e Wi-Fi Multimedia (WMM)
WLAN Security
WLAN Security Threats
• 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
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
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
Evil Twin APLegitimate
Client
LegitimateAP
Duped Association
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
Evil Twin APLegitimate
Client
LegitimateAP
1. Associates
2.Associates
As LegitimateClient
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
Evil Twin APLegitimate
ClientLegitimate
AP
802.11 WLAN Management
Wireless LAN Management
• 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
Wireless LAN Management
• 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
Wireless Access Point Management Alternatives
UTP
Manageable SmartAccess Point
Ethernet Switch
Central ManagementStation
DumbAccess Point
DumbAccess Point
ManageableWLANSwitch
Management intelligence can be placedin the access point or the WLAN switch
Wireless LAN Management
• 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
Bluetooth Personal Area Networks (PANs)
• 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.)
Bluetooth Personal Area Networks (PANs)
• Cable Replacement Technology
– For example, with a Bluetooth PDA, print wirelessly to a nearby Bluetooth-enabled printer
– No access points are used
• Direct device-to-device communication
Print Job
Bluetooth Personal Area Networks (PANs)
• Disadvantages Compared to 802.11
– Short distance (10 meters)
– Low speed (3 Mbps, with a slower reverse channel)
– Insufficient for WLAN in a building
Bluetooth Personal Area Networks (PANs)
• 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
Bluetooth Personal Area Networks (PANs)
• 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
Other Wireless Communication
• 3G Cellular phones
• VoIP on wireless
• RFID and embedded wireless technology, e.g. credit/ID cards
• Wireless IPODs?