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/
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