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Capacity, QoS, and Security Related Advances in IEEE 802.11

Capacity, QoS, and Security Related Advances in IEEE 802.11

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Capacity, QoS, and Security Related Advances in IEEE 802.11

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  1. Capacity, QoS, and Security Related Advances in IEEE 802.11 Kaustubh S. Phanse K. N. Gopinath AirTight Networks, Inc. National Conference on Communications (NCC 2008) Indian Institute of Technology, Bombay February 1, 2008 www.airtightnetworks.net

  2. Outline • Introduction: 802.11 overview: history and basic concepts • 802.11n: MIMO concepts, channelization, frame aggregation, frame formats, performance • 802.11e: Coordination functions for QoS support, service classes • 802.11i, 802.11w: Authentication and encryption; protection of management and broadcast frames • What this tutorial will NOT cover… • Communication and information theory: modulation and demodulation techniques, estimation, … • Details of certain optional features in 802.11 standards AirTight Networks

  3. IEEE 802.11 • Working group established in 1990 • First standard in 1997 (already 10 years ago!) • Frequency: 2.4 GHz band • Physical layer: DSSS, FH, IR • MAC layer: CSMA/CA • Data rate: 2 Mbps AirTight Networks

  4. 802.11 protocol suite AirTight Networks

  5. 802.11 MAC and PHY enhancements 802.11i Security QoS 802.11e MAC Data link 802.11w 802.11n Capacity & Coverage PLCP 802.11n Physical PMD AirTight Networks

  6. Two-slide primer on 802.11 MAC (1/2) • Distributed coordination function (DCF) using carrier sense • multiple access (CSMA/CA) AirTight Networks

  7. Two-slide primer on 802.11 MAC (2/2) AirTight Networks

  8. Example of DCF CSMA/CA (1) AirTight Networks

  9. Example of DCF CSMA/CA (2) AirTight Networks

  10. Example of DCF CSMA/CA (3) AirTight Networks

  11. Example of DCF CSMA/CA (4) AirTight Networks

  12. Example of DCF CSMA/CA (5) AirTight Networks

  13. Example of DCF CSMA/CA (6) AirTight Networks

  14. Example of DCF CSMA/CA (7) AirTight Networks

  15. Motivation for multicarrier modulation • Large delay spread (due to multipath reception) can cause • significant inter-symbol interference (ISI) • Burst errors • Limits maximum achievable data rate τ τ AirTight Networks

  16. Multicarrier modulation • Divide a high-rate sequence of symbols into several low-rate • sequences • Symbol duration (TN) becomes large • Transmit low-rate symbols simultaneously over multiple sub- • channels or subcarriers • Total bandwidth B is divided into subchannels each with bandwidth B/N AirTight Networks

  17. Orthogonal frequency division multiplexing (OFDM) • Tighter packing of subcarriers than traditional FDM • Subcarriers are orthogonal to enable demodulation • Spacing ∆f is at least 1/TN AirTight Networks

  18. OFDM in 802.11 • Each 20 MHz channel divided into 52 subcarriers • Bandwidth of 16.6 MHz actually used for transmission • Subcarriers spaced 312.5 KHz • 48 subcarriers for data transmission • 4 pilot subcarriers for monitoring AirTight Networks

  19. 802.11n PHY Enhancements AirTight Networks

  20. What is MIMO? • SISO: Single Input (transmit) Single Output (receive) • MIMO: Multiple Input Multiple Output • Spatial diversity (transmitter and receiver) • Spatial multiplexing Tx Rx Tx Rx M x N system (N >1, M>1) AirTight Networks

  21. Spatial diversity • Use multiple independently fading signal paths to reduce the error • probability • Low probability of independent fading signal paths to simultaneously experience deep fades • Need multiple antennas spaced sufficiently apart (~ λ/2) • Maximum diversity gain (D) for M x N system = MN AirTight Networks

  22. Receiver diversity r1ejθ1s(t) r2ejθ2s(t) r3ejθ3s(t) rMejθMs(t) • Let noise at each antenna = N0 Combined output SNR ηΣ = a1e-jθ1 aNe-jθM a2e-jθ2 a3e-jθ3 x x x x Σ Combiner Output SNR = ηΣ AirTight Networks

