CWNA Chapter 19 – Very High Throughput (VHT) and 802.11ac

My Notes from chapter 19 of the CWNA study guide

802.11ac-2013 amendment

  • 802.11ac technology does not operate in the 2.4 GHz ISM band, only the 5 GHz U-NII bands.

5 GHz only

  • 802.11ac expands channel widths even further than 802.11n, with channel widths of 80 MHz and 160 MHz.
  • Due to the limited frequency space in the 2.4 GHz band, 802.11ac is designed to operate only in the 5 GHz band where much more frequency space is available.

20, 40, 80, and 160 MHz channels

  • 802.11ac introduced two new channel widths: 80 MHz and 160 MHz
  • 40 MHz channel is created by combining two 20 MHz channels, an 80 MHz channel combines two 40 MHz channels
  • The two 40 MHz channels that make up the 80 MHz channel must be adjacent.
  • the 160 MHz channel is made up of two 80 MHz channels; however, the two 80 MHz channels do not have to be adjacent
  • If the channels are adjacent, then it is referred to as a 160 MHz channel.
  • If they are not adjacent, then it is referred to as an 80+80 MHz channel.
  • With 802.11ac, a new feature has been added that allows the AP to choose the channel width on a per-frame basis. This feature is known as dynamic bandwidth operation.

256-QAM modulation

  • 256-QAM is an evolutionary upgrade that was introduced with 802.11ac
  • 256-QAM identifies 256 unique values, using 16 different levels of phase shift and 16 different levels of amplitude shift.
  • Because there are 256 distinct values, each value is able to represent 8 bits
  • 256-QAM is more sensitive to noise and interference. Because of this, 802.11ac receiver performance requires about 5 dB of additional gain as compared to 64-QAM
  • 256-QAM is used for the highest modulation coding sets. To achieve these higher data rates, higher signal-to-noise ratios are needed

Modulation and coding schemes

  • 802.11n (HT) defined 77 different modulation and coding schemes (MCSs)
  • 802.11ac simplified this by defining only 10 MCS options
  • The first eight modulation and coding schemes are mandatory; however, most vendors will support the last two, which provide 256-QAM modulation

Single-user MIMO

  • To distinguish the MIMO technology that was introduced with 802.11n from MU-MIMO, we will refer to it as single-user MIMO (SU-MIMO)
  • The 802.11ac amendment doubles the total number of supported spatial streams to eight.

802.11ac data rates

  • The first enhancement toward the increased data rates of 802.11ac is 256-QAM
  • The second enhancement is  the increase in data rates is the increase in the number of spatial streams
  • Third is the channel width


  • A-MPDU
    • All 802.11ac frames are transmitted using the Aggregate MAC Protocol Data Unit (A-MPDU) frame format, even if only a single frame is being transmitted
    • Aggregation also shifts some of the frame information from the Physical Layer Convergence Protocol (PLCP) header to the MPDU header.
    • Since PLCP information is transmitted at the lowest supported data rate, and the MPDU information is transmitted at the higher data rates, this will improve performance
    • The higher transmission speeds of 802.11ac make Reduced Interframe Space (RIFS) obsolete
    • An A-MPDU frame reduces the per-frame overhead and only requires a single block ACK
    • 802.11ac can use RTS/CTS to perform dynamic bandwidth operations
    • Uses it to dynamically change channel width


