The Group Key Handshake consists of two frames excluding any Ack frames that may be required. The Group Key Handshake is used to distribute and update the Group Temporal Key (GTK) for encrypting broadcast and multicast traffic. The AP initiates the Group Key Handshake by sending a Group Key Message 1 frame to a STA, which contains the new GTK and other information. The STA responds with a Group Key Message 2 frame to the AP, which confirms the receipt of the GTK and other information. After this, both the AP and the STA can use the new GTK for encryption and decryption of broadcast and multicast traffic . References: CWAP- 405 Certified Wireless Analysis Professional Study and Reference Guide, Chapter 7: 802.11 Security, page 246; CWAP- 405 Certified Wireless Analysis Professional Study and Reference Guide, Chapter 7: 802.11 Security, page 247.

An HE TB PPDU (High Efficiency Trigger-Based Packet Data Unit) is used to respond with an uplink transmission to an MU-RTS trigger frame in the 802.11ax PHY (Physical Layer). An MU-RTS trigger frame is a frame that initiates a multi-user transmission opportunity (MU-TXOP) by requesting multiple stations (STAs) to send clear-to-send (CTS) frames on different spatial streams or resource units (RUs). An HE TB PPDU is a frame that contains data from multiple STAs that have been allocated RUs by an MU-RTS trigger frame or another type of trigger frame. An HE SU PPDU (High Efficiency Single User Packet Data Unit) is a frame that contains data from a single STA using all available spatial streams or RUs. An HE MU PPDU (High Efficiency Multi User Packet Data Unit) is a frame that contains data from multiple STAs using different spatial streams or RUs without being triggered by another frame. A VHT PPDU (Very High Throughput Packet Data Unit) is a frame that uses the 802.11ac PHY and does not support multi-user transmissions.