wireless lans (wlans) chapter 5 panko’s business data networks and telecommunications, 6th edition...

Wireless LANs (WLANs)

Chapter 5

Panko’sBusiness Data Networks and Telecommunications, 6th edition

Copyright 2007 Prentice-HallMay only be used by adopters of the book

5-2

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

5-3

Figure 5-1: Local Wireless Technologies

• Physical-Layer Transmission

– Uses radio transmission

– Gives mobility

5-4

Figure 5-1: Local Wireless Technologies, Continued

• 802.11

– The dominant WLAN technology today

– Standardized by the 802.11 Working Group

802.11

5-5

Figure 5-2: 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

5-6

Figure 5-3: Access Router with Wireless Access Point and Wireless NICs

PC CardWNIC

for a NotebookComputer

InternalWNIC

For Desktop PC

USB WNIC

Access Routerwith Access Point

5-7


• 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

5-8


• 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

5-9


• Other Local Wireless Technologies

– Ultra-wideband (UWB)Ultra-wideband (UWB): Up to 250 Mbps (fast) over a distance of 10 meters (short)

– Ideal for video networking in homes

– ZigBeeZigBee for almost-always-off sensor networks at low speeds

– Allows battery lives of months or years

– Radio Frequency ID (RFID) tagsRadio 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

UPC: Universal Product Code

5-10RFID tagRFID tag

RFIDRFIDReaderReader

PDAPDA

BluetootBluetoothh HeadsetHeadset

WLANWLANAPAP

Web ServerWeb Server

Guide DB

1122

44

33

席德進 [正坐少年 ]

結合WLAN 與 RFID之無線導覽系統

Network

5-11


• Other Local Wireless Technologies

– Mesh networkingMesh networking: multiple access points can route frames to their destination (Figure 5-4) without using a wired LAN

– Being standardized at 802.11s

FrameForwardingA

D

C E

F

HostA

HostB

802.11Frame

Radio Propagation

5-13

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).

5-14

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)

5-15

Figure 5-6: 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

5-16

Figure 5-7: Wireless Propagation Problems

2.Attenuation: signal gets

weaker with distance

3.Shadow

Zone(Dead Spot)

1.Electromagnetic

Interference(EMI) from

Other stations,Microwaveovens, etc.

BlockingObject

Reflected Signal

Direct Signal

4. MultipathInterference

Direct and reflected signals may interfere

5-17

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

5-18

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

折射

浮標

5-19

Figure 5-8: 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.Multiple Channelswithin a ServiceBand; Each Channelcarries a differentSignal.

4.Signals in different channels do not

interfere with one another

5-20

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

Frequency

Signal

5-21


• 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)

88.0 MHz 88.2 MHz

Bandwidth = 0.2 MHz = 200 kHz

Amplitude

Frequency

5-22


• Shannon Equation

– Specifies the connection between channel bandwidth and the channel’s maximum signal transmission speed

– C = B [ LogC = B [ Log22(1+S/N) ](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

5-23


• 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

5-24


• 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

5-25


• 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

匱乏

5-26


• 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

5-28

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

• 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

5-29

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 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

展頻

5-30

Figure 5-12: 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

timefrequency

5-31

Figure 5-12: 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

5-32

Figure 5-11: Spread Spectrum Transmission, Continued

• There are Several Spread Spectrum Transmission Methods (Figure 5-13)

– Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum (FHSS) up to 4 Mbps

• The book says 2 Mbps, but it is now higher.

– Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum (DSSS) is used at 11 Mbps

– Orthogonal Frequency Division MultiplexingOrthogonal Frequency Division Multiplexing (OFDM) is used at 54 Mbps

– MIMO for speeds of 100 Mbps to 600 Mbps

– We will look at these in term

5-33

Figure 5-13: Spread Spectrum Transmission Methods

Frequency HoppingSpread Spectrum

(FHSS)

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)

time

5-34


Wideband butLow-Intensity Signal

Direct SequenceSpread Spectrum

(DSSS)

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

5-35


OrthogonalFrequency

DivisionMultiplexing

(OFDM)

Subcarrier 1

Subcarrier 3

Subcarrier 2

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)

多餘的

5-36

Figure 5-20: Multiple Input/Multiple Output (MIMO) Transmission

A BX

Y

Signal 2

Signal 1Reflected

Signal 2Reflected

Signal 1

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.

802.11 WLAN Operation

5-38

Figure 5-14: 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)

5-39

Figure 5-14: Typical 802.11 WLAN Operation, Continued

Server

EthernetSwitch

LaptopAPA

Large Wired LAN

Client PCAPB

UTP

Handoff or Roaming(if mobile computermoves to another

access point,it switches service

to that access point)

802.11 Frame802.3 Frame

5-40

Figure 5-15: 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

5-41

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)

Box

5-42

Figure 5-16: CSMA/CA+ACK 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

Box

5-43


• 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

Box

5-44


• 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!

Box

5-45

RTS/CTS

A BRTS

CTS

C

D

CSMA/CA

http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csma-ca/withhidden.html

http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csma-ca/withhidden.html

5-46

Figure 5-17: 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

5-47

Figure 5-17: 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

5-48

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

Box

802.11 WLAN Standards

5-50

Figure 5-18: 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

5-51


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.

