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Wireless LANs (WLANs)

Wireless LANs (WLANs). Chapter 5 Updated January 2009 XU Zhengchuan Fudan University. 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)

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Wireless LANs (WLANs)

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  1. Wireless LANs (WLANs) Chapter 5 Updated January 2009 XU Zhengchuan Fudan University

  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. Figure 5-1: Local Wireless Technologies, Continued • 802.11 • The dominant WLAN technology today • Standardized by the 802.11 Working Group 802.11

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

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

  6. Figure 5-1: Local Wireless Technologies, Continued • 802.11 Wireless LANs • Today, mostly speeds of tens of megabits per second with distances of 30 to 100 meters or more • Can serve many users in a home or office • Increasingly, 100 Mbps to 600 Mbps with 802.11n • Organizations can provide coverage throughout a building or a university campus by installing many access points

  7. Radio Propagation

  8. 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).

  9. Figure 5-5: Frequency Measurement, Continued • Measuring Frequencies • Frequency measures increases by factors of 1,000 (not 1,024) • Kilohertz (kHz) [Note the lower-case k] • Megahertz (MHz) • Gigahertz (GHz)

  10. 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 Figure 5-6: Omnidirectional and Dish Antennas

  11. Figure 5-7: Wireless Propagation Problems 1. Electromagnetic Interference (EMI) from Other stations, Microwave ovens, etc. 2. Attenuation: signal gets weaker with distance Blocking Object 3. Shadow Zone (Dead Spot)

  12. Figure 5-7: Wireless Propagation Problems Blocking Object Direct Signal Laptop 4. Multipath Interference Reflected Signal Direct and reflected signals may interfere

  13. Inverse Square Law Attenuation • Inverse square law attenuation • To compare relative power at two distances • Divide the longer distance by the shorter distance • Square the result; this is the relative power ratio • Examples • 100 mW (milliwatts) at 10 meters • At 20 meters, 100 / (20/10)2 = 100 mW / 4 = 25 mW • At 30 meters, 100 / (30/10)2 = 100 mW / 9 = 11 mW • Much faster attenuation than UTP or fiber

  14. Frequently-Depended Propagation Problem • Some Problems are Frequency-Dependent • Higher-frequency signals attenuate faster • Absorbed more rapidly by water in the air • Higher-frequency signals blocked more by obstacles • At lower frequencies, signal refract (bend) around obstacles like an ocean wave hitting a buoy • At higher frequencies, signals do not refract; leave a complete shadow behind obstacles

  15. Figure 5-8: The Frequency Spectrum, Service Bands, and Channels 1. Frequency Spectrum (0 Hz to Infinity) 4.Signals in different channels do not interfere with one another 3. Multiple Channels within a Service Band; each Channel carries a different signal Channel 5, Signal A 2. Service Band (FM Radio, Cellular Telephony, etc.) Channel 4, Signal D Channel 3, Signal B Channel 2, No Signal Channel 1, Signal E 0 Hz

  16. Figure 5-9: Channel Bandwidth and Transmission Speed (Study Figure) • Signal Bandwidth • Chapter 3 discussed a wave operating at a single frequency • However, most signals are spread over a range of frequencies • The higher the speed, the greater the spread of frequencies Signal Amplitude Frequency

  17. Figure 5-9: Channel Bandwidth and Transmission Speed (Study Figure) • Channel Bandwidth • Higher-speed signals need wider-bandwidth channels • Channel bandwidth is the highest frequency in a channel minus the lowest frequency • An 88.0 MHz to 88.2 MHz channel has a bandwidth of 0.2 MHz (200 kHz) Amplitude 88.0 MHz 88.2 MHz Frequency Bandwidth = 0.2 MHz = 200 kHz

  18. Figure 5-9: Channel Bandwidth and Transmission Speed (Study Figure) • Shannon Equation • Specifies the connection between channel bandwidth and the channel’s maximum signal transmission speed • C = B [ Log2(1+S/N) ] • C = Maximum possible transmission speed in the channel (bps) • B = Bandwidth (Hz) • S/N = Signal-to-Noise Ratio • Measured as a ratio • If given in dB, must convert to ratio

  19. Figure 5-9: Channel Bandwidth and Transmission Speed (Study Figure) • Shannon Equation • C = B [ Log2 (1+S/N) ] • Note that doubling the bandwidth doubles the maximum possible transmission speed • Increasing the bandwidth by X increases the maximum possible speed by X • Wide bandwidth is the key to fast transmission • Increasing S/N helps slightly but usually cannot be done to any significant extent

  20. Figure 5-9: Channel Bandwidth and Transmission Speed (Study Figure) • Broadband and Narrowband Channels • Broadband means wide channel bandwidth and therefore high speed • Narrowband means narrow channel bandwidth and therefore low speed • Narrowband is below 200 kbps • Broadband is above 200 kbps

  21. Figure 5-9: Channel Bandwidth and Transmission Speed (Study Figure) • Channel Bandwidth and Spectrum Scarcity • Why not make all channels broadband? • There is only a limited amount of spectrum at desirable frequencies • Making each channel broader than needed would mean having fewer channels or widening the service band • Service band design requires tradeoffs between speed requirements, channel bandwidth, and service band size

  22. Figure 5-9: Channel Bandwidth and Transmission Speed (Study Figure) • The Golden Zone • Most organizational radio technologies operate in the golden zone in the high megahertz to low gigahertz range • At higher frequencies,propagation problemsare severe • At lower frequencies,there is not enoughtotal bandwidth Higher Frequency Golden Zone Lower Frequency

  23. Spread Spectrum Transmission

  24. Figure 5-11: Spread Spectrum Transmission (Study Figure) • Unlicensed Bands • WLANs operate in unlicensed service bands • You do not need a license to have or move your stations • Two unlicensed bands are widely used: the 2.4 GHz band and the 5 GHz band • 5 GHz has worse propagation characteristics • 2.4 GHz has fewer available channels

