High Throughput (HT) and 802.11n Module-10B

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High Throughput (HT) and 802.11nModule-10B

• Jerry Bernardini
• Community College of Rhode Island

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Presentation Reference Material

• CWNA Certified Wireless Network Administration
  Official Study Guide
• (PWO-104), David Coleman, David Westcott,
  2009, Chapter-18
• 802.11n Demystified Companion Guide, Xirrus Inc
• USING MIMO-OFDM TECHNOLOGY TO BOOST WIRELESS LAN
  PERFORMANCE TODAY - DATACOMM RESEARCH COMPANY

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802.11 Summary Characteristics
Protocol Release Date Op. Frequency Throughput (Typ) Data Rate (Max) Modulation Technique Range (Radius Indoor) Depends, and type of walls Range (Radius Outdoor) Loss includes one wall
802.11a 1999 5 GHz 23 Mbps 54 Mbps OFDM 35 Meters 120 Meters
802.11b 1999 2.4 GHz 4.3 Mbps 11 Mbps DSSS -CCK 38 Meters 140 Meters
802.11g 2003 2.4 GHz 19 Mbps 54 Mbps OFDM DSSS 38 Meters 140 Meters
802.11n June 2009(est.) 2.4 GHz5 GHz 74 Mbps 248 Mbps OFDM MIMO 70 Meters 250 Meters
CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output CCK-Complementary Code Keying OFDM-Orthogonal Frequency Division Multiplexing DSSS-Direct Sequence Spread Spectrum MIMO-Multi-Input Multi-Output
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802.11n Requirements

• Backward compatible with 802.11abg
• Higher throughput than 802.11abg
• Mixed mode operation

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Review 802.11g Protection

• Before an 802.11g client can transmit to an
  802.11g AP it must reserve the medium.
• Must transmit so 802.11b will understand.
• Two Protection Methods
• CTS-to self at 802.11b modulation (slow Clear to
  Send)
• RTS-CTS at 802.11b modulation
• CTS-to-self is more efficient but may not be
  seen by hidden-node
• RTS-CTS is more reliable but has more overhead
• Both Methods dramatically reduce the 802.11g
  throughput

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802.11b/g Mixed Mode Operation
AP 802.11g
Station802.11g
Station802.11g
Station802.11b
1-Slow CTS
2-Slow CTS
2-Slow CTS
3-Fast Data
3-Fast Data
3-Fast Data
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Protection Throughput Effect
Technology Transactions per second Mbps of TCP payload throughput Transactional speed relative to 802.11b
11b, 11 Mbps 479 5.6 1.0
11a, 54 Mbps 2,336 27.3 4.9
11g, 54 Mbps/no protection 2,336 27.3 4.9
11g, 54 Mbps/CTS-to-self protection 1,113 13.0 2.3
11g, 54 Mbps/RTS/CTS protection 750 8.8 1.6
Based on Matthew Gast, 802.11 Wireless Networks The Definitive Guide Based on Matthew Gast, 802.11 Wireless Networks The Definitive Guide Based on Matthew Gast, 802.11 Wireless Networks The Definitive Guide Based on Matthew Gast, 802.11 Wireless Networks The Definitive Guide
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802.11g Conclusions

• 802.11g is significantly faster then 802.11b for
  all conditions
• 802.11b station associating with a 802.11g
  network drops throughput due to protection
• 802.11b station does not have to be active to
  reduce throughput (just associated )
• Mixed 802.11b/g deployments are likely to be
  common for the foreseeable future
• Mixed 802.11b/a deployments will have higher
  throughput
• 802.11b/g/n will also have to provide protection

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802.11n History

• In 2004 IEEE 802.11 Group-n formed to improve
  802.11 standards
• 2009 802.11n draft
• Main objectives
• Increase data rates and throughput
• Operate in 2.4 GHz and 5 GHz bands
• 802.11n Draft defines High Throughput (HT)
• Defines PHY and MAC enhancements
• Can provide data rates up to 600Mbps
• Wi-Fi Alliance Certification

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802.11n Draft Amendment

• Defines HT
• Uses Multiple-input Multiple-output (MIMO)
• OFDM
• MAC layer enhancements
• Backward compatible to 802.11abg

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Wi-Fi Alliance Certification802.11n

• Most vendors say draft 2.0 software can be
  upgraded to 802.11n final
• Two spatial stream support-mandatory
• Two spatial receive stream support-mandatory
• A-MPDU and A-MSDU support- mandatory
• Block ACK support-mandatory
• Dual Band support-optional
• 40 MHz band support-optional
• Greenfield support optional
• Short guard interval-optional
• Concurrent 2.4 GHz and 5Ghz--optional

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MIMO

• Multiple-input Multiple-output (MIMO)
• Takes advantage of multipath
• Multiple radios and antennas

Transmitter x Receiver MIMO Tx Rx MIMO
2 3 2x3 3 3 3x3 4
4 4x4
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Antenna Beamforming and Diversity
Beamforming (beam steering) employs two transmit
antennas to deliver the best multipath signal
Diversity (receive combining) uses two receive
antennas to capture the best multipath signal
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Multi-Antenna Systems not the Same
Multi-antennas beam steering/diversity approach,
only one signal is sent over the channel.
MIMO uses multiple transmitters, receivers and
antennas to send multiple signals over the same
channel, multiplying spectral efficiency.
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MIMO and Multi-path

• Normally when a signal is transmitted from A to B
  the signal will reach the receiving antenna via
  multiple paths, causing interference.
• MIMO uses this multipath propagation to increase
  the data rate by using a technique known as
  spatial division multiplexing.
• The data is split into a number of spatial
  streams and these are transmitted through
  separate antennas to corresponding antennas at
  the receiver.
• Doubling the number of spatial streams doubles
  the raw data rate, enabling a far greater
  utilization of the available bandwidth.
• The current 802.11n standard allows for up to
  four spatial streams.

