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CWNA Guide to Wireless LANs, Second Edition

CWNA Guide to Wireless LANs, Second Edition. Chapter Four IEEE 802.11 Physical Layer Standards. Objectives. List and describe the wireless modulation schemes used in IEEE WLANs Tell the difference between frequency hopping spread spectrum and direct sequence spread spectrum

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CWNA Guide to Wireless LANs, Second Edition

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  1. CWNA Guide to Wireless LANs, Second Edition Chapter Four IEEE 802.11 Physical Layer Standards

  2. Objectives • List and describe the wireless modulation schemes used in IEEE WLANs • Tell the difference between frequency hopping spread spectrum and direct sequence spread spectrum • Explain how orthogonal frequency division multiplexing is used to increase network throughput • List the characteristics of the Physical layer standards in 802.11b, 802.11g, and 802.11a networks CWNA Guide to Wireless LANs, Second Edition

  3. Introduction Figure 4-2: OSI data flow CWNA Guide to Wireless LANs, Second Edition

  4. Introduction (continued) Table 4-1: OSI layers and functions CWNA Guide to Wireless LANs, Second Edition

  5. Telecommunication Channel • Channel - a path along which information in the form of an electrical signal passes. Usually a range of contiguous frequencies involved in supporting information transmission. Center Channel Frequency Amplitude Bandwidth Frequency Channel CWNA Guide to Wireless LANs, Second Edition

  6. Narrow and Wide Band • Narrow and Wide Band – a relative comparison of a group or range of frequencies used in a telecommunications system. Narrow Band would describe a small range of frequencies as compared to a larger Wide Band range. Amplitude NB WB Frequency Freq. Range fL fH CWNA Guide to Wireless LANs, Second Edition

  7. Noise Floor • Noise –A disturbance, especially a random and persistent disturbance, that obscures or reduces the clarity of a signal. Anything you don’t want. Amplitude Channel Signal Noise Floor Shot Thermal Freq. CWNA Guide to Wireless LANs, Second Edition

  8. Introduction to Spread Spectrum • Spread Spectrum – a telecommunications technique in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information. Narrowband Amplitude Wideband Frequency CWNA Guide to Wireless LANs, Second Edition

  9. Wireless Modulation Schemes • Four primary wireless modulation schemes: • Narrowband transmission • Frequency hopping spread spectrum • Direct sequence spread spectrum • Orthogonal frequency division multiplexing • Narrowband transmission used primarily by radio stations • Other three used in IEEE 802.11 WLANs CWNA Guide to Wireless LANs, Second Edition

  10. Uses of Spread Spectrum • Military - For low probability of interception of telecommunications. • Civil/Military - Range and positioning measurements. GPS – satellites. • Civil Cellular Telephony. • Civil Wireless Networks – 802.11 and Bluetooth. CWNA Guide to Wireless LANs, Second Edition

  11. Narrowband Transmission • Radio signals by nature transmit on only one radio frequency or a narrow portion of frequencies • Require more power for the signal to be transmitted • Signal must exceed noise level • Total amount of outside interference • Vulnerable to interference from another radio signal at or near same frequency • IEEE 802.11 standards do not use narrowband transmissions CWNA Guide to Wireless LANs, Second Edition

  12. Narrowband Transmission (continued) Figure 4-3: Narrowband transmission CWNA Guide to Wireless LANs, Second Edition

  13. Spread Spectrum Transmission Figure 4-4: Spread spectrum transmission CWNA Guide to Wireless LANs, Second Edition

  14. Spread Spectrum Transmission (continued) • Advantages over narrowband: • Resistance to narrowband interference • Resistance to spread spectrum interference • Lower power requirements • Less interference on other systems • More information transmitted • Increased security • Resistance to multipath distortion CWNA Guide to Wireless LANs, Second Edition

  15. Frequency Hopping Spread Spectrum (FHSS) • Uses range of frequencies • Change during transmission • Hopping code: Sequence of changing frequencies • If interference encountered on particular frequency then that part of signal will be retransmitted on next frequency of hopping code • FCC has established restrictions on FHSS to reduce interference • Due to speed limitations FHSS not widely implemented in today’s WLAN systems • Bluetooth does use FHSS CWNA Guide to Wireless LANs, Second Edition

  16. Frequency Hopping Spread Spectrum (continued) Figure 4-6: FHSS error correction CWNA Guide to Wireless LANs, Second Edition

  17. FHSS • FHSS - Acronym for frequency-hopping spread spectrum. Bluetooth & HomeRF. Amp. 1 3 2 4 Freq. Channel Wide Band Frequency Hop Sequence: 1, 3, 2, 4 CWNA Guide to Wireless LANs, Second Edition

