802.11ac: A Survival Guide

Chapter 3. The MAC

If you cannot get rid of the family skeleton, you might as well make it dance.

—George Bernard Shaw

Most of the work in the 802.11ac MAC is evolutionary. In contrast with the major efficiency enhancements introduced in 802.11n, most of the MAC work in 802.11ac consists of supporting new physical layer features. Frames are bigger, but the aggregation framework in place handles those larger frames without significant change. One of the few protocol features to see large changes was around sharing radio resources between channels of different sizes.

Framing

For the most part, 802.11ac maintains the frame format used by its predecessors. There are two major changes, shown in Figure 3-1. First, 802.11ac extends the maximum frame size from almost 8,000 bytes to over 11,000 bytes, further increasing the ability to aggregate frames from higher layers. 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.^[21] When the HT Control field begins with a 1, the HT Control field is of the VHT-variant type. Figure 3-1 shows the format of the VHT-variant HT Control field. It is composed of fields that are used to communicate MCS feedback, a seldom-implemented procedure that enables two devices to exchange information on how well transmissions are received to find the best data rate for the connection.