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

2. 2 Objectives List and describe the wireless modulation schemes used in IEEE WLANs List and describe the wireless modulation schemes used in IEEE WLANs Tell the difference between Tell the difference between frequency hoppingfrequency hopping spread spectrumspread spectrum direct sequence spread spectrumdirect sequence spread spectrum Explain how orthogonal frequency division multiplexing is used to increase network throughput Explain how orthogonal frequency division multiplexing is used to increase network throughput List the characteristics of the Physical layer standards in List the characteristics of the Physical layer standards in 802.11b networks802.11b networks 802.11g networks802.11g networks 802.11a networks802.11a networks

3. 3 Introduction Figure 4-2: OSI data flow

4. 4 Introduction (continued) Table 4-1: OSI layers and functions

5. 5 Wireless Modulation Schemes Four primary wireless modulation schemes: Four primary wireless modulation schemes: Narrowband transmissionNarrowband transmission Frequency hopping spread spectrumFrequency hopping spread spectrum Direct sequence spread spectrumDirect sequence spread spectrum Orthogonal frequency division multiplexingOrthogonal frequency division multiplexing Narrowband transmission used primarily by radio stations Narrowband transmission used primarily by radio stations Other three used in IEEE 802.11 WLANs Other three used in IEEE 802.11 WLANs

6. 6 Narrowband Transmission Radio signals by nature transmit on only one radio frequency or a narrow portion of frequencies 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 Require more power for the signal to be transmitted Signal must exceed noise levelSignal must exceed noise level Total amount of outside interference Total amount of outside interference Vulnerable to interference from another radio signal at or near same frequency Vulnerable to interference from another radio signal at or near same frequency IEEE 802.11 standards do not use narrowband transmissions IEEE 802.11 standards do not use narrowband transmissions

7. 7 Narrowband Transmission Figure 4-3: Narrowband transmission

8. 8 Spread Spectrum Transmission Figure 4-4: Spread spectrum transmission

9. 9 Spread Spectrum Transmission Advantages over narrowband: Advantages over narrowband: Resistance to narrowband interferenceResistance to narrowband interference Resistance to spread spectrum interferenceResistance to spread spectrum interference Lower power requirementsLower power requirements Less interference on other systemsLess interference on other systems More information transmittedMore information transmitted Increased securityIncreased security Resistance to multipath distortion (e.g. reflections off of buildings and structures)Resistance to multipath distortion (e.g. reflections off of buildings and structures)

10. 10 Frequency Hopping Spread Spectrum (FHSS) Uses range of frequencies Uses range of frequencies Change during transmissionChange during transmission Hopping code: Sequence of changing frequencies 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 codeIf 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 FCC has established restrictions on FHSS to reduce interference Due to speed limitations FHSS not widely implemented in today’s WLAN systems Due to speed limitations FHSS not widely implemented in today’s WLAN systems Bluetooth does use FHSSBluetooth does use FHSS

11. 11 Frequency Hopping Spread Spectrum (continued) Figure 4-6: FHSS error correction

12. 12 Direct Sequence Spread Spectrum (DSSS) Uses expanded redundant code to transmit data bits Uses expanded redundant code to transmit data bits Chipping code: Bit pattern substituted for original transmission bits Chipping code: Bit pattern substituted for original transmission bits Advantages of using DSSS with a chipping code:Advantages of using DSSS with a chipping code: Error correction Error correction Less interference on other systems Less interference on other systems Shared frequency bandwidth Shared frequency bandwidth Co-location: Each device assigned unique chipping codeCo-location: Each device assigned unique chipping code Security Security

13. 13 Direct Sequence Spread Spectrum (continued) Figure 4-7: Direct sequence spread spectrum (DSSS) transmission

14. 14 Orthogonal Frequency Division Multiplexing (OFDM) With multipath distortion, receiving device must wait until all reflections received before transmitting With multipath distortion, receiving device must wait until all reflections received before transmitting Puts ceiling limit on overall speed of WLANPuts ceiling limit on overall speed of WLAN OFDM: Send multiple signals at same time OFDM: Send multiple signals at same time Split high-speed digital signal into several slower signals running in parallelSplit high-speed digital signal into several slower signals running in parallel OFDM increases throughput by sending data more slowly OFDM increases throughput by sending data more slowly Avoids problems caused by multipath distortion Avoids problems caused by multipath distortion Used in 802.11a networks Used in 802.11a networks

15. 15 Orthogonal Frequency Division Multiplexing (continued) Figure 4-8: Multiple channels

16. 16 Orthogonal Frequency Division Multiplexing Figure 4-9: Orthogonal frequency division multiplexing (OFDM) vs. single-channel transmissions

17. 17 Comparison of Wireless Modulation Schemes FHSS transmissions less prone to interference from outside signals than DSSS 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 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 DSSS has potential for greater transmission speeds over FHSS Throughput much greater for DSSS than FHSS Throughput much greater for DSSS than FHSS Amount of data a channel can send and receiveAmount of data a channel can send and receive

