CWNA Chapter 18 – 802.11n

My Notes from chapter 18 of the CWNA study guide

802.11n-2009 amendment

  • Defines High Throughput (HT) Clause 20 radios that use multiple-input, multiple-output (MIMO) technology in unison with Orthogonal Frequency Division Multiplexing (OFDM) technology
  • Benefits of using MIMO are increased throughput and even greater range
  • 802.11n radios are backward compatible with legacy 802.11a/b/g radios
  • A dual-frequency 802.11n Wi-Fi radio is usually referred to as an 802.11a/b/g/n radio.
  • It should be noted that the technology defined for use by 802.11n radios is not frequency dependent

Wi-Fi Alliance certification

  • 802.11n products are tested for both mandatory and optional baseline capabilities
  • All certified products must also support both Wi-Fi Multimedia (WMM) quality of service (QoS) mechanisms and WPA/WPA2 security mechanisms
  • Wi-Fi CERTIFIED n devices can operate in both the 2.4 GHz and 5 GHz frequency bands and are also backward compatible with 802.11a/b/g certified devices.

Multiple-Input, Multiple-Output (MIMO)

  • Requires the use of multiple radios and antennas, called radio chains
  • transmit multiple radio signals at the same time to take advantage of multipath.
  • Multipath is a propagation phenomenon that results in two or more paths of the same signal arriving at a receiving antenna at the same time or within nanoseconds of each other.
  • MIMO systems, however, take advantage of multipath and, believe it or not, multipath then becomes your friend.
  • The MIMO receiver will then use advanced digital signal processing (DSP) techniques to sort out the originally transmitted signals.
  • Transmitting multiple streams of data with a method called spatial multiplexing (SM) provides for greater throughput and takes advantage of the old enemy known as multipath.


  • Radio chains
    • Conventional 802.11 radios transmit and receive RF signals by using a single-input single output (SISO) system
    • SISO systems use a single radio chain
    • A radio chain is defined as a single radio and all of its supporting architecture, including mixers, amplifiers, and analogue/ digital converters
    • A MIMO system consists of multiple radio chains, with each radio chain having its own antenna
    • A MIMO system is characterized by the number of transmitters and receivers used by the multiple radio chains
    • or example, a 2Å~3 MIMO system would consist of three radio chains with two transmitters and three receivers. A 3Å~3 MIMO system would use three radio chains with three transmitters and three receivers
    • In a MIMO system, the first number always references the transmitters (TX), and the second number references the receivers (RX).
    • The use of multiple transmitters in a MIMO system provides for the transmission of more data via spatial multiplexing
    • The 802.11n standard allows for MIMO systems up to 4Å~4 using four radio chains
  • Spatial multiplexing (SM)
    • A MIMO radio also has the ability to send independent unique data streams.
    • Each independent data stream is known as a spatial stream, and each unique stream can contain data that is different from the other streams transmitted by one or more of the other radio chains\
    • The fact that the 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).
    • Do not confuse the independent unique streams of data with the number of transmitters.
    • In a MIMO system, the first number always references the transmitters (TX), and the second number references the receivers (RX). The third number represents how many unique streams of data can be sent or received.
    • For example, a 3X:2 MIMO system would use three transmitters and three receivers, but only two unique data streams are utilized.
    • not all 802.11n radios have the same MIMO capabilities
    • If good RF conditions exist, when a 3Å~3:3 access point and a 3X3:3 client device are communicating with each other, three spatial streams can be used for unicast transmissions.
    • However, when a 3X3:3 access point and a 2X2:2 client device are communicating with each other, only two spatial streams will be used for unicast transmissions
    • The 802.11n amendment does allow for the use of up to a 4X4:4 MIMO system
    • Multiple spatial streams can be sent with the same (equal) modulation or they can be sent using different (unequal) modulation
    • Although unequal modulation is theoretically and technically possible, WLAN vendors have never implemented unequal modulation with 802.11n radios
  • MIMO diversity
    • Do you think you would be able to hear more clearly if you had three or four ears instead of just two?
    • MIMO systems employ advanced antenna diversity capabilities that are analogous to having multiple ears.
    • Simple antenna diversity is a method of compensating for multipath as opposed to utilizing multipath.
    • When receive diversity is used, the signals may also be linearly combined by using a signal processing technique called maximal ratio combining (MRC).
    • MRC algorithms are used to combine multiple received signals by looking at each unique signal and optimally combining the signals in a method that is additive as opposed to destructive.
    • MIMO systems using MRC will effectively raise the SNR level of the received signal.
    • MRC uses a receive-combining function that assesses the phase and SNR of each incoming signal.
    • Each received signal is phase-shifted so that they can be combined.
    • The amplitude of the incoming signals is also modified to focus on the signal with the best SNR.
  • Space-time block coding (STBC)
    • Is a method where the same information is transmitted on two or more antennas.
    • It is a type of transmit diversity.
    • can be used when the number of radio chains exceeds the number of spatial streams.
    • STBC does, however, increase the receiver’s ability to detect signals at a lower SNR than would be otherwise possible
    • TBC and cyclic shift diversity (CSD) are transmit diversity techniques where the same transmit data is sent out of multiple antennas
    • STBC communication is possible only between 802.11n devices
  • Cyclic shift diversity (CSD)
    • Is another transmit diversity technique specified in the 802.11n standard
    • CSD diversity signals can be received by either 802.11n or legacy devices.
    • For mixed mode deployments, where 802.11n coexists with 802.11g and 802.11a devices, there is a need to have a way of transmitting the symbols in the legacy OFDM preamble over multiple transmit antennas
    • The cyclic delay is chosen to be within the limits of the guard interval (GI) so that it does not cause excessive intersymbol interference (ISI)
    • An 802.11n system has no problem using the multiple signals to improve the overall SNR of the preamble
  • Transmit beamforming (TxBF)
    • The 802.11n amendment also proposes an optional PHY capability called transmit beamforming (TxBF), which uses phase adjustments.
    • Transmit beamforming can be used when there are more transmitting antennas than there are spatial data streams
    • Transmit beamforming is a method that allows a MIMO transmitter using multiple antennas to adjust the phase and amplitude of the outgoing transmissions in a coordinated method.
    • f the transmitter (TX) knows about the RF characteristic of the receiver’s location, the phase of the multiple signals sent by a MIMO transmitter can be adjusted. When the multiple signals arrive at the receiver, they are in phase, resulting in constructive multipath instead of the destructive multipath caused by out-of-phase signals.
    • Transmit beamforming will also result in higher throughput because of the higher SNR that allows for the use of more complex modulation methods that can encode
    • more data bits.
    • The higher SNR also results in fewer layer 2 retransmissions.
    • Transmit beamforming could be used together with spatial multiplexing (SM)
    • In practice, transmit beamforming will probably be used when spatial multiplexing is not the best option
    • Transmitters that use beamforming will try to adjust the phase of the signals based on feedback from the receiver by using sounding frames.
    • The transmitter is considered the beamformer, while the receiver is considered the beamformee
    • Transmit beamforming relies on implicit feedback or explicit feedback from both the transmitter and receiver
    • With some vendor-specific exceptions, 802.11n transmit beamforming has not been utilized due to the lack of client-side support for the technology
    • Even though transmit beamforming never really caught on with 802.11n radios, it is widely believed that 802.11ac will make use of the technology in the near future.

