Chapter 18 High Throughput and 802.11n

1. Chapter 18 High Throughput and 802.11n • 802.11 n history • MIMO • HT Channels • HT PHY • HT MAC • HT Operation

2. Exam Essentials • Define the differences between MIMO and SISO. • Understand that SISO devices use only one radio chain, whereas MIMO systems use multiple radio chains. • Understand spatial multiplexing. • Describe how SM takes advantage of multipath and sends multiple spatial streams resulting in increased throughput. • Explain MIMO diversity. • Be able to explain the differences between simple switched diversity and the advanced diversity used by MIMO antenna systems. Explain the use of maximal ratio combining with MIMO diversity. • Understand transmit beamforming. • Explain how optional transmit beamforming can be used to steer beams in an optimal path toward a receiving radio and the benefts of the beamforming process.

3. Exam Essentials • Understand 20 MHz and 40 MHz channels. • Understand legacy 20 MHz channels, 20 MHz HT channels, and 40 MHz channels and how they use OFDM. Explain why 40 MHz channels work best in the 5 GHz UNII bands. Explain primary and secondary channels. • Explain the guard interval. • Describe how the guard interval compensates for intersymbol interference. Discuss the use of both 800- and 400-nanosecond GIs. • Understand modulation coding schemes. • Explain how modulation coding schemes are used to define data rates and all the variables that can affect the data rates.

4. Exam Essentials • Explain the three HT PPDU formats. • Describe the differences between non-HT legacy, HT Mixed, and HT Greenfield. • Understand HT MAC enhancements. • Explain how the use of A-MSDU, A-MPDU, block ACKs, and RIFS are used to increase throughput at the MAC sublayer. Define the two new power-management methods used by HT radios. • Explain the HT protection modes. • Describe the differences between protection modes 0–3. Explain the use of Dual-CTS.

5. 802.11 n HT • High Throughput technology • New PHY and MAC specs • 100 Mbps or greater • Use MIMO • Multiple radios and antennas • USE multipath to advantage • Different modes of operations • Co exists with older networks Pg 589

6. 802.11 n History • Define data rates with modulation and coding schemes (MCS) • Goal is to increase data rates in both 2.4 and 5 ghz • Potential of 600 mbps Pg 589

7. 802.11 n Draft • HT clause 20 radio with MIMO and OFDM • Must be backward compatible with • HR-DSSS • ERP Pg 590

8. WiFi Alliance • 802.11 n draft 2 certified • Mandatory and tested implementations • Many vendors had pre 802.11n equipment Pg 590

9. MIMO • Multiple in, Multiple out • Mutiple radios AND antennas • Radio Chains • Also allows for spatial multiplexing • Transmit beamforming can be used to “steer” beams for greater coverage Pg 592

10. Radio Chains • Radio chain is single radio and associated antennas • Previous technologies were single input single output • MIMO has multiple radio chains • Each radio with own antenna • 2x3 MIMO 3 radios, with 2 transmitters and 3 receivers Pg 592

11. Radio Chains • Multiple Transmitters provides for more data transmission through Spatial Multiplexing • Multiple Receivers gives increased signal to noise ration because of MIMO antenna diverstiy • Up to 4x4 • Each Radio chain takes power • 2x2 needs less power than 4x4 Pg 592

12. Spatial Multiplexing • Multipath is caused by two or more paths of same signal arriving in close time, but out of phase • MIMO uses the variation in arrivals to transmit MORE data Pg 593

13. Spatial Multiplexing • MIMO radios transmit multiple radio signals at the same time and take advantage of multipath. • Each radio signal is transmitted by a unique radio and antenna of the MIMO system. • Each signal is known as a spatial stream, • each unique stream can contain different data than the other streams transmitted by one or more of the other radios. • Each stream will also travel a different path, because there is at least a half-wavelength of space between the multiple transmitting antennas. • Multiple streams follow different paths to the receiver because of the space between the transmitting antennas is known as spatial diversity. • Sending multiple independent streams of unique data using spatial diversity is often also referred to as spatial multiplexing (SM) or spatial diversity multiplexing (SDM). Pg 593