18. 18 Comparison of Wireless Modulation Schemes DSSS preferred over FHSS for 802.11b WLANs DSSS preferred over FHSS for 802.11b WLANs OFDM is currently most popular modulation scheme OFDM is currently most popular modulation scheme High throughputHigh throughput Supports speeds over 100 Mbps for 802.11a WLANsSupports speeds over 100 Mbps for 802.11a WLANs Supports speeds over 54 Mbps for 802.11g WLANsSupports speeds over 54 Mbps for 802.11g WLANs

19. 19 IEEE 802.11 Physical Layer Standards IEEE wireless standards follow OSI model, with some modifications IEEE wireless standards follow OSI model, with some modifications Data Link layer divided into two sublayers: Data Link layer divided into two sublayers: Logical Link Control (LLC) sublayer: Provides common interface, reliability, and flow controlLogical Link Control (LLC) sublayer: Provides common interface, reliability, and flow control Media Access Control (MAC) sublayer: Appends physical addresses to framesMedia Access Control (MAC) sublayer: Appends physical addresses to frames

20. 20 IEEE 802.11 Physical Layer Standards (continued) Physical layer divided into two sublayers: 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 dataPhysical 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 functionsPhysical Layer Convergence Procedure (PLCP) sublayer: Performs two basic functions Reformats data received from MAC layer into frame that PMD sublayer can transmit Reformats data received from MAC layer into frame that PMD sublayer can transmit “Listens” to determine when data can be sent “Listens” to determine when data can be sent

21. 21 IEEE 802.11 Physical Layer Standards (continued) Figure 4-10: Data Link sublayers

22. 22 IEEE 802.11 Physical Layer Standards (continued) Figure 4-11: PHY sublayers

23. 23 IEEE 802.11 Physical Layer Standards (continued) Figure 4-12: PLCP sublayer reformats MAC data

24. 24 IEEE 802.11 Physical Layer Standards (continued) Figure 4-13: IEEE LANs share the same LLC

25. 25 Legacy WLANs Two “obsolete” WLAN standards: Two “obsolete” WLAN standards: Original IEEE 802.11: FHSS or DSSS could be used for RF transmissionsOriginal IEEE 802.11: FHSS or DSSS could be used for RF transmissions But not both on same WLAN But not both on same WLAN HomeRF: Based on Shared Wireless Access Protocol (SWAP)HomeRF: Based on Shared Wireless Access Protocol (SWAP) Defines set of specifications for wireless data and voice communications around the home Defines set of specifications for wireless data and voice communications around the home Slow Slow Never gained popularity Never gained popularity

26. 26 IEEE 802.11b Physical Layer Standards Physical Layer Convergence Procedure Standards: Based on DSSS Physical Layer Convergence Procedure Standards: Based on DSSS PLCP must reformat data received from MAC layer into a frame that the PMD sublayer can transmitPLCP must reformat data received from MAC layer into a frame that the PMD sublayer can transmit Figure 4-14: 802.11b PLCP frame

27. 27 IEEE 802.11b Physical Layer Standards (continued) PLCP frame made up of three parts: PLCP frame made up of three parts: Preamble: prepares receiving device for rest of framePreamble: prepares receiving device for rest of frame Header: Provides information about frameHeader: Provides information about frame Data: Info being transmittedData: Info being transmitted Synchronization field Synchronization field Start frame delimiter field Start frame delimiter field Signal data rate field Signal data rate field Service field Service field Length field Length field Header error check field Header error check field Data field Data field

28. 28 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 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 MbpsCan transmit at 11, 5.5, 2, or 1 Mbps 802.11b uses ISM band802.11b uses ISM band 14 frequencies can be used 14 frequencies can be used Two types of modulation can be usedTwo types of modulation can be used Differential binary phase shift keying (DBPSK): For transmissions at 1 Mbps Differential binary phase shift keying (DBPSK): For transmissions at 1 Mbps Differential quadrature phase shift keying (DQPSK): For transmissions at 2, 5.5, and 11 Mbps Differential quadrature phase shift keying (DQPSK): For transmissions at 2, 5.5, and 11 Mbps

29. 29 IEEE 802.11b Physical Layer Standards (continued) Table 4-2: 802.11b ISM channels

30. 30 IEEE 802.11b Physical Layer Standards (continued) Table 4-3: IEEE 802.11b Physical layer standards

31. 31 IEEE 802.11a Physical Layer Standards IEEE 802.11a achieves increase in speed and flexibility over 802.11b primarily through OFDM IEEE 802.11a achieves increase in speed and flexibility over 802.11b primarily through OFDM Use higher frequencyUse higher frequency Accesses more transmission channelsAccesses more transmission channels More efficient error-correction schemeMore efficient error-correction scheme

32. 32 U-NII Frequency Band Table 4-5: U-NII characteristics Table 4-4: ISM and U-NII WLAN characteristics

