1. Wireless LANs (WLANs) Chapter 5 Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of the book

2. 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

3. 5-3 Figure 5-1: Local Wireless Technologies Physical-Layer Transmission –Uses radio transmission –Gives mobility

4. 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. 5-5 Figure 5-2: Wireless LAN (WLAN) Access Point Server Internet Router Ethernet Switch Laptop Mobile Client Wireless Access Point Large Wired Ethernet LAN UTP Radio Transmission Wireless access point (WAP) bridges wireless stations to resources on wired LAN—servers and routers for Internet access Communication

6. 5-6 Figure 5-3: Access Router with Wireless Access Point and Wireless NICs PC Card WNIC for a Notebook Computer Internal WNIC For Desktop PC USB WNIC Access Router with Access Point

7. 5-7 Figure 5-1: 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

8. 5-8 Figure 5-1: 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 USB Bluetooth Adapter

9. 5-9 Figure 5-1: Local Wireless Technologies, Continued 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 –ZigBee –ZigBee for almost-always-off sensor networks at low speeds –Allows battery lives of months or years –Radio Frequency ID (RFID) tags –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 UPC: Universal Product Code

10. 5-10 RFID tag RFIDReader PDA Bluetooth Headset Headset WLANAP Web Server Guide DB 1 2 4 3 席德進 [ 正坐少年 ] 結合 WLAN 與 RFID 之無線導覽系統 Network

11. 5-11 Figure 5-1: Local Wireless Technologies, Continued Other Local Wireless Technologies –Mesh networking –Mesh networking: multiple access points can route frames to their destination (Figure 5-4) without using a wired LAN –Being standardized at 802.11s

12. Radio Propagation

13. 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).

14. 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)

15. 5-15 Figure 5-6: Omnidirectional and Dish Antennas Omnidirectional Antenna Spread signals in all directions Rapid signal attenuation ----- No need to point at receiver Good for mobile subscribers Dish Antenna Focuses signals in a narrow range Signals can be sent over long distances ----- Must point at the sender Good for fixed subscribers

16. 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, Microwave ovens, etc. Blocking Object Reflected Signal Direct Signal 4. Multipath Interference Direct and reflected signals may interfere

17. 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

18. 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

19. 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. Service Band (FM Radio, Cellular Telephony, etc.) 1. Frequency Spectrum (0 Hz to Infinity) 3. Multiple Channels within a Service Band; Each Channel carries a different Signal. 4. Signals in different channels do not interfere with one another

20. 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

21. 5-21 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) 88.0 MHz88.2 MHz Bandwidth = 0.2 MHz = 200 kHz Amplitude Frequency

22. 5-22 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 [ Log 2 (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

23. 5-23 Figure 5-9: Channel Bandwidth and Transmission Speed (Study Figure) Shannon Equation –C = B [ Log 2 (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

24. 5-24 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

25. 5-25 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 匱乏

26. 5-26 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 Golden Zone Higher Frequency Lower Frequency

27. Spread Spectrum Transmission

28. 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

29. 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

30. 5-30 Figure 5-12: Normal Radio Transmission and Spread Spectrum Transmission Channel Bandwidth Required for Signal Speed Normal Radio: Bandwidth Is No Wider than Required 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 bandwidth required by your signaling speed time frequency

31. 5-31 Figure 5-12: Normal Radio Transmission and Spread Spectrum Transmission Channel Bandwidth Required for Signal Speed Note: Height of Box Indicates Bandwidth of Channel Spread Spectrum Transmission: Channel Bandwidth Is 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

32. 5-32 Figure 5-11: Spread Spectrum Transmission, Continued There are Several Spread Spectrum Transmission Methods (Figure 5-13) –Frequency Hopping Spread Spectrum –Frequency Hopping Spread Spectrum (FHSS) up to 4 Mbps The book says 2 Mbps, but it is now higher. –Direct Sequence Spread Spectrum –Direct Sequence Spread Spectrum (DSSS) is used at 11 Mbps –Orthogonal Frequency Division Multiplexing –Orthogonal 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

33. 5-33 Figure 5-13: Spread Spectrum Transmission Methods Frequency Hopping Spread 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

34. 5-34 Figure 5-13: Spread Spectrum Transmission Methods Wideband but Low-Intensity Signal Direct Sequence Spread 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

35. 5-35 Figure 5-13: Spread Spectrum Transmission Methods Orthogonal Frequency Division Multiplexing (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

36. 5-36 Figure 5-20: Multiple Input/Multiple Output (MIMO) Transmission 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.

37. 802.11 WLAN Operation

38. 5-38 Figure 5-14: Typical 802.11 WLAN Operation Server Ethernet Switch Laptop WAP Large Wired LAN Client PC UTP Radio Transmission 802.11 Frame802.3 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)

39. 5-39 Figure 5-14: Typical 802.11 WLAN Operation, Continued Server Ethernet Switch Laptop AP A Large Wired LAN Client PC AP B UTP Handoff or Roaming (if mobile computer moves to another access point, it switches service to that access point) 802.11 Frame 802.3 Frame

40. 5-40 Figure 5-15: Stations and Access Points Transmit in a Single Channel Collision if 2 Devices send Simultaneously

41. 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

42. 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

43. 5-43 Figure 5-16: CSMA/CA+ACK 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 Box

44. 5-44 Figure 5-16: CSMA/CA+ACK 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! Box