14. Spatial Multiplexing • Using spatial multiplexing can greatly increase throughput • Each transmission is a multiplier of speed • IF full transmission is received • You WANT the signals to arrive at different times • Take advantage of multipath Pg 593

15. Spatial Multiplexing • Each stream can use the same, or different modulation techniqus Pg 593

16. MIMO Diversity • Antenna Diversity helps to reduce effect of multipath • Single radio with multiple antennas • MIMO takes advantage of multipath • Multiple radios with own antennas • Radio Chains • Receive Diversity looks for best received signal • Maximal Ratio Combining will look for best signal by adding the received information together Pg 594

17. MIMO Diversity • MRC is best when going from Non-MIMO to MIMO Pg 594

18. Transmit Beamforming (TxBF) • Optional PHY capability in 802.11n • Phased array or smart antenna • Switched array • Fixed beam patterns • Adaptive Array • Maneuvers beam to targeted receiver • Allows transmitter to “focus” signal • Arrange transmissions to create constructive multipath • Transmitter must know details about receiver Pg 595

19. Transmit Beamforming (TxBF) • Emulate a high gain unidirectional antenna • Results in higher throughput • Could be used in conjuction with Spatial Diversity Multiplexing (SDM) • Restricted to situations with matching antennas numbers • Most likely to be used where SDM is not an option Pg 595

20. Transmit Beamforming (TxBF) • Transmitter (beamformer) will use sounding frames to gather information from receiver (beamformee) • Implicit feedback requires the transmitter to analyze the receivers stream • Explicit feedback will have the receiver do some of the thinking as well Pg 595

21. Transmit Beamforming (TxBF) Pg 595

22. HT Channels • OFDM is used in both 2.4 Ghz and 5 Ghz range • Clause 20 radios-HT • 802.11a and g use 20 Mhz OFDM channels • 52 subcarriers with 4 pilot channels • HT can use 20 or 40 Mhz channels • 20 Mhz Channel has 56 subcarriers with 4 being pilots • Slightly higher througput Pg 597

23. HT Channels Pg 597

24. 40 Mhz Channels • Creates 114 subcarriers • Six used for pilot • Effectively doubles throughput • Combines two 20 Mhz channels (bonded) • Primary and secondary channels • Positive is one channel above • Negative is one channel below • Allows use of additional bandwidth • Reserved space at top of primary and bottom of secondary Pg 599

25. 40 Mhz Channels • Works well for 5 Ghz range • Not as well for 2.4 Ghz Pg 599

26. Guard Interval (GI) • Each OFDM Symbol contains 288 bits • 216 of data and 72 error correction • 800 nanosecond Guard interval between symbols is designed to counteract intersymbol interference • Normal delay spread is 50 to 100 nanaoseconds, max of 200 Pg 602

27. Guard Interval (GI) • HT can use 400 nanosecond GI • Increase throughput • Risk of intersymbol interference • Look for retransmissions Pg 602

28. Modulation and Coding • Data rates are defined by modulation and coding scheme (MCS) • Based on modulation, number of spatial streams (antennas) channel size and guard interval • 77 schemes exist • 8 mandatory modulation schemes • Like basic/required rates • Up to 600 mbps • With 400 ns GI,4 spatial streams and 64-QAM Pg 603

29. Modulation and Coding Pg 603

30. HT PHY • The MSDU is data from layer 3-7 • MPDU is MSDU with 802.11 header (layer 2) • With Physical layer preamble and PHY header, this is the PPDU • Preamble is used to synchronize radios • PHY Header gives info about transmitting MPDU • 3 PPDU structures Pg 605

31. HT PHY Pg 605

32. HT PHY • Non-HT Legacy • Same as 802.11a and g formats • HT Mixed • Contains non-HT short and long training symbols so legacy systems can understand • Also has HT symbols • Broadcast traffic must go out on 20 Mhz channels for backward compatibility • HT Greenfield • HT only • optional Pg 605

33. HT MAC • New enhancements to MAC for throughput and power management • Frame aggregation • Power management Pg 607

34. A-MSDU • MSDU aggregation • Send multiple MSDU with single MAC header • Creates new MPDU • Single destination • Must be same 802.11e service access category Pg 607

