June 1997: 
802.11-1997 
September 1 999: 
802.11m 802.11b 
March 2007: 
802.11-2007 
March 2012: 
802.11-2012 
February 2014? 
802.1 
June 2003: 
802.1 lg 
September 2005: 
802.11e 
June 2004: 
802.1 li 
September 2009: 
802.11 n 
September 2009: 
802.11w 
2006 
May 2008: 
802.11k, 802.1 Ir 
2007 2008 2009 
September 2011: 
802.11 v,802.11u 
2010 2011 2012 2013 2014 
802.11ax 
Ratified Late 2020 
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 
Figure 1-1. 802.11 timeline

This blog post will be focusing on 802.11ac in particular. We visited the aspects of 802.11n in the last blog post.

802.11ac introduced the VHT (Very High Throughput) along with some core technological advancement like MU-MIMO, 256 QAM addition & support for 80MHz/160MHz channels. One of the key differences also lie in the support of only 5GHz band. So there is still a dependency on 802.11n for 2.4Ghz support, however the upcoming 802.11ax will support both 2.4GHz and 5GHz.

Table 1. 
PHY 
Calculating the speed of 802.1 lac and 802.1 lax 
802.1 lac 
802.1 lax 
Bandwidth 
(as number of 
data subcarriers) 
234 (80 MHz) 
2 234 (160 MHz) 
980 (80 MHz) 
2 x 980 (1 60 MHz) 
Data bits per 
subcarrier 
5/6 log2(256) 
= 6.67 
x 
5/6 x log2(1 024) 
= 8.33 
Time per 
OFDM symbol 
(800ns G') 
4 ps 
13.6 vs 
390 
Mbps 
780 
Mbps 
600 
Mbps 
1.2 
Gbps 
1.17 
Gbps 
1.8 
Gbps 
3.6 
Gbps 
1 .56 
G bps 
3.12 
G bps 
2.4 
G bps 
4.8 
G bps 
4.8 
Gbps
Table 1. 
PHY 
Calculating the speed of 802.1 lac and 802.1 lax 
802.1 lac 
802.1 lax 
Bandwidth 
(as number of 
data subcarriers) 
234 (80 MHz) 
2 234 (160 MHz) 
980 (80 MHz) 
2 x 980 (1 60 MHz) 
Data bits per 
subcarrier 
5/6 log2(256) 
= 6.67 
x 
5/6 x log2(1 024) 
= 8.33 
Time per 
OFDM symbol 
(800ns G') 
4 ps 
13.6 vs 
390 
Mbps 
780 
Mbps 
600 
Mbps 
1.2 
Gbps 
1.17 
Gbps 
1.8 
Gbps 
3.6 
Gbps 
1 .56 
G bps 
3.12 
G bps 
2.4 
G bps 
4.8 
G bps 
4.8 
Gbps

Multi-user MIMO

  • One of the greatest potential of 802.11ac
  • Prior to this all the 802.11 standards used single user.
  • If there are two receivers located in sufficiently different directions, a beamformed transmission may be sent to each of them at the same time.
  • Enables better spatial reuse. As per the below example, the MU-MIMO builds on small-cell approach by enabling even more tightly packed networks. As a result AP can send independent transmissions within its own coverage area. Just as 802.3(Ethernet) reduces collision domains, MU-MIMO intends to reduce spatial contention of transmissions.
Downlink Multi-User MIMO

802.11ac Wave 1 and 2 – The first wave of 802.11ac products will be driven by the enthusiasm for higher speeds. APs will typically have three stream capabilities, but with 802.11ac providing 80 MHz channels and 256-QAM modulation, the speed will go from 450 Mbps to 1.3 Gbps. The second wave of 802.11ac products will add even wider channels and possibly even multi-user MIMO support, as outlined in the figure below.

Attribute 
Maximum number of spatial streams 
Channel width 
Maximum modulation 
Typical maximum speed 
Beamforming support 
MU-MIMO support 
First wave 
3 
80 MHz 
256-QAM 
1.3 Gbps 
Varies (depending on vendor) 
Second wave 
3 or 4 
160 MHz 
256-QAM 
26 Gbps 
Yes 
Yes

The PHY

#Channels

  • OFDM based transmission, 802.11ac divides the channel into OFDM sub carriers each 312.5kHz
  • To increase throughput, 802.11ac introduces two new channel widths. Supports 80MHz and further added 160MHz channel option for even higher speeds.
  • 802.11ac channels have exactly the same shape as previous OFDM channels (802.11a,g,n)

MCS & GI

  • MCS Index tends to be much simpler than 802.11n. First 7 are mandatory and others are supported.
MCS index value 
2 
3 
4 
5 
6 
7 
8 
9 
Modulation 
BPSK 
QPSK 
QPSK 
16-QAM 
16-QAM 
64-QAM 
64-QAM 
64-QAM 
256-QAM 
256-QAM 
Code rate (R) 
1/2 
3/4 
1/2 
3/4 
3/4 
3/4
  • 802.11ac retains the ability to select a shortened OFDM guard interval if both Tx and Rx are capable of processing it. The GI shrinks from 800ns to 400ns, providing a 10% boost in the throughput.