  • Explicit beamforming
    • 802.11ac only uses explicit beamforming and requires support by both the transmitter and receiver in order for beamforming to be used
    • To begin the process, the beamformer transmits a null data packet (NDP) announcement frame, which notifies the beamformee of the intent to send a beamformed transmission.
    • The beamformer then follows this with an NDP frame
    • The beamformee processes each OFDM subcarrier and creates feedback information. The feedback contains information regarding power and the phase shift between each pair of transmit and receive antennas
    • The beamformer uses the feedback matrix to calculate a steering matrix that is used to direct the data transmission to the beamformee.
  • Multiuser MIMO
    • The 802.11ac technology that changes one of the fundamental concepts of wireless networks, multiuser MIMO (MU-MIMO).
    • Up until now, an 802.11 AP was only able to communicate with one device at a time.
    • With 802.11ac, it is possible to communicate with up to four devices
    • The goal of MU-MIMO is to use as many spatial streams as possible, whether the transmission is with one client using four spatial streams or with four clients using one spatial stream each.
    • Due to the advanced signal processing that is required, MU-MIMO is only supported for downstream transmission from an AP to multiple clients
    • Beamforming is a critical part of MU-MIMO
  • Multiuser beamforming
    • With MU-MIMO, the task of beamforming is not just performed for transmitting to a single client, it’s performed for transmitting to up to four clients at a time.
    • To begin the MU-MIMO beamforming process, the AP performs a channel sounding procedure, similar but more complex than with SU-MIMO
    • the AP transmits a null data packet (NDP) announcement frame, notifying multiple beamformees of the intent to send a beamformed transmission
    • As with beamforming to a single user, each beamformee processes each OFDM subcarrier and creates feedback information, creating a compressed feedback matrix
    • The first beamformee responds to the AP with its compressed feedback matrix.
    • The AP then polls each additional beamformee sequentially using Beamforming Report Poll frames
    • The AP then uses the feedback matrix from each of the beamformees to create a single steering matrix
    • the AP is sending 16 transmissions, 4 from each antenna. Of those 16 transmissions, the receiving antenna needs to be able to distinguish and interpret the signal that is directed toward it while trying to ignore the
    • other 12 transmissions.
    • Beamformees that are too close to each other could experience inter-user interference from signals directed toward other users
    • After the AP transmits the multiuser frame, the client stations must each acknowledge its frame
    • The AP keeps track of the capabilities of each client and goes through the beamforming and block acknowledgment processes for each transmission that it performs
    • The AP will periodically need to update its steering matrix in order to redirect or refocus the signal to the new location of a moving beamformee
  • Quality of service
    • With the implementation of MU-MIMO, the implementation of the queuing and transmission of QoS frames is handled differently than in single-user wireless environments
    • The AP takes the fi st AC_VO frame for Station 1 and begins to construct a multiuser frame.
    • The AP takes frames for the other stations and adds them to the multiuser frame, providing that the stations are spatially distinct and the frames are shorter than the primary frame.
    • These other frames can be from the primary or secondary access category. Any shorter frames are padded
    • After the multiuser frame is transmitted to the three stations, block ACKs are used to confirm their successful transmission


Infrastructure requirements

  • Ethernet
    • With the transition to the second phase of 802.11ac, the data throughput of the wireless network may exceed Gigabit Ethernet speeds.
    • So, if second phase 802.11ac is capable of data rates of up to 1.3 Gbps, what options are available for providing this throughput on the distribution system?
      • 10Gig copper
      • 10 Gig Fibre
      • 2 x 1Gig Cooper
      • mGig (NBaseT)
  • Power
    • The greater the performance of the AP, the more likely that additional power will be needed.
    • The industry is transitioning to PoE+.
    • Upgrading the network to provide 802.3at (PoE+) power of 30 watts is highly recommended and most likely will become a necessity.

802.11ac in a SOHO or home

  • Device radios
    • One of the first things to determine is whether the client devices that you will be using support 802.11ac
    • If you do plan to upgrade, you will need to purchase a dual-radio AP to continue to provide support for older 2.4 GHz devices
  • Data flow/usage
    • Unless you have an unusual network configuration, your communications will be either between your client device and the Internet or between your client device and some type of server within your network
    • If communications with the Internet, it is not likely that your Internet connection is fast enough to support the throughput that an 802.11ac AP can provide.
    • If it can, you will need to ensure that the entire data path between the AP and the server supports at least 1 Gbps Ethernet.
  • Spatial streams
    • Many personal mobile devices do not support multiple spatial streams due to the power requirements.
    • You will need to consider how your faster devices are affected and whether the performance you achieve is worth the upgrade costs
  • Wider 802.11ac channels
    • With channel widths up to 80 MHz and 160 MHz with the second phase of 802.11ac, the wider channel width is a technology that all 802.11ac devices can benefit from
    • MU-MIMO does have the potential of improving performance in the SOHO or home environment
    • With MU-MIMO, the AP can transmit to up to four devices simultaneously
    • You need to remember that this can only occur if the devices are spatially separated, since beamforming will not work properly if two devices are near each other

Wi-Fi Alliance certification

  • Prior to the ratification of the 802.11ac amendment, the Wi-Fi Alliance published its vendor certification program for 802.11ac, Wi-Fi CERTIFIED ac.
  • Wi-Fi CERTIFIED ac products must support both Wi-Fi Multimedia (WMM) quality-of-service mechanisms and WPA2/WPA2 security mechanisms
  • They only need to be backward compatible with 5 GHz 802.11a/n certified devices



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