  18. FHSS Timing Time Data Amplitude Hop Time Dwell Time Hop Sequence 1 2 3 4 Channels Frequency CWNA Guide to Wireless LANs, Second Edition

  19. FHSS System Block Diagram Antenna FHSS Data Buffer 1 3 2 4 Mixer Carrier Frequency Sequence Generator 1 3 2 4 Frequency Synthesizer CWNA Guide to Wireless LANs, Second Edition

  20. FHSS Channel Allocation 2.480 GHz 2.403 GHz 2.479 GHz 2.402 GHz CH 79 CH 80 CH 2 CH 3 Amplitude 1 MHz 1 MHz 2.401.5 GHz 2.401.5 GHz 2.402.5 GHz 2.402.5 GHz Freq. 2.4835 GHz 2.400 GHz CWNA Guide to Wireless LANs, Second Edition

  21. FCC Rules for FHSS • Prior to 8-31-00 • Use 75 of the 79 channels • Output Powermax = 1 Watt • Bandwidthmax = 1 MHz • Data Ratemax = 2 Mbps • After 8-31-00 • Only 15 of the 79 channels required • Output Powermax = 125 mW • Bandwidthmax = 5 MHz • Data Ratemax = 10 Mbps CWNA Guide to Wireless LANs, Second Edition

  22. Direct Sequence Spread Spectrum (DSSS) • Uses expanded redundant code to transmit data bits • Chipping code: Bit pattern substituted for original transmission bits • Advantages of using DSSS with a chipping code: • Error correction • Less interference on other systems • Shared frequency bandwidth • Co-location: Each device assigned unique chipping code • Security CWNA Guide to Wireless LANs, Second Edition

  23. Direct Sequence Spread Spectrum (continued) Figure 4-7: Direct sequence spread spectrum (DSSS) transmission CWNA Guide to Wireless LANs, Second Edition

  24. DSSS • DSSS - Acronym for direct-sequence spread spectrum. WLAN, 802.11. Amp. Signal 1 1 3 2 4 Freq. Channel DSSS Band CWNA Guide to Wireless LANs, Second Edition

  25. DSSS Channel Allocation Amplitude Channels 1 2 3 4 5 6 7 8 9 10 11 Freq. 2.401 GHz 2.473 GHz CWNA Guide to Wireless LANs, Second Edition

  26. DSSS 3 Non-overlap Channels Amplitude Ch 1 Ch 6 Ch 11 (2.412 GHz) (2.437GHz) (2.462 GHz) Freq. 22 MHz 2.473 GHz 3MHz 2.401 GHz 2401 MHz 2423 MHz 2426 MHz CWNA Guide to Wireless LANs, Second Edition

  27. DSSS System Block Diagram Carrier Frequency Antenna DSSS Mixer Carrier Generator 11-bit Barker Code Pseudo – Noise Generator Data Buffer Modulator Chipping Code CWNA Guide to Wireless LANs, Second Edition

  28. Comparing FHSS and DSSS CWNA Guide to Wireless LANs, Second Edition

  29. Orthogonal Frequency Division Multiplexing (OFDM) • With multipath distortion, receiving device must wait until all reflections received before transmitting • Puts ceiling limit on overall speed of WLAN • OFDM: Send multiple signals at same time • Split high-speed digital signal into several slower signals running in parallel • OFDM increases throughput by sending data more slowly • Avoids problems caused by multipath distortion • Used in 802.11a networks CWNA Guide to Wireless LANs, Second Edition

  30. Orthogonal Frequency Division Multiplexing (continued) Figure 4-8: Multiple channels CWNA Guide to Wireless LANs, Second Edition

  31. Orthogonal Frequency Division Multiplexing (continued) Figure 4-9: Orthogonal frequency division multiplexing (OFDM) vs. single-channel transmissions CWNA Guide to Wireless LANs, Second Edition

  32. Comparison of Wireless Modulation Schemes • FHSS transmissions less prone to interference from outside signals than DSSS • WLAN systems that use FHSS have potential for higher number of co-location units than DSSS • DSSS has potential for greater transmission speeds over FHSS • Throughput much greater for DSSS than FHSS • Amount of data a channel can send and receive CWNA Guide to Wireless LANs, Second Edition

  33. Comparison of Wireless Modulation Schemes (continued) • DSSS preferred over FHSS for 802.11b WLANs • OFDM is currently most popular modulation scheme • High throughput • Supports speeds over 100 Mbps for 802.11a WLANs • Supports speeds over 54 Mbps for 802.11g WLANs CWNA Guide to Wireless LANs, Second Edition