  25. Figure 5-11: Spread Spectrum Transmission, Continued • Spread Spectrum Transmission • You are REQUIRED BY LAW to use spread spectrum transmission in unlicensed bands • Spread spectrum transmission uses much larger channels than transmission speed requires • Spread spectrum transmission is required to reduce propagation problems at high frequencies • Especially multipath interference • Spread spectrum transmission is NOT used for security in WLANs • This surprises many people

  26. Figure 5-11: Spread Spectrum Transmission, Continued Not Used in 802.11 • There are Several Spread Spectrum Transmission Methods (Figure 5-13) • Older Techniques • Frequency Hopping Spread Spectrum (FHSS) up to 4 Mbps (The book says 2 Mbps) • Direct Sequence Spread Spectrum (DSSS) up to 11 Mbps • Orthogonal Frequency Division Multiplexing (OFDM) is used at 54 Mbps • MIMO for speeds of 100 Mbps to 600 Mbps

  27. Figure 5-13: Spread Spectrum Transmission Methods Only used in Old 802.11 systems And Bluetooth 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)

  28. Figure 5-13: Spread Spectrum Transmission Methods Only used in old 802.11 networks 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

  29. Figure 5-13: Spread Spectrum Transmission Methods Orthogonal Frequency Division Multiplexing (OFDM) Subcarrier 1 Subcarrier 2 Subcarrier 3 OFDM divides the broadband channel into subcarriers Sends part of the signal in each subcarrier The subcarrier transmissions are redundant so that if some carriers are lost, the entire signal still gets through Used in 802.11a and 802.11g at 54 Mbps (later)

  30. 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.

  31. 802.11 WLAN Operation

  32. Figure 5-14: Typical 802.11 WLAN Operation Ethernet Switch 802.3 Frame 802.11 Frame UTP Radio Transmission WAP Laptop 802.3 Frame Wireless access points (WAPs) bridge the networks (translate between the 802.11 wireless frame and the Ethernet 802.3 frame used within the LAN) Client PC Server Large Wired LAN

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

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

  35. Media Access Control Box • Only one station or the access point can transmit at a time • To control access (transmission), two methods can be used • CSMA/CA+ACK (mandatory) • RTS/CTS (optional unless 802.11b and g stations share an 802.11g access point)

  36. Figure 5-16: CSMA/CA+ACK in 802.11 Wireless LANs Box • CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) • CSMA • Sender Always Listens for Traffic • Carrier is the signal; sense is to listen • If there is traffic, the sender waits • If there is no traffic … • If the time since the last transmission is more than a critical value, the station may send immediately

  37. Figure 5-16: CSMA/CA+ACK in 802.11 Wireless LANs Box • CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) • If there is no traffic • If the time since the last transmission is less than a critical value, the station sets a random timer and waits • If there is no traffic at the end of the waiting time, the station sends • If there is traffic, CSMA starts over again

  38. Figure 5-16: CSMA/CA+ACK in 802.11 Wireless LANs Box • ACK (Acknowledgment) • Receiver immediately sends back an acknowledgment when it receives a frame • Does not wait to send an ACK • This avoids interference with other stations, which must wait • If sender does not receive the acknowledgment, it retransmits the frame using CSMA/CA • 802.11 with CSMA/CA+ACK is a reliable protocol!

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

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

  41. Recap Box • CSMA/CA+ACK is mandatory • RTS/CTS is optional • However, it is mandatory if 802.11b and 802.11g NICs share the same 802.11g access point

  42. 802.11 WLAN Standards

  43. Figure 5-18: Specific 802.11 Wireless LAN Standards 802.11a 802.11b 802.11g 802.11g if 802.11g access point serves an 802.11b station Unlicensed Band 5 GHz 2.4 GHz 2.4 GHz 2.4 GHz Crowded Band? No Yes Yes Yes Attenuation Higher Lower Lower Lower Price Higher Lower Lower Lower Market Acceptance Very Low High Higher Lower

  44. Figure 5-18: Specific 802.11 Wireless LAN Standards Source for throughput data: Broadband.com 802.11a 802.11b 802.11g 802.11g if 802.11g access point serves an 802.11b station 802.11a, operating at a higher frequency, has more attenuation Than 802.11b Rated Speed* 54 Mbps 11 Mbps 54 Mbps Not Specified Throughput, 3 m 25 Mbps 6 Mbps 25 Mbps 12 Mbps Throughput, 30 m 12 Mbps 6 Mbps 20 Mbps 11 Mbps *Maximum rated speed. There are slower modes if propagation is poor.

  45. Figure 5-18: Specific 802.11 Wireless LAN Standards, Continued • Transmission Speed and Distance • As a station moves away from an access point, transmission speed falls • There are several modes of operation specified in each standard • The fastest mode only works with a very strong signal • As the user moves away, the signal strength becomes too low • That station and the access point switch to a slower mode • This slows things down for all users

  46. Figure 5-18: Specific 802.11 Wireless LAN Standards 802.11b 802.11a 802.11g 802.11g if 802.11g access point serves an 802.11b station Unlicensed Band 2.4 GHz 5 GHz 2.4 GHz 2.4 GHz Number of Non- Overlapping Channels 3 Up to 24 3 3 2.4 GHz non-overlapping channels are 1, 6, and 11

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

  48. 802.11n • Under Development • Rated speeds of 100 Mbps to 600 Mbps • Will operate in both the 2.4 GHz and 5 GHz bands • May use twice current bandwidth per channels (~20 MHz) to roughly double speed • Will use MIMO • Currently a draft standard

  49. WLAN Security

  50. 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

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