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Spatial Multiplexing (SM or SDM)

• MIMO employs multiple independent radio
  transmitter-receiver pairs
• Radio pairs send independent signals
• Transmitter antennas spaced by half-wave length
  or more insuring different paths
• Each independent signal is a spatial stream
• Spatial streams are combined at the access point
• Referred to as
• Spatial Multiplexing (SM) or
• Spatial Diversity Multiplexing (SDM)

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MIMO Diversity

• Increasing the number of receiving antennas can
  improve overall signal to noise
• Pre-802.11n used switched diversity to select
  best multipath signal
• Increasing antennas (3 or 4) increases the
  receiver choice for a good signal
• MIMO maximal ratio combining (MRC) allows for
  additive effective of multipath signals
  increasing signal to noise ratio

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Transmit Beamforming (TxBF)

• 802.11n optional feature
• Multiple transmitter antennas focus the signal
  to a receiver
• Used by radar phased-array antenna systems
• Transmitter is the beamformer
• Receiver is the beamformee
• Feedback from the beamformee allows the
  beamformer to adjust the antennas and signal to
  improve SNR

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802.11n HT Channel Technology

• 802.11n uses OFDM (just as 802.11ag)
• 802.11n has option to use 20 MHz and 40 MHz
  channels
• 802.11n can use can combine channels for Channel
  Bonding
• 802.11n can use variable Guard Interval (GI)
• 802.11n can use various Modulation and Coding
  Schemes (MCS)

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Non-HT and HT Channels (clause 20)

• 802.11ag use 20 MHz OFDM channels
• Each channel are made of 52 subcarriers
• 48-subcarriers transmit data
• 4-subcarriers transmit pilot tones for
  transmitter-receiver calibrations
• 802.11n can use either 20 MHz or 40 MHz channels
• Each HT 20 MHz channel has 56 subcarriers
• 52-subcarriers transmit data
• 4-subcarriers transmit pilot tones for
  transmitter-receiver calibrations
• Each HT 40 MHz channel has 114 subcarriers
• 108-subcarriers transmit data
• 6-subcarriers transmit pilot tones for
  transmitter-receiver calibrations

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Channel Bonding

• 40 MHz channels are formed by bonding two 20MHz
  channels
• When bonding two channels there no need for a
  guard band
• 5 GHz UNNI band allows twenty three 20 MHz
  channels to be bonded
• 2.4 GHz ISM band allows only one bonding of two
  20 MHz channels (only 3 non-overlapping channels)

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Channel Bonding
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Guard Interval (GI)

• Digital Symbol is a collection of bits
• If the bits overlap Inter-symbol Interference
  (ISI) is experienced
• 802.11ag uses a 800 ns guard interval between
  symbols
• 802.11n can use a 800 ns or 400 ns guard interval
  between symbols
• 400 ns GI improves throughput by 10
• The 400 ns GI should only be used in a good RF
  environment

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Modulation and Coding Schemes (MCS)

• 802.11n defines data rates as Modulation and
  Coding Schemes (MCS)
• MCS are based upon
• Modulation technique (BPSK, QPSK, 16-QAM, 64-QAM)
• Spatial streams (1, 4)
• Channel size (20 MHz, 40 MHz)
• Guard Interval (400 ns, 800 ns)
• 802.11n requires Eight mandatory 20 MHz MCSs
• Total of 78 MCSs
• Data rates vary from 6.5 Mbps to 600 Mbps

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HT PHY and MPDU

• 802.11 frame is a MAC Protocol Data Unit (MPDU)
• The payload is the MAC service Unit (MSDU) (layer
  7-3 data)
• MPDU is made up of the header and body
• At the PHY layer is the Physical Layer Protocol
  Data Unit (PPDU)
• PPDU MPDU PHY preamble-header
• 802.11n defines three PHY preamble-headers
• Legacy format, HT Mixed, HT Greenfield

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HT PPDU Formats

• Non-HT Legacy
• Mandatory for 802.11n
• Only 20 MHz channels
• Same format as 802.11ag
• HT Mixed
• Two part preamble
• First part can be decoded by 802.11ag
• Second part can not be decoded by 802.11ag
• HT Greenfield
• Preamble can not be decoded by 802.11ag
• Can use both 20 MHz and 40 MHz channels

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HT MAC
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HT Operation

• 20/40 Channel Operation
• Legacy 802.11 communication using 20 MHz only
• 802.11n can use 20MHz or 40 MHz
• HT protection Modes
• Four modes
• Mode 0, 1, 2, 3
• Dual-CTS protection
• Send both legacy and HT RTS/CTS combinations
• Phased Coexistence Operation (PCO)
• Time slices between 20MHz or 40 MHz channel usage

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Through Put Comparison