  34. IEEE 802.11 Physical Layer Standards • IEEE wireless standards follow OSI model, with some modifications • Data Link layer divided into two sublayers: • Logical Link Control (LLC) sublayer: Provides common interface, reliability, and flow control • Media Access Control (MAC) sublayer: Appends physical addresses to frames CWNA Guide to Wireless LANs, Second Edition

  35. IEEE 802.11 Physical Layer Standards (continued) • Physical layer divided into two sublayers: • Physical Medium Dependent (PMD) sublayer: Makes up standards for characteristics of wireless medium (such as DSSS or FHSS) and defines method for transmitting and receiving data • Physical Layer Convergence Procedure (PLCP) sublayer: Performs two basic functions • Reformats data received from MAC layer into frame that PMD sublayer can transmit • “Listens” to determine when data can be sent CWNA Guide to Wireless LANs, Second Edition

  36. IEEE 802.11 Physical Layer Standards (continued) Figure 4-10: Data Link sublayers CWNA Guide to Wireless LANs, Second Edition

  37. IEEE 802.11 Physical Layer Standards (continued) Figure 4-11: PHY sublayers CWNA Guide to Wireless LANs, Second Edition

  38. IEEE 802.11 Physical Layer Standards (continued) Figure 4-12: PLCP sublayer reformats MAC data CWNA Guide to Wireless LANs, Second Edition

  39. IEEE 802.11 Physical Layer Standards (continued) Figure 4-13: IEEE LANs share the same LLC CWNA Guide to Wireless LANs, Second Edition

  40. Legacy WLANs • Two “obsolete” WLAN standards: • Original IEEE 802.11: FHSS or DSSS could be used for RF transmissions • But not both on same WLAN • HomeRF: Based on Shared Wireless Access Protocol (SWAP) • Defines set of specifications for wireless data and voice communications around the home • Slow • Never gained popularity CWNA Guide to Wireless LANs, Second Edition

  41. IEEE 802.11b Physical Layer Standards • Physical Layer Convergence Procedure Standards: Based on DSSS • PLCP must reformat data received from MAC layer into a frame that the PMD sublayer can transmit Figure 4-14: 802.11b PLCP frame CWNA Guide to Wireless LANs, Second Edition

  42. IEEE 802.11b Physical Layer Standards (continued) • PLCP frame made up of three parts: • Preamble: prepares receiving device for rest of frame • Header: Provides information about frame • Data: Info being transmitted • Synchronization field • Start frame delimiter field • Signal data rate field • Service field • Length field • Header error check field • Data field CWNA Guide to Wireless LANs, Second Edition

  43. IEEE 802.11b Physical Layer Standards (continued) • Physical Medium Dependent Standards: PMD translates binary 1’s and 0’s of frame into radio signals for transmission • Can transmit at 11, 5.5, 2, or 1 Mbps • 802.11b uses ISM band • 14 frequencies can be used • Two types of modulation can be used • Differentialbinary phase shift keying (DBPSK): For transmissions at 1 Mbps • Differential quadrature phase shift keying (DQPSK): For transmissions at 2, 5.5, and 11 Mbps CWNA Guide to Wireless LANs, Second Edition

  44. IEEE 802.11b Physical Layer Standards (continued) Table 4-2: 802.11b ISM channels CWNA Guide to Wireless LANs, Second Edition

  45. IEEE 802.11b Physical Layer Standards (continued) Table 4-3: IEEE 802.11b Physical layer standards CWNA Guide to Wireless LANs, Second Edition

  46. IEEE 802.11a Physical Layer Standards • IEEE 802.11a achieves increase in speed and flexibility over 802.11b primarily through OFDM • Use higher frequency • Accesses more transmission channels • More efficient error-correction scheme CWNA Guide to Wireless LANs, Second Edition

  47. U-NII Frequency Band Table 4-4: ISM and U-NII WLAN characteristics Table 4-5: U-NII characteristics CWNA Guide to Wireless LANs, Second Edition

  48. U-NII Frequency Band (continued) • Total bandwidth available for IEEE 802.11a WLANs using U-NII is almost four times that available for 802.11b networks using ISM band • Disadvantages: • In some countries outside U.S., 5 GHz bands allocated to users and technologies other than WLANs • Interference from other devices is growing • Interference from other devices one of primary sources of problems for 802.11b and 802.11a WLANs CWNA Guide to Wireless LANs, Second Edition

  49. Channel Allocation Figure 4-16: 802.11a channels CWNA Guide to Wireless LANs, Second Edition

  50. Channel Allocation (continued) Figure 4-17: 802.11b vs. 802.11a channel coverage CWNA Guide to Wireless LANs, Second Edition

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