  23. Receiver diversity: Selection combining • Choose the branch with the highest SNR ηΣ = ηk = • Often implemented as a single receiver that switches to the chosen antenna branch • But it is still a single transmit-receive chain (SISO) Bit stream Bit stream DSP DSP Radio Radio Tx Rx AirTight Networks

  24. Radio Radio Bit stream DSP DSP Bit stream Radio Radio Tx Rx Receiver diversity: Maximum Ratio Combining (MRC) • Give higher weights to branches with high SNR and lower weights • to branches with low SNR AirTight Networks

  25. Receiver diversity: MRC • Optimal weight ak = • rk is the energy per symbol = • Then, SNR = • Combined received SNR ηΣ = • Array gain: M-fold increase in SNR versus a SISO system • Maximum array gain (A) for M x N system = MN AirTight Networks

  26. Transmitter diversity: Channel-aware • Transmitter has knowledge of channel state information (CSI) • Feedback from receiver • Assume channel is reciprocal • Similar to receiver diversity with coherent combining, e.g., MRC • Assign weights to antenna branches depending on channel conditions AirTight Networks

  27. Transmitter diversity: Channel-unaware • Space-time block codes (STBC): Alamouti scheme • Assume channel gain is constant over two symbol periods • Transmit symbols s1 and s2 during first symbol period • Transmit -s2* and s1* during next symbol period • Let each antenna have a channel gain hk = rkejθk • Received signal is r(t) = • Symbol received during first symbol period y1 = h1s1 + h2s2 • Symbol received second symbol period y2 = -h1s2* + h2s1* AirTight Networks

  28. Transmitter diversity: Alamouti scheme • Let sequence of received symbols be represented as a vector • y = [y1 y2*]T • y = = Hs • Let z = HHy = HHHs = (|h12| + |h22|)I2s • Then • z1 = h1*z1 + h2z2 = (|h12| + |h22|)s1 • z2 = h2*z1 – h1z2 = (|h12| + |h22|)s2 AirTight Networks

  29. Transmitter diversity: Alamouti scheme • Received SNR ηk for zk= • Total SNR ηΣ= • Array gain = 1 • Diversity gain = 2 AirTight Networks

  30. Practical significance: array gain and diversity gain • Maximum: array gain A = MN, diversity gain D = MN • For a Rayleigh channel: error probability (Pe) α • For M x N system,Peα Diversity gain Array gain AirTight Networks

  31. Practical significance: array gain and diversity gain Pe Diversity gain determines the slope of the curve Array gain shifts the curve SNR AirTight Networks

  32. Spatial multiplexing • Multiplexing • Time (TDM), frequency (FDM), code (CDM) • SDM: using space as another dimension to multiplex data • Degrees of freedom • Rich scattering environment • Transmit unique data streams over separate RF chains AirTight Networks

  33. b1 b1 b3 b3 b5 b5 b2 b2 b4 b4 b6 b6 b1 b1 b2 b2 b3 b3 b4 b4 b5 b5 b6 b6 Spatial multiplexing • Maximum multiplexing gain = min (M,N) • Use training symbols to estimate channel matrix H • Linear systems theory analogy: min (M,N) variables with min (M,N) • equations Radio Radio Split Merge DSP DSP Radio Radio Rx Tx AirTight Networks

  34. Spatial multiplexing gain vs. diversity gain trade-off 0, MN 1, (M-1)(N-1) Diversity gain 2, (M-2)(N-2) k, (M-k)(N-k) Min(M, N), 0 Spatial multiplexing gain AirTight Networks

  35. 802.11n channels • 40 MHz operation (channel bonding) • Primary channel plus secondary (upper/lower) channel • Primary for management frames, both channels for data frames • Higher bandwidth, higher data rates! • …but higher interference • Only one non-overlapping channel in 2.4 GHz • Implications for legacy WLANs AirTight Networks

  36. 802.11n Modes of Operation PLCP Enhancements AirTight Networks

  37. 802.11n: Modes of Operation • 3 Modes: Non-HT, Mixed, Greenfield (distinguished by their PLCP headers) • Mixed • Full support for legacy clients • Broadcast control frames always in 20 Mhz • Perf degradation for .11n stations • Greenfield • No backward compatibility • Short & more efficient PLCP format • No performance degradation for .11n devices MIMO estimation: D-LTF 1 per stream providing channel estimation for data portion of the frame Detection of PPDU, timing & coarse freq acquisition Staggered preambles (e.g., sounding packets) Additional optional estimation info for channels For use of legacy devices also Signalling (See next slide) AirTight Networks