VHT Signal Fields

The purpose of the Signal Field is to help the receiver decode the data payload, which is done by describing the parameters used for transmission. 802.11ac separates into Signal A and Signal B fields. For CWAP purposes this has not been dealt in depth. There are 2 parts in VHT Signal A field are referred as VHT-SIG-A1 & VHT-SIG-A2.

SIGNAL A

  • Bandwidth
    • 0 – 20MHz, 1 – 40MHz, 2- 80MHz & 3 – 160MHz
  • STBC
    • If the payload is encoded with STBC (Space-time block coding may be used when the number of radio chains exceed the number of spatial streams, it tx a single data stream across 2 spatial streams.) for extra robust-ness, this field is set to 1, otherwise will be 0.
  • Group ID
    • Frames to AP > group ID =0
    • Frames sent to STA Client > group ID = 63
  • Number of space-time streams
    • Starts from 0, e.g. if field is set to 3, then there are 4 space time streams.
  • Partial AID
    • Last 9 bits of the BSSID.
  • Transmit power save forbidden
    • Field will be 0, if AP in network allows client to power off radios when they have opportunity to transmit frames. Otherwise will be 1.
  • Short GI – Field set to 1 for 400ns, 0 for otherwise.
  • Short GI disambiguation – Extra symbol may be required denoting 1 or 0 for not required.
  • Coding – Field is 0 when convolutional coding is used to protect the data field, 1 when LDPC is used.
  • LDPC Extra Symbol – Field is set to 1 if extra symbol is required.
  • MCS – MCS Index value of the payload.
  • Beamformed – If matrix is applied to the transmission, the bit is set to 1 otherwise set to 0.
  • CRC – Error correction
  • Tail – 6 zeros are included to terminate the convolutional coder that protects the Signal A field.

SIGNAL B

  • Used to setup the data rate, as well as tune in the MIMO reception.
  • VHT Signal B Length (17, 19 or 21 Bits)
  • Reserved bits – Set to 1.
  • Tail bits
< IEEE 802.11ac Figure 22-19—VHT-SIG-A2 structure > 
32 
83 
84-87 
SU VHT-MCS/MU[1-3] coding 
SU VHT-MCS 
eam 
Formed 
Formed 
Rese rved 
Variable 
818-823 
BIO-B17 
u 
Composite Name 
SU Name 
MU Name 
Bits 
8 us 
L-STF 
BO-BI 
Composite Name 
SU Name 
MU Name 
Bits 
Coding 
OFDM PHY Modulation 
MU[2] 
Coding 
MUC3] 
Rese rved 
Coding 
VHT Modulation 
Bus 
L-LTF 
83 
4us 
VHT 
L-SIG 
84 ag 
8 us 
VHT-SIG-A 
NSTS 
MUCO] 
NSTS 
Bus 
BIO-B2 
NSTS/Partial AID 
Partial AID 
822 
823 
MU[I] 
NSTS 
MU[2] 
N STS 
MUC3] 
N STS 
< IEEE 802.1 lac Figure 22-18— 
VHT-SIG 
-Al structure >

Air Magnet Pro can help you scan through the PHY frames

The MAC

Frame aggregation was introduced in 802.11n, 802.11ac however adds an interesting new take on the aggregation. All frames transmitted use the aggregated MPDU (A-MPDU) format. Even the single frame transmitted in one shot is transmitted as aggregate frame.

Table 3-1. Size comparisons of transmissions for different 802.11 PHYs 
Attribute 
MSDU (MAC payload) size 
MPDU (MAC frame) size 
A-MSDU (aggregate MAC payload) 
sue 
PSDU (PLO payload) size 
PPDU frame) size 
802.11a 
2,304 
Implied by maximum MSDU 
Size 
Not used with 802.1 la 
4,095 bytes 
Implied by maximum PSDU 
size 
802.11n 
2,304 
Implied by A-MSDU size 
7,935 
65,535 bytes 
5.484 ms (mixed mode) or 10 
ms (greenfield mode) 
802.1 lac 
2,304 
11,454 
Implied by maximum 
MPDU size 
bytes 
5.484 ms

