WLAN 11ac frame format

This page of WLAN 802.11ac tutorial covers 802.11ac frame format used in 11ac physical layer to create 802.11ac compliant frame.

802.11ac frame format or structure has been designed to take care of compatibility with previous standards viz. 11a and 11n. To meet this specifications, 11ac frame format is similar to 11n mixed format. Unlike 11n, 11ac has only one frame format and hence physical layer is simple to implement. In 11ac header has been changed to take care of identifying multiuser transmission or single user. Let us examine each fields as described in the figure one by one.

802.11ac physical layer frame format

As shown in the figure, 802.11ac frame consists of L-STF,L-LTF,L-SIG, VHT-SIG-A,VHT-STF,VHT-LTF,VHT-SIG-B and Data part.

L-STF and L-LTF: These fields are similar to the one as used in 11a. They are total 12 symbols and are used for frame identification and front end synchronization.

L-SIG: This field is also same as one used in 802.11a. It describes data rate and length of the frame in unit of bytes.

VHT-SIG-A and VHT-SIG-B: These fields are for similar purpose as header field(HT-SIG) of 11n and SIG field of 11a. In 11ac signal fields are two SIG-A and SIG-B. They describe channel width,modulation-coding and also whether the frame is meant for single user ro multiple user. These fields are used by only 11ac devices and are ignored by 11a and 11n devices.

VHT-STF: This has same function as that of non HT STF field. It assists 11ac receiver detect the repeating pattern.

VHT-LTF: This consists of sequence of symbols and will help demodulation of the rest of the frame. It's length depends on number of transmitted streams. It can be 1,2,4,6 or 8 symbols. This is mainly help in channel estimation and correction purpose.

Data field: It carry data from upper layers. If there are no data from upper layers than the field is referred as null data packet (NDP) and is used mainly for measurement and beamforming set up purpose by the physical layer.

VHT Signal A field:

It is transmitted initially in the 11ac frame. It is BPSK1/2 modulated and coded. It holds rate information to decode the payload of the 11ac physical layer frame.

VHT signal A fields= { Bandwidth(2bits), STBC(1), Group ID(6), Number of space time streams(3), partial AID(9), Transmit power save forbidden(1), short GI(1), Short GI disambiguation(1), coding(1), LDPC extra symbol(1), MCS(4), beamformed(1),CRC(8), tail(8) }

VHT signal B field:

It helps set up the data rate in 11ac and also help in tuning MIMO. It is also BPSK1/2 coded similar to VHT signal A.

VHT signal B fields={ Length(17,19 or 21 bits), reserved(2 or 3 bits) , tail bits(6)}

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

Similar posts on 802.11ac (WiFi 5)

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


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

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