HT channels

  • 20 MHz non-HT and HT channels
    • 802.11n (HT) radios also use the same OFDM technology and have the capability of using either 20 MHz channels or 40 MHz channels
  • 40 MHz channels
    • 802.11n (HT) radios also have the capability of using 40 MHz OFDM channels
    • The 40 MHz HT channels use 128 OFDM subcarriers; 108 of the subcarriers transmit data, whereas 6 of the subcarriers are used as pilot tones for dynamic calibration between the transmitter and receiver.
    • The 40 MHz channels used by HT radios are essentially two 20 MHz OFDM channels that are bonded together
    • Deploying 40 MHz HT channels at 2.4 GHz unfortunately does not scale well in multiple channel architecture.
  • 40 MHz Intolerant
    • Any 802.11n AP using a 40 MHz channel will be forced to switch back to using only 20 MHz channels if they receive the frames from nearby 802.11n 2.4 GHz stations that are intolerant.
  • Guard interval (GI)
    • Data is modulated onto the carrier signal in bits or collections of bits called symbols
    • 802.11a/g radios use an 800-nanosecond guard interval (GI) between OFDM symbols.
    • In a multipath environment, symbols travel different paths, and therefore some symbols arrive later. A “new” symbol may arrive at a receiver before a “late” symbol has been completely received. This is known as intersymbol interference (ISI) and often results in data corruption.
    • 802.11n also uses an 800-nanosecond guard interval; however, a shorter 400-nanosecond guard interval is optional. A shorter guard interval results in a shorter symbol time, which has the effect of increasing data rates by about 10 percent.
  • Modulation and coding scheme (MCS)
  • HT PHY
    • The 802.11n amendment defines the use of three PPDU structures that use three different preambles. One of the preambles is a legacy format, and two are newly defined HT preamble formats.
  • Non-HT legacy
    • Often also referred to as a legacy format because it was originally defined by Clause 18 of the 802.11-2012 standard for OFDM transmissions.
    • The header contains the signal field, which indicates the time needed to transmit the payload of the non-HT PPDU, which of course is the MPDU (802.11 frame).
    • Support for the non-HT legacy format is mandatory for 802.11n radios, and transmissions can occur in only 20 MHz channels
    • The non-HT format effectively is the same format used by legacy 802.11a and 802.11g radios
  • HT Mixed
    • PPDU formats defined in the 802.11n amendment is the HT Mixed format.
    • the beginning of the preamble contains the non-HT training symbols and legacy signal field that can be decoded by legacy 802.11a and 802.11g radios.
    • The rest of the HT Mixed preamble and header cannot be decoded by legacy 802.11a/g devices
    • The HT Signal (HT-SIG) contains information about the MCS, frame length, 20 MHz or 40 MHz channel size, frame aggregation, guard interval, and STBC. The HT Short
    • Training Field (HT-STF) and HT Long Training Field (HT-LTF) are used for synchronization between MIMO radios
  • HT Greenfield
    • An 802.11n radio in HT Greenfield mode can receive frames from legacy devices; however, legacy devices cannot understand the HT Greenfield preamble.
    • Therefore, any legacy device will interpret an HT Greenfield transmission as noise.