  38. L-SIG (MM) & HT-SIG (MM & GF) Encoded value indicating Duration of rest of the packet Always 6 Mbps L-SIG of Mixed Mode Refer to next slides AirTight Networks

  39. HT-SIG AirTight Networks

  40. HT-SIG AirTight Networks

  41. Modulation & Coding Scheme (MCS) • MCS is a compact representation (index) indicating • Modulation (BPSK, QPSK, QAM,…) • Coding (1/2, ¾,…) • Number of Spatial Streams (1,2,3,4) • MCS index can be from 0 to 127 • Mandatory MCS • MCS 0 to 15 at 20 Mhz (at AP) • MCS 0 to 7 at 20 Mhz (at client STA) • Rest all optional • MCS 16 to 76 are optional • All MCS at 40 Mhz • MCS 77 to 127 are reserved for future use AirTight Networks

  42. Rate Dependent Parameters (20 MHz and Mandatory MCS) NSS = 1 NSS = 2 AirTight Networks

  43. Rate Dependent Parameters (40 Mhz & Mandatory MCS) NSS = 1 NSS = 2 AirTight Networks

  44. Other Optional MCSs • Other MCSs • HT Duplicate • MCS 32 • Useful under very high noise • Lowest rate of 40 Mhz (bpsk) • 6.7 Mbps max rate • MCSs with unequal modulation • Use with • Tx beamforming • STBC • MCS 33 – 38 (4 SS) • Max rate 495 Mbps • MCS 39 – 52 (4 SS) • Max rate 495 Mbps • MCS 53 – 76 (4 SS) • Max rate 495 Mbps • MCSs with SS=3 • MCS 16 – 23 • Max rate (MCS 23) • 216.7 Mbps (20 Mhz) • 450 Mbps (40 Mhz) • MCSs with SS=4 • MCS 24 – 31 • Max rate (MCS 23) • 288.9 Mbps (20 Mhz) • 600 Mbps (40 Mhz) AirTight Networks

  45. MAC Enhancements AirTight Networks

  46. Frame Aggregation AirTight Networks

  47. Motivation DCF PLCP MPDU1 PLCP ACK DCF PLCP MPDU2 PLCP ACK • Amortize PLCP, MAC overheads by sending bigger packets • Can be implemented in several ways (as discussed next) SIFS DCF PLCP MPDU PLCP ACK AirTight Networks

  48. Physical Level Aggregation (A-MPDU) • Consists of several MPDUs addressed to the same receiver • Identified by the HT SIG PLCP field ‘Aggregation’ of a received packet • Each MPDU embedded in a subframe • Subframes consists of a delimiter followed by an MPDU (and padding in some cases) • Except last subframe, others are padded so that they are multiple of 4 byte octet • Delimiter • Delimiters (ASCII N) useful for recovery during errors • CRC protects reserved and length fields • When an invalid Delimiter is obtained, de-aggregation process skips forward 4 bytes and restarts its search for a new MPDU AirTight Networks

  49. Physical Level Aggregation (A-MPDU) • Parameters negotiated using “A-MPDU parameters set” of HT capabilities IE field in a mgmt frame • Max length (64k is the limit) • Min MPDU start spacing • 0 indicates no restriction • Else, ranges from 1/4 to 16 usecs • Realized by using Delimiters with MPDU length 0 • Can be limited by a station using its Assoc packet • Examples frames that an A-MPDU can contain • QoS data frames • Block ack • Block ACK req frames • Action management frames of subtype “Action No ACK” (e.g., carrying MIMO info) Max Rx Factor(x): 0 to 3 [2^13+x] Min spacing: 0.25 to 16 usecs AirTight Networks

  50. A-MSDU • A-MSDU consists of multiple subframes • All MSDUs are intended to be received by the same receiver • A-MSDU of length is 4095 – QoS data overheads = 4065 bytes cannot be Tx in an A-MPDU (as A-MPDU cannot carry fragments) AirTight Networks