45. 5-45 RTS/CTS AB RTS CTS C D CSMA/CA http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csma-ca/withhidden.html

46. 5-46 Figure 5-17: Request to Send/Clear to Send (RTS/CTS) Server Switch Laptop Access Point B Large Wired LAN Radio Link Client PC RTS 1. Device that wishes to transmit may send a Request-to-Send message Box

47. 5-47 Figure 5-17: Request to Send/Clear to Send (RTS/CTS) Server Switch May Send Frames WAP Large Wired LAN Radio Link Client PC 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 CTS Box Must Wait

48. 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

49. 802.11 WLAN Standards

50. 5-50 Figure 5-18: Specific 802.11 Wireless LAN Standards 802.11b802.11g if 802.11g access point serves an 802.11b station 2.4 GHz Unlicensed Band Lower Attenuation Yes 802.11a 5 GHz Higher NoYes Crowded Band? Lower PriceHigher LowerMarket AcceptanceVery LowHigh

51. 5-51 Figure 5-18: Specific 802.11 Wireless LAN Standards 802.11b802.11g if 802.11g access point serves an 802.11b station 11 Mbps54 Mbps Not Specified Rated Speed* 6 Mbps25 Mbps12 MbpsThroughput, 3 m 6 Mbps 802.11a 54 Mbps 25 Mbps 12 Mbps20 Mbps11 MbpsThroughput, 30 m Source for throughput data: Broadband.com 802.11a, operating at a higher frequency, has more attenuation Than 802.11b *Maximum rated speed. There are slower modes if propagation is poor.

52. 5-52 Figure 5-18: Specific 802.11 Wireless LAN Standards 802.11g if 802.11g access point serves an 802.11b station Aggregate throughputs; Individual throughputs are lower Are These Aggregate Or Individual Throughputs? 20 Mbps11 MbpsThroughput, 30 m 802.11a 12 Mbps 11 Mbps54 Mbps Not Specified Rated Speed 802.11b 6 Mbps 54 Mbps

53. 5-53 Figure 5-18: Specific 802.11 Wireless LAN Standards 802.11a802.11b802.11g if 802.11g access point serves an 802.11b station 12 to 24333 Number of Non- Overlapping Channels 5 GHz2.4 GHz Unlicensed Band 2.4 GHz non-overlapping channels are 1, 6, and 11

54. 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) 1241224012423 2241724052428 3242224112433 4242724162438 5243224212443 6243724262448 7244224312453 8244724362458 9245224412463 10245724462468 11246224512473 5 MHz 22 MHz

55. 5-55 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

56. 5-56 Figure 5-18: 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

57. 5-57 Figure 5-19: Interference Between Nearby Access Points Operating on the Same Channel Access Point Channels Should be Selected to Minimize Mutual Interference

58. 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

59. 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)

60. 802.11 WLAN Security

61. 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

62. 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

63. 5-63 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 Evil Twin AP Legitimate Client Legitimate AP Duped Association 詐騙

64. 5-64 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 Evil Twin AP Legitimate Client Legitimate AP 1. Associates 2. Associates As Legitimate Client

65. 5-65 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 Evil Twin AP Legitimate Client Legitimate AP

66. 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

67. 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

68. 5-68

69. 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

70. 5-70 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

71. 5-71 Figure 5-22: 802.11 Security Standards, Continued 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

72. 5-72 WPA WPA’s encryption scheme: TKIP (Temporal Key Integrity Protocol)

73. 5-73 Figure 5-22: 802.11 Security Standards, Continued 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

74. 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 WPA802.11i (WPA2) Can use 802.1X Mode? Yes Can use PSK Mode? Yes

75. 5-75 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

76. 5-76

77. 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 Authentication Server Access Points Client Credentials OK Accept 憑據

78. 5-78 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 Central Authentication Server Access Points Client Credentials OK Accept

79. 5-79 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 Central Authentication Server Access Points Client Credentials OK Accept

80. 5-80 Figure 5-23: 802.11 Security in 802.1X (Enterprise Mode) Keys –Central authentication also provides keys to clients –Changes the keys frequently Central Authentication Server Access Points Client Key

81. 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”

82. 5-82 Wii Wireless Connection Setting http://www.nintendo.com/consumer/systems/wii/en_na/online.jsp WPA: Wi-Fi Protected Access PSK: pre-shared key WEP: Wired Equivalent Privacy TKIP: Temporal Key Integrity Protocol Reference: IEEE 802.11i Wi-Fi Alliance

83. 802.11 WLAN Management

84. 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

85. 5-85 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

86. 5-86 Figure 5-25: Wireless Access Point Management Alternatives Management intelligence can be placed in the access point or the WLAN switch 啞的, 愚笨的

87. 5-87 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

88. Bluetooth For Personal Area Networks (PANs)

89. 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.)

90. 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

91. 5-91 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

92. 5-92 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

93. 5-93 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

94. Topics Covered

95. 5-95 Local Wireless Technologies 802.11 for Corporate WLANs Bluetooth for PANs Ultrawideband (UWB) RFIDs ZigBee Mesh Networks

96. 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

97. 5-97 Radio Propagation Shannon’s Equation and the Importance of Channel Bandwidth –C = B Log 2 (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)

98. 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

99. 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

100. 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

101. 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

102. 5-102 Bluetooth PANs Cable Replacement Technology Limited Speeds and Distance Application Profiles UWB in the Future?