  • A-MSDU
    • Every time a unicast 802.11 frame is transmitted, a certain amount of fixed overhead exists as a result of the PHY header, MAC header, MAC trailer, Interframe spacing, and acknowledgment frame.
    • Medium contention overhead also exists because of the time required when each frame must contend for the medium.
    • Frame aggregation is a method of combining multiple frames into a single frame transmission.
    • The fixed MAC layer overhead is reduced, and overhead caused by the random backoff timer during medium contention is also minimized.
  • A-MPDU
    • Aggregate MAC Protocol Data Unit (A-MPDU).
  • Block Acknowledgment
    • An A-MSDU contains multiple MSDUs all wrapped in a single frame with one MAC header and one destination.
    • Block ACKs were first introduced by the 802.11e amendment as a method of acknowledging multiple individual 802.11 frames during a frame burst
  • RIFS
    • 802.11e QoS amendment introduced the capability for a transmitting radio to send a burst of frames during a transmit opportunity (TXOP).
    • The 802.11n amendment defines a new Interframe space that is even shorter in time, called a reduced Interframe space (RIFS)
    • A RIFS interval can be used in place of a SIFS interval, resulting is less overhead during a frame burst.
    • It should be noted that RIFS intervals can be used only when a Greenfield HT network is in place.
  • HT power management
    • The 802.11e QoS amendment introduced unscheduled automatic power save delivery (U-APSD), which is the mechanism used by WMM Power Save (WMM-PS).
    • The 802.11n power-management mechanisms are meant as supplements to WMM-PS when MIMO radios are used.
    • spatial multiplexing power save (SM power save). The purpose of SM power save is to allow a MIMO 802.11n device to power down all but one of its radios
    • Power Save Multi Poll (PSMP), has also been defined for use by 802.11n (HT) radios. PSMP is an extension of automatic power save delivery (APSD) that was defined by the 802.11e amendment

HT operation

  • 20/40 channel operation
    • 802.11n radios can operate in either a 20 MHz–only channel mode or a 20/40 MHz channel operation mode
    • The HT radios that are 20/40 capable can use 40 MHz transmissions when communicating with each other; however, they would need to use 20 MHz transmissions when communicating with the legacy stations
    • The 802.11n access point must declare 20-only or 20/40 support in the beacon management frame.
    • 802.11n client stations must declare 20-only or 20/40 in the association or reassociation frames.
    • Client stations must reassociate when switching between 20-only 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.
  • HT protection modes (0–3)
    • To ensure backward compatibility with older 802.11a/b/g radios, 802.11n (HT) access  points may signal to other 802.11n stations when to use one of four HT protection modes:
      • Mode 0—Greenfield (No Protection) Mode
        • Only HT radios are in use.
        • All the HT client stations must also have the same operational capabilities.
      • Mode 1—HT Non-member Protection Mode
        • In this mode, all the stations in the BSS must be HT stations. Protection mechanisms kick in when a non-HT client station or non-HT access point is heard that is not a member of the BSS
      • Mode 2—HT 20 MHz Protection Mode
        • In this mode, all the stations in the BSS must be HT stations and are associated to a 20/40 MHz access point.
        • If a 20 MHz–only HT station associates to the 20/40 MHz AP, protection must be used.
      • Mode 3—Non-HT Mixed Mode
        • This protection mode is used when one or more non-HT stations are associated to the HT access point
        • If any 802.11a/b/g radios associate to the BSS, protection will be used.
        • Mode 3 will probably be the most commonly used protection mode because most basic service sets will most likely have legacy 802.11a/b/g devices as members
  • RTS/CTS and CTS-to-self
    • When HT protection is enabled within an HT BSS, an HT STA will precede HT transmissions with either an RTS/CTS control frame exchange or a CTS-to-Self control frame using
    • modulation and coding understandable to the STAs that are being protected against
    • The Duration ID within these control frames causes STAs to update their network allocation vector (NAV).




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