WLAN 802.11ac MAC Layer Explained

This page of the WLAN 802.11ac tutorial covers the 802.11ac MAC layer. The 802.11ac MAC layer has a similar frame format to that used in earlier 802.11n, but there are a few changes as outlined below:

• Extension of maximum frame size to about 8K to 11K bytes.
• Frame aggregation was introduced in 802.11n to increase network efficiency. 802.11ac added a new feature to this by requiring all frames to be transmitted using the A-MPDU format. Hence, even a single frame needs to be transmitted in A-MPDU.
• The HT control field is kept the same as in 11n. When it begins with 0, it will be an 11n frame. If it begins with 1, it will be a VHT frame (i.e., an 11ac frame).

Data payload for aggregated frames consists of the following:

MSDU contains IP layer information.

MPDU delimiter + MAC Header + MSDU1 + MPDU delimiter + MAC Header + MSDU2 + MPDU delimiter + MAC Header + MSDU3

The maximum length of A-MPDU is controlled by the A-MPDU length exponent.

Max. length of A-MPDU = $2^{13+Exp} - 1$ bytes

Exp can range from 0 to 7, this yields A-MPDU to be of length 8KB to about 1MB.

Following are some of the useful attributes of the 11ac MAC layer:

• MSDU size for MAC payload - 2304
• MPDU size for MAC frame - 11,454
• PSDU size (PLCP payload) - About 4MB
• PPDU size (PLCP frame) - 5.484 ms

802.11ac MAC frame format

Management Frames

802.11ac has a VHT capabilities Information Element (IE), which is the format used in the management frame required to establish a connection in 802.11ac networks.

VHT capabilities IE fields = {

Max. MPDU Length (2 bits), Supported Channel Width set (2 bits),

Rx LDPC (1bit), Short GI for 80 MHz (1bit), short GI for 160 MHz (1bit), Tx STBC (1bit), Rx STBC (3bits), single user beamformer (1bit), beamformee (1bit),

Sounding dimensions (3bits), multiuser beamformer (1bit), beamformee (1bit),

VHT TxOp power saving (1bit), VHT variant HT control (1bit), Max. A MPDU length exponent (3), VHT link adaptation capable (2), Rx and Tx antenna pattern consistency (1 each),

Rx and Tx VHC MCS MAP (16 bits each), Rx and Tx Highest data rate supported (13 each)

}

VHT operation Information Element in 802.11ac is used for the operation of the 802.11 network. Basically, it describes channel-related information such as width, frequency, and rates supported by the transmitter. Rates help in determining MCS and the number of spatial streams.

VHT IE = { Channel width (1byte), channel center frequency 0 (1byte), channel center frequency 1 (1byte) }

MAC Features

A-MPDU for transmit and receive, Clear Channel Assessment (CCA) on secondary channels, and bandwidth signaling in RTS/CTS are mandatory features in 802.11ac WLAN networks.

There are a few optional features, which include single-user beamforming and multi-user beamforming and GCMP.

This tutorial also provides a link to 802.11ac data rate, beamforming, frame format, physical layer, radio network planning, spectrum mask.