35. A-MPDU • MPDU aggregation • Send multiple MPDU with single PLCP header • Single PHY preamble and header • Must be same 802.11e service access category • Each MPDU has separate encryption • Less saved overhead Pg 607

36. MTBA and RIFS • Each unicast frame needs acknowledge ment • With A-MPDU, each MPDU would need an ACK • Multiple traffic ID block acknowledgement frame (MTBA) • Similar to the 802.11e ack for frame bursts • RIFS is a new reduced interframe space of only 2 nanoseconds • Only for greenfield Pg 609

37. HT Power management • Basic Power Save • APs will buffer traffic-legacy power save • Spatial Multiplexing Power Save (SM power save) • Power down all but one radio • Static-power down all but one. • Acts like an 802.11a/g station • Tell AP when powered down or up • Dynamic allows power up much faster • AP can trigger the client to wake up with a RTS • Client sends CTS when powered up Pg 610

38. HT Power management • Power Save Multi Poll (PSMP) is an extension of the APSD • Same benefits Pg 610

39. HT Operation • 20, 40 or 20/40 • APs can also support HT and non-HT in same cell • RTS and CTS as well as Phased Coexistence Pg 611

40. 20/40 Channel operation • 20 for legacy • 40 for HT • The HT access point must declare 20 or 20/40 support in the beacon management frame • Client stations must declare 20 or 20/40 in the association or reassociation frames. • Client stations must reassociate when switching between 20 and 20/40 modes. • If 20/40-capable stations transmit by using a single 20 MHz channel, they must transmit on the primary channel and not the secondary channel. Pg 611

41. HT Protection • Mode 0-Greenfield-HT Only-no protection • Mode 1-HT nonmember-All stations are HT • If non-HT client/AP is heard, but not part of BSS • Interference • Mode 2-HT 20 Mhz-all stations must be HT and are with a 20/40 AP • If a 20 MHZ HT stations joins, 40 Mhz will protect to prevent that station from transmitting • Mode 3-HT Mixed-when one or more non-HT stations join an HT service set • 20 or 20/40 Pg 612

42. Dual CTS Protection • When using protection, station will send RTS • AP will send two CTS, one on 20 Mhz and one on 40 Mhz • AP will send two CTS to self • One 20 Mhz and one 40 Mhz Pg 613

43. Phased Coexistence Operation(PCO) • Separate timeslots for 20 and 40 Mhz transmissions • No Protection needed • Could increase jitter-no good for VoWiFi Pg 613

44. Exam Essentials • Define the differences between MIMO and SISO. • Understand that SISO devices use only one radio chain, whereas MIMO systems use multiple radio chains. • Understand spatial multiplexing. • Describe how SM takes advantage of multipath and sends multiple spatial streams resulting in increased throughput. • Explain MIMO diversity. • Be able to explain the differences between simple switched diversity and the advanced diversity used by MIMO antenna systems. Explain the use of maximal ratio combining with MIMO diversity. • Understand transmit beamforming. • Explain how optional transmit beamforming can be used to steer beams in an optimal path toward a receiving radio and the benefts of the beamforming process.

45. Exam Essentials • Understand 20 MHz and 40 MHz channels. • Understand legacy 20 MHz channels, 20 MHz HT channels, and 40 MHz channels and how they use OFDM. Explain why 40 MHz channels work best in the 5 GHz UNII bands. Explain primary and secondary channels. • Explain the guard interval. • Describe how the guard interval compensates for intersymbol interference. Discuss the use of both 800- and 400-nanosecond GIs. • Understand modulation coding schemes. • Explain how modulation coding schemes are used to define data rates and all the variables that can affect the data rates.

46. Exam Essentials • Explain the three HT PPDU formats. • Describe the differences between non-HT legacy, HT Mixed, and HT Greenfeld. • Understand HT MAC enhancements. • Explain how the use of A-MSDU, A-MPDU, block ACKs, and RIFS are used to increase throughput at the MAC sublayer. Define the two new power-management methods used by HT radios. • Explain the HT protection modes. • Describe the differences between protection modes 0–3. Explain the use of Dual-CTS.