Very High Throughput (VHT) in 11ac

802.11ac maintains the frame format used by its predecessors. There are two major changes. First, 802.11ac extends the maximum frame size from almost 8,000 bytes to over 11,000 bytes. Second, it reuses the HT Control field from 11n, but does so by defining a new form of the Control field. When the HT Control field begins with a 0, the format is identical to 802.11n and the HT Control field is of the HT-variant type. When the HT Control field begins with a 1, the HT Control field is of the VHT-variant type.

Management Frames

Management frames signal that they are capable of building an 802.11ac network or participating in an 802.11ac network by including the VHT Capabilities Information element. This element is placed in Probe request and Probe response frames to enable client devices to match their capabilities to those offered by a wireless network. The VHT Capabilities Information element, as shown in the following picture, is the core information element used in management frames to set up operation of 802.11ac networks. 

The VHT Operation Information element 

All 802.11 physical layers have an information element (IE) that describes their operation, and the VHT PHY is no exception. The VHT Operation IE, show the following picture, describes the channel information and the basic rates supported by the transmitter.

The following figure shows VHT Capabilities information element in Beacon. The key thing to look for is the number of lines that read "10". A line that reads "10" indicates that a spatial stream is available. A line that reads "11" indicates that no spatial stream is available. That means if three lines read "10" and the remaining five lines read "11", then there are three spatial streams available for my AP.

What does that mean for data rates?

1 spatial stream: 6.5 Mbps to 433 Mbps data rates

2 spatial stream: 6.5 Mbps to 867 Mbps data rates

3 spatial stream: 6.5 Mbps to 1.3 Gbps data rates

Reference 

https://sniffwifi.wordpress.com/

WiFi for iPhone6

The big news about the iPhone 6 is 802.11ac.  Yippee!  Apple has finally adopted the latest and greatest WiFi standard in a mobile device.

802.11ac has data rates as high as 6.9 Gbps in the standard, but wireless LAN folks know that’s not what happens in real life.  Real 802.11ac devices top out at a 1.3 Gbps data rate when multiple input-multiple output (MIMO) antenna systems are supported, while non-MIMO devices top out at 433 Mbps.

The iPhone 6 and iPhone 6 Plus are non-MIMO 802.11ac devices.  That means a top rate of 433 Mbps.  That is higher than the top rate of the 802.11n-supporting iPhone 5 and iPhone 5S, which is 150 Mbps.  And that is where Apple gets the justification for putting this on their site:

You see, 433/150 = almost 3.  That means three times faster wireless!  Except it doesn’t.

802.11ac is basically the same thing as 802.11n.  I know that 433 and 150 seem like very different numbers, but in most real world cases, they’re actually the same.

Here’s how it works:

802.11n = 150 Mbps –>

–> Normal 802.11ac = 150 Mbps –>

–> 802.11ac with clear line of sight and a distance less than 30ft/10m = 200 Mbps –>

(That’s because when you’re that close and there are no obstructions, then 802.11ac can use a technology called 256-QAM, which allows waves to carry 8 bits of data rather than 6 bits.  8/6 = 200/150, so that means that adding 256-QAM boosts the top data rate to 200 Mbps.)

–> 802.11ac with clear line of sight and a distance less than 30ft/10m and 80 MHz channels enabled = 433 Mbps

(80 MHz channels are no good for high capacity WiFi.  Instead of being able to split users up amongst 9 [if disabling dynamic frequency selection {DFS}] or 21 [if enabling DFS] channels, an 80 MHz wireless network only has 2 [non-DFS] or 4 [DFS] channels. 

Think about the average high-capacity wireless network.  Do the users have a line of sight to the APs?  Usually, No.  Are the users within thirty feet (ten meters) of the APs?  Often, No.  Is it better to spread users out among four or five times as many channels?  Definitely, Yes.  If you agree with these answers, then 802.11ac in the iPhone 6 and iPhone 6 Plus reverts to the 150 Mbps data rates used in the iPhone 5 and iPhone 5S.

A year ago, yours truly wrote that non-MIMO devices were going to be around for a while.  MIMO drains battery life faster and it can cause a device to heat up.  So, the lack of MIMO in the new iPhones is no surprise.  But it is disappointing.  And it comes down to this:

802.11n w/ MIMO > 802.11ac w/o MIMO

The iPad Air, which has been out for about a year now, is a mobile device from Apple that supports MIMO.

Reference 

https://sniffwifi.wordpress.com/category/802-11ac/