Wireless networks

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


• 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


802.11b 802.11g

802.11gif 802.11g

accesspoint


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.


802.11g

802.11gif 802.11g

accesspoint


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


802.11b 802.11a 802.11g

802.11gif 802.11g

accesspoint


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.


• 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


• 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


• 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


• 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


• 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 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


• 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 (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


• 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


• 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


• 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



– 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


Client

LegitimateAP

1. Associates

2.Associates

As LegitimateClient



– 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


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


• 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


• 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

For Personal Area Networks (PANs)

https://www.bluetooth.org/

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.)

https://www.bluetooth.org/


• 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


• Disadvantages Compared to 802.11

– Short distance (10 meters)

– Low speed (3 Mbps, with a slower reverse channel)

– Insufficient for WLAN in a building


• 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 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?

Wireless networks

Documents

wireless data

wireless penetration

wireless availability

wireless lan standards

mbps throughput

g access point

wireless propagation

wireless municipal networks