f spatial streams it can receive. 802. 11ac Interface 5.1.2. Control frames RTS (Request To Send) and CTS (Clear To Send) control frames are used to negotiate the value of the available bandwidth. The TA (Transmitter Address) field of the RTS control frame may contain information relating to the bandwidth of the radio channel. The RA field of the CTS control frame copies the value of the TA field of the RTS control frame if the required bandwidth is available. In the opposite case, the value of the RA field indicates the value of the bandwidth actually available. When an 802.11ac device sends an RTS control frame, it must verify that the bandwidth (80 MHz) of the radio channel is available. The control frame is transmitted in a 20 MHz radio channel and replicated to fill the 80 MHz radio channel (Figure 5.1). Each device, whether it is 802.11a/n/ac equipment, receives the RTS control frame and interprets the Duration field to determine the occupancy time of the 20 MHz radio channel. RTS (80MHz) CTS (80MHz) Block Ack RTS (80MHz) CTS (80MHz) Block Ack Traffic data RTS (80MHz) CTS (80MHz) Block Ack RTS (80MHz) CTS (80MHz) Block Ack a) The bandwidth of 80 MHz is available RTS (80MHz) CTS (40MHz) Block Ack Traffic data RTS (80MHz) CTS (40MHz) Block Ack RTS (80MHz) RTS (80MHz) b) The bandwidth of 40 MHz is available Figure 5.1. Bandwidth negotiation Wi-Fi Integration to the 4G Mobile Network Before the equipment addressed by the RTS control frame (the receiver) responds with the CTS control frame, it checks whether a transmission is taking place on its main radio channel or any other radio channel at 20 MHz in the band of 80 MHz (Figure 5.1). If the bandwidth of 80 MHz is free, then the recipient reports it with four duplicate CTS control frames. If a portion of the bandwidth is used, the recipient responds with a CTS control frame only on the available 20 MHz radio channels indicating the overall bandwidth (e.g. 40 MHz). 5.1.3. MAC header structure The 802.11ac interface maintains the structure of the MAC header defin