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WLAN 802.11ac MAC layer

This tutorial section on WLAN-11ac basics covers following sub topics:
Main page  frame  PHY layer  MAC layer  data rates  spectral mask  beamforming  radio planning

This page of WLAN 802.11ac tutorial covers 802.11ac MAC layer.

The 802.11ac MAC layer has similar frame format as earlier used in 802.11n but there are few changes as outlined below.

•  extension of maximum frame size to about 8K to 11K bytes.
•  Frame aggregation concept was added in 802.11n so that network efficiency can be increased. 11ac added new feature to this by allowing all frames to be transmitted should use A-MPDU format. Hence even though there is one frame that too needs to be transmitted in A-MPDU.
•  HT control field is kept same as 11n, when it begins with 0 it will be 11n frame, if it begins with 1 it will be VHT frame(i.e. 11ac frame).

Data payload for aggregated frames consists of following. MSDU contains IP layer information.
MPDU delimiter+MAC Header+MSDU1+ MPDU delimiter+MAC Header+MSDU2+ MPDU delimiter+MAC Header+MSDU3
Max. length of A-MPDU is controlled by A-MPDU length exponent.
Max. length of A-MPDU = 213+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 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 VHT capabilities Information Element(IE) , which is the format used in management frame required to establish 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 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 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 signalling in RTS/CTS are mandatory features in 802.11ac WLAN network. There are few optional features which include single user beamforming and multiuser beamforming and GCMP.

This tutorial also provide link to 802.11ac data rate, beamforming, frame format, physical layer, radio network planning, spectrum mask. Refer links mentioned on the left side panel.

Frame structure of wireless standards, technologies

Frame structure of various wireless standards/technologies are mentioned below. It include WiMAX, WLAN, Zigbee, GSM, GPRS, UMTS, LTE, TD-SCDMA, GPS, SDH, 11ac WLAN, AMPS, Ethernet, VLAN etc.

WiMAX physical layer Frame Structure as per 802.16d and 802.16e standards
WiMAX MAC layer Frame Structure as per OFDM 802.16d standard
Zigbee RF4CE Frame Structure
Zigbee physical layer Frame Structure
Zigbee MAC layer Frame Structure
GPRS Frame Structure
GPS Frame Structure
LTE Frame Structure
TD-SCDMA Frame Structure
UMTS Frame Structure
SONET Frame Structure
SDH Frame Structure
802.11ac PHY Frame Structure
802.11ac MAC layer Frame Structure
WLAN Frame Structure as per 802.11a,11b,11n,11ac standards
AMPS Frame Structure
Ethernet Frame Structure
VLAN Frame Structure
GSM Frame Structure


What is wlan?
WLAN standards-11a,11b,11g,11n,11ac
11a WLAN Physical layer
11b WLAN Physical layer
11n WLAN Physical layer
WLAN 802.11-ac
WLAN 802.11-ad
Difference between 11a,11b,11g,11n
Difference between 11-n,11-ac and 11-ad
WLAN router providers
WLAN providers


Other Standard MAC layer protocol


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