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HT/VHT – #CWAP – Exam specific revision

CWAP-403 covers this topic under the 802.11 Frame Exchanges section. I’ve found this topic a bit hard to grasp. I have attempted to read the CWAP-403 study guide but honestly I could not get closer to fully grasp the topic. Much of the explanation in that book is direct copy/paste from standards documents. There is not much of attempt made to break it down and help us understand the topic better. I’ve attempted to find certain whitepapers to get deeper understanding. Hope you find it useful.

1.0 Analyse HT/VHT-specific transmission methods
1.0.1 MIMO
1.0.2 Transmit Beamforming (TxBF)
1.0.3 MU-MIMO
1.0.4 Frame aggregation (A-MSDU and A-MPDU)


  • Introduced in 802.11n & also used in 802.11ac wave 1.
  • Allows AP to multiply throughput with the use of multiple antenna.
  • AP send traffic to one client at a time and the airtime is shared between the clients.
  • When a device reports 3×3:3 MIMO, it has 3 transmit chains, three receive chains and 3 spatial steams in that order.
  • TxBF – Allows MIMO Tx (transmitter) using multiple antenna to focus the transmission on best Rx (receiver)
  • STBC – Space-Time Block Code – Technique to improve the reliability of the data transfer by transmitting redundant copies of the data stream from different antennas.
  • Spatial Multiplexing – Sending multiple independent streams of unique data using spatial diversity

Transmit Beam Forming (TxBF)

  • Use of multiple antenna to transmit a signal strategically with varying phases thereby increasing the overall throughput towards the receiver
  • The increased power improves SNR and data rates to those receiver devices.
  • TxBF is most effective for medium range transmissions. At short range, there is enough power to support max data rates. Beamforming helps overcome the problem by extending the range and so improving the data rates.
  • Beamforming uses multiple antenna arrays to change the transmission pattern of the AP on the fly, per frame basis.
  • Device transmitting the frames is called beamformer, the one receiving it is called beamformee. Both AP/Client STA can be the beamformer/beamformee depending on the points of conversation.

TxBF Explained

  • AP communicating with a client laptop.
  • AP begins exchanging frames to measure the channel.
  • Channel measurement is used to derive the “steering matrix” which determines how to direct the transmission to the receiver.
  • Once this process is completed, AP is now the beamformer and begins transmission.
  • After the transmission is completed, laptop acknowledges the frames which makes it beamformer and AP the beamformee.

Null Data Packet (NDP Sounding) Beamforming

  • Before 802.11n, all beamforming techniques were proprietary resulting in lower usage.
  • In 802.11ac, IEEE mandated NDP Sounding as the beamforming to be used for explicit feedback.
  • Lot of factors come into play for steering the beams, hence channel calibration procedures (sounding) must be determined
  • 802.11ac use multi-carrier OFDM, the analysis allow weak paths to be avoided and strong paths to be taken advantage of.
  • STEP 1: Transmitter (Typically AP), sends NDP announcement frame with the AP and the target recipients.
  • STEP 2: The transmitter sends NDP to the target recipients.
  • STEP 3: Each target receiver uses the preamble in NDP to measure the RF channel properties and returns the measurements as a compressed beamforming steering matrix to the transmitter.
  • STEP 4: The transmitter uses the data from all the recipients
  • STEP 5: The beamformee analyses the training fields in the NDP and calculates the feedback matrix.
  • STEP 6: The beamformer receives the feedback matrix and calculate steering matrix to direct transmissions toward the beamformee in a CBF (compressed beamforming frame)
  • The NDP generally does not show up in pcap because it only has PLCP preamble and does not have a mac header. Packets of this nature are not decodable by sniffer tools.


  • Access Points which are capable of simultaneously transmitting data to multiple groups of devices.
  • 802.11ac standard specifies that up to 4 different groups can be formed by the AP during a single transmission.
  • An elaborate version SU-MIMO channel sounding process is used to achieve the beamforming for MU-MIMO transmissions.
  • The key distinction, The MU-MIMO beamformer requires a response from all beamformees in order to conclude channel sounding.
  • Each client sends response packets along with channel state information in form of feedback matrix as discussed before.
  • The beamformer uses the feedback matrix to form a steering matrix for the beamformees.
Machine generated alternative text:
Single-User MIMO 
one client at a time 
Access Point 
Multi-User MIMO 
Service multiple clients at the 
Access pent 
same time 
Stream 2

CBF (Compressed Beamforming Feedback)

  • 802.11 action frame which contains channel matrix that specifies the channel state information for each client. The CBF is the largest contributor to the overhead caused by MU-MIMO transmission and its size is determined by 3 factors.
  • Channel Width, Number of radio chain pairs & Bit count of each CSI unit.

Spatial Multiplexing

  • Spatial Division Multiplexing (SDM) was first introduced with 802.11n, became Spatial Division Multiple Access (SDMA) with 802.11ac (MU-MIMO).
  • In Spatial Multiplexing same information is placed across two or more available antenna in an AP/client STA.
  • There is no channel sounding procedure that takes place in order to determine optimal phase of spatial streams.

Frame Aggregation (A-MSDU and A-MPDU)

  • Was introduced for improvements in QoS transmissions in 802.11e. Used in HT/VHT transmissions as well. First seen in 802.11n transmissions.
  • Increases/improves throughput by sending multiple MSDU in a single transmission.
  • The reduction of fixed mac layer overhead improves throughput along with
  • odds of collision and overhead caused by the random backoff timer during medium contention is also minimized.


  • The upper layer information for the MAC layer in 802.11 is called MSDU.
  • A-MSDU is a method by which AP receives multiple 802.3 frames for transmission to a wireless client STA as efficiently as possible.
  • This is done by removing 802.3 headers and trailers and then encapsulates the multiple MSDU payload into a single 802.11 frame for transmissions to the client STA.
  • If encryption is enabled all MSDU are encrypted together in single payload.
  • The A-MSDU serves as one packet as its passed down from higher layers to the MAC sub layer. The CRC is calculated for each A-MSDU as if it were a regular data frame. So, if an A-MSDU transmission fails, the entire A-MSDU must be retransmitted reducing its effort.
  • An A-MSDU contain only MSDUs where DA/SA parameter values map to the same RA/TA values. Also, it can contain MSDUs which are potentially from different source as long as they are of same traffic identifier (TID).


  • A-MPDU has similar goal to that of A-MSDU.
  • The data payload of each MPDU is encrypted separately.
  • MPDU aggregation has more overhead than A-MSDU. As each MPDU has individual MAC header and trailer.
  • If retransmission is required only individual MPDU is retransmitted.
  • The inclusion of A-MSDUs as a part of A-MPDU is more efficient over just using A-MPDU. The inclusion results in lower CRC errors.

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