33. 33 U-NII Frequency Band Total bandwidth available for IEEE 802.11a WLANs using U-NII is almost four times that available for 802.11b networks using ISM band 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: Disadvantages: In some countries outside U.S., 5 GHz bands allocated to users and technologies other than WLANsIn some countries outside U.S., 5 GHz bands allocated to users and technologies other than WLANs Interference from other devices is growingInterference from other devices is growing Interference from other devices one of primary sources of problems for 802.11b and 802.11a WLANs Interference from other devices one of primary sources of problems for 802.11b and 802.11a WLANs

34. 34 Channel Allocation Figure 4-16: 802.11a channels

35. 35 Channel Allocation (continued) Figure 4-17: 802.11b vs. 802.11a channel coverage

36. 36 Error Correction 802.11a has fewer errors than 802.11b 802.11a has fewer errors than 802.11b Transmissions sent over parallel subchannelsTransmissions sent over parallel subchannels Interference tends to only affect one subchannelInterference tends to only affect one subchannel Forward Error Correction (FEC): Transmits secondary copy along with primary information Forward Error Correction (FEC): Transmits secondary copy along with primary information 4 of 52 channels used for FEC4 of 52 channels used for FEC Secondary copy used to recover lost dataSecondary copy used to recover lost data Reduces need for retransmission Reduces need for retransmission

37. 37 Physical Layer Standards PLCP for 802.11a based on OFDM PLCP for 802.11a based on OFDM Three basic frame components: Preamble, header, and data Three basic frame components: Preamble, header, and data Figure 4-18: 802.11a PLCP frame

38. 38 Physical Layer Standards Table 4-6: 802.11a Rate field values

39. 39 Physical Layer Standards Modulation techniques used to encode 802.11a data vary depending upon speed Modulation techniques used to encode 802.11a data vary depending upon speed Speeds higher than 54 Mbps may be achieved using 2X modes Speeds higher than 54 Mbps may be achieved using 2X modes Table 4-7: 802.11a characteristics

40. 40 IEEE 802.11g Physical Layer Standards 802.11g combines best features of 802.11a and 802.11b 802.11g combines best features of 802.11a and 802.11b Operates entirely in 2.4 GHz ISM frequency Operates entirely in 2.4 GHz ISM frequency Two mandatory modes and one optional mode Two mandatory modes and one optional mode CCK mode used at 11 and 5.5 Mbps (mandatory)CCK mode used at 11 and 5.5 Mbps (mandatory) OFDM used at 54 Mbps (mandatory)OFDM used at 54 Mbps (mandatory) PBCC-22 (Packet Binary Convolution Coding): Optional modePBCC-22 (Packet Binary Convolution Coding): Optional mode Can transmit between 6 and 54 Mbps Can transmit between 6 and 54 Mbps

41. 41 IEEE 802.11g Physical Layer Standards (continued) Table 4-8: IEEE 802.11g Physical layer standards

42. 42 IEEE 802.11g Physical Layer Standards (continued) Characteristics of 802.11g standard: Characteristics of 802.11g standard: Greater throughput than 802.11b networksGreater throughput than 802.11b networks Covers broader area than 802.11a networksCovers broader area than 802.11a networks Backward compatibleBackward compatible Only three channelsOnly three channels If 802.11b and 802.11g devices transmitting in same environment, 802.11g devices drop to 11 Mbps speedsIf 802.11b and 802.11g devices transmitting in same environment, 802.11g devices drop to 11 Mbps speeds Vendors can implement proprietary higher speedVendors can implement proprietary higher speed Channel bonding and Dynamic turbo Channel bonding and Dynamic turbo

43. 43 Summary Three modulation schemes are used in IEEE 802.11 wireless LANs: frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM) Three modulation schemes are used in IEEE 802.11 wireless LANs: frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM) Spread spectrum is a technique that takes a narrow, weaker signal and spreads it over a broader portion of the radio frequency band Spread spectrum is a technique that takes a narrow, weaker signal and spreads it over a broader portion of the radio frequency band Spread spectrum transmission uses two different methods to spread the signal over a wider area: FHSS and DSSS Spread spectrum transmission uses two different methods to spread the signal over a wider area: FHSS and DSSS

44. 44 Summary OFDM splits a single high-speed digital signal into several slower signals running in parallel OFDM splits a single high-speed digital signal into several slower signals running in parallel IEEE has divided the OSI model Data Link layer into two sublayers: the LLC and MAC sublayers IEEE has divided the OSI model Data Link layer into two sublayers: the LLC and MAC sublayers The Physical layer is subdivided into the PMD sublayer and the PLCP sublayer The Physical layer is subdivided into the PMD sublayer and the PLCP sublayer The Physical Layer Convergence Procedure Standards (PLCP) for 802.11b are based on DSSS The Physical Layer Convergence Procedure Standards (PLCP) for 802.11b are based on DSSS

45. 45 Summary IEEE 802.11a networks operate at speeds up to 54 Mbps with an optional 108 Mbps IEEE 802.11a networks operate at speeds up to 54 Mbps with an optional 108 Mbps The 802.11g standard specifies that it operates entirely in the 2.4 GHz ISM frequency and not the U-NII band used by 802.11a The 802.11g standard specifies that it operates entirely in the 2.4 GHz ISM frequency and not the U-NII band used by 802.11a