5-52


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

5-53


802.11a 802.11b 802.11g

802.11gif 802.11g

accesspoint


12 to 24 3 3 3Number of Non-Overlapping Channels

5 GHz 2.4 GHz 2.4 GHz 2.4 GHzUnlicensed Band

2.4 GHz non-overlapping channels are 1, 6, and 11

5-54

Figure 5-18: Specific 802.11 Wireless LAN Standards, Continued

• 802.11b and 802.11g: Nonoverlapping channels are 1, 6, and 11.

Channel Nominal Frequency

(MHz)Minimum

(MHz)Maximum

(MHz)

1 2412 2401 2423

2 2417 2405 2428

3 2422 2411 2433

4 2427 2416 2438

5 2432 2421 2443

6 2437 2426 2448

7 2442 2431 2453

8 2447 2436 2458

9 2452 2441 2463

10 2457 2446 2468

11 2462 2451 2473

5 MHz

22 MHz

5-55


• 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

5-56


• 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

5-57

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

5-58

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

5-59

802.11e

• Standard for Quality of Service (QoS)

– Needed for voice and video transmission

– Wi-Fi Alliance calls 802.11e Wi-Fi Multimedia (WMM)

802.11 WLAN Security

5-61

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

5-62

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

5-63


• 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詐騙

5-64



– 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

5-65



– 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

5-66

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

– WEP protected the key with a poorly implemented initialization vector (IV)

– Not even turned on by default on early products

5-67

Figure 5-22: 802.11 Security Standards, Continued

• Wired Equivalent Privacy (WEP)

– Because secret key was shared, it does not seem to be secret

• Users often give out freely

– Even if not given away, could be cracked.

• Initially could be cracked in 1-2 hours;

• Now can be cracked in 3-10 minutes using software readily available on the Internet

– NEVER use WEP!!!

– By 2001, WEP security was in crisis

5-69

WEP Encryption

Plain Text: 010111001011100101001101...

key IV 110101100101010011001001...

⊕

Cipher Text: 100010101110110110000100...

key IV 110101100101010011001001...

IV 100010101110110110000100...

⊕

010111001011100101001101...Plain Text:

⊕ : XOR

( A B B = A )⊕ ⊕

RC4

RC4

5-70


• 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

5-71


• WPA was designed for upgrading old equipment

– Old equipment has limited memory and processing power

– WPA uses a subset of 802.11i that can run on older wireless NICs and access points

– WPA added simpler security algorithms for 802.11i functions that could not run on older machines

• Equipment that cannot be upgraded to WPA should be discarded

5-72

WPA

WPA’s encryption scheme: TKIP (Temporal Key Integrity Protocol)

5-73


• 802.11i (WPA2)

– Created by the IEEE

– Uses powerful AES-CCMP encryption with 128-bit keys for confidentiality and key management

– Robust Security Network (RSN): a network with all devices compliant to 802.11i

– Wi-Fi Alliance calls 802.11i “WPA2”

– Should be used if equipment supports it.

• Vendor support has been slow in coming.

CBC-MAC: Cipher Block Chaining-Message Authentication Code

AES-CCMP: AES-Counter Mode CBC-MAC Protocol

5-74

Modes of Operation

• Both 802.11i and WPA (as a subset of 802.11i) operate in two modes

– 802.1X mode and

– Pre-Shared Key (PSK) Mode

WPA 802.11i(WPA2)

Can use 802.1XMode?

Yes Yes

Can use PSKMode?

Yes Yes

5-75


• 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

5-77

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

Central AuthenticationServer

Access Points

Client

Client

Credentials

Credentials

OK

Accept

憑據

5-78


• 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


Access Points

Client

Client

Credentials

Credentials

OK

Accept

5-79


• 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


AccessPoints

Client

Client

Credentials

Credentials

OK

Accept

5-80


• Keys

– Central authentication also provides keys to clients

– Changes the keys frequently


Access Points

Client

Client

Key

Key

5-81

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”

5-82

Wii Wireless Connection Setting

http://www.nintendo.com/consumer/systems/wii/en_na/online.jsp

WPA: Wi-Fi Protected AccessPSK: pre-shared keyWEP: Wired Equivalent PrivacyTKIP: Temporal Key Integrity Protocol

Reference: IEEE 802.11i Wi-Fi Alliance

802.11 WLAN Management

5-84

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

5-85


• 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

5-86

Figure 5-25: 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

啞的 ,愚笨的

5-87


• 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/

5-89

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.)

https://www.bluetooth.org/

5-90

Figure 5-26: Bluetooth Personal Area Networks (PANs), Continued

• 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

5-91


• Disadvantages Compared to 802.11

– Short distance (10 meters)

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

– Insufficient for WLAN in a building

5-92


• 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

5-93


• 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

Topics Covered

5-95

Local Wireless Technologies

• 802.11 for Corporate WLANs

• Bluetooth for PANs

• Ultrawideband (UWB)

• RFIDs

• ZigBee

• Mesh Networks

5-96

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

5-97

Radio Propagation

• Shannon’s Equation and the Importance of Channel Bandwidth

– C = B Log2(1+S/N)

• 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)

5-98

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

5-99

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

– 802.11a: 54 Mbps, OFDM, 2.4 GHz band

– 802.11b: 11 Mbps, DSSS, 2.4 GHz band

– 802.11g: 54 Mbps, OFDM, 5 GHz band

– 802.11n: 100 Mbps – 600 Mbps, MIMO, Dual-Band

5-100

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

5-101

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

5-102

Bluetooth

• PANs

• Cable Replacement Technology

• Limited Speeds and Distance

• Application Profiles

• UWB in the Future?

wireless lans (wlans) chapter 5 panko’s business data networks and telecommunications, 6th edition...

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