Management Frames

  • VHT Capabilities Information element.
v VHT Capabilities Info: 
ØxØ39179b1 
. . .01 
= Maximum MPDU Length: 7 991 
00.. = Supported Channel Width Set: Neither 160MHz nor 80+80 supported (OXO) 
. 1 = Rx LDPC: Supported 
1. = Short Gl for Supported 
.0.. 
= Short GI for 160MHz and 80+80MHz: Not supported 
1... = Tx STBC: Supported 
. = Rx ST BC: 1 Spatial Stream Supported (Oxl) 
. . .01 
. 1... = SU Beamformer Capable: Supported 
...1 
. = SU Beamformee Capable: Supported 
. = Beamformee STS Capability: 4 (Ox3) 
. 011. . 
= Number of Sounding Dimensions: 2 (Oxl) 
.01 
= MU Beamformer Capable: Not supported 
. = MU Beamformee Capable: Supported 
. ..Ø. = TXOP PS: Not supported 
.0.. 
. = +HTC—VHT Capable: Not supported 
.. 11 1... . .. 
. . = Max A-MPDU Length Exponent: 1 048 575 
. = VHT Link Adaptation: No Feedback (OXO) 
...o 
= Rx Antenna Pattern Consistency: Not supported 
= Tx Antenna Pattern Consistency: Not supported 
00.. = Extended NSS BW Support:
  • VHT Operations Information element
v Tag: VHT Operation 
Tag Number: VHT Operation (192) 
00 = Basic 
ll.. = Basic 
11 = Basic 
= Basic 
= Basic 
= Basic 
= Basic 
= Basic 
Tag length: 5 
v VHT Operation Info 
Channel Width: 20 MHz or 40 
Channel Center Segment 0: 
Channel Center Segment 1: 
Basic MCS Map: Oxfffc 
. ll.. 
.. 11 
. ll.. 
.. 11 
ll.. 
MHz 
1 
2 
3 
4 
5 
6 
7 
8 
SS: 
SS: 
SS: 
SS: 
SS: 
SS: 
SS: 
MCS 
Not 
Not 
Not 
Not 
Not 
Not 
Not 
0-7 (OXO) 
Suppo r ted 
Suppo r ted 
Suppo r ted 
Suppo r ted 
Suppo r ted 
Suppo r ted 
Suppo r ted

NOTE: Greenfield mode was offered with 802.11n. The efficiency gains from greenfield mode were often lost because airtime-devouring CTS-to self

messages were required before transmitting in the greenfield mode. As a result, greenfield mode was removed from 802.11ac.

Beamforming Basics

  • As 802.11ac beamforming is based on explicit channel measurements, both the transmitter and receiver must support it.
  • Any device that shapes its transmitted frames is called beamformer, receiver of such frames is called beamformee.
  • The AP initiates frame exchange with the STA, which helps it to measure the channel. The result of the channel measurement is a derivation of the steering matrix.
  • Steering Matrix describes how to setup each element of transmitter’s antenna system to precisely overlap transmissions to reach farther.
  • To steer transmissions in a particular direction, a beamformer will subtly alter what is transmitted by each array. A simple phase shift can alter/steer the transmission.

Null Data Packet (NDP) – Standardizes beamforming methods. 802.11ac method of beamforming is termed as null data packet sounding. Sounding is the term used to denote the process  performed by the transmitter to acquire channel state information (CSI) from each of the different users by sending training symbols and waiting for the receivers to provide explicit feedback containing a measure of the channel.

VHT beamformer shall initiate a sounding feedback sequence by transmitting VHT NDP announcement frame followed by a VHT NDP after a SIFS.

Beam 
rormer 
formec 
for mec 
NDP 
-=ment 
Frame 
F 
Frame 
Beamfcgm— I 
•ing Relx»rt r 
Fr ame S 
Beunfu•u• 
—ing 
IS Frmne 
. 1. AVHT

SU Beamforming

  • Begins with the beamformer sending a NDP announcement packet followed by NDP. The NDP has fixed known format. The beamformee receives the NDP, analyzes it and computes back in form of feedback matrix. The feedback matrix is sent in reply to the NDP in the form of compressed beamforming frame (CBF).
SIFS 
NDP 
Announcement 
Beamformer 
Compressed 
Beamtorming 
Beamfor mee 
SIFS

MU Beamforming

  • As opposed to Tx to one device, MU-MIMO Aps are capable of simultaneously transmitting data to multiple device groups.
  • The key distinction between them is that with MU-MIMO beamforming and beamformer requires a response from all beamformees in order to conclude channel sounding.
  • The CBF packet is 802.11 action frame which contains a channel matrix that specifies the CSI for each client. The CBF is the largest contributor to the overhead caused by MU-MIMO transmission and is size is determined by
    • Channel Width
    • Number of radio chain pairs
    • Bit count of each CSI unit
SIFS 
Beamformer 
Beamformee 1 
Be amforming 
Bea 
SIFS 
mforming 
Report poll 
ieai\ 
SIFS 
Beamformee 2 
Beamformee 3 
SIFS 
Compressed 
Beam f 
SIFS

Recommended Reading

Cisco 802.11ax White Paper
Wifi Certified 6 Highlights
802.11 Framing in Detail
802.11ac Channel Planning
802.11ac VHT PHY
Research Paper on VHT MU-MIMO
802.11ac – A Survival Guide

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