The 802.11 data frame is only slightly changed by 802.11n. Figure 5-1 shows the format of an 802.11 Data frame as modified by 802.11n. The major changes from the traditional 802.11n Data frame are the increase in size, the addition of the optional HT Control subfield, and the fact that the QoS Control field is utilized extensively in block acknowledgment. The payload of the MAC is increased about fourfold, which can be used to aggregate higher-layer frames together for efficiency.
Management frames signal that they are part of an 802.11n network by including the HT Capabilities information element, shown in Figure 5-2. When a station includes the HT IE in transmissions, it is declaring that it is an 802.11n device. The HT IE is included in Beacon frames so that stations can determine that a network supports 802.11n. An 802.11 station will insert the HT IE into Probe Request frames to seek out 802.11n networks and declare to an AP that it is capable of 802.11n operation. The HT IE is also included in Association and Reassociation exchanges so that an 802.11n device associating to an AP can exchange information on capabilities.
