tion/AID: this field is coded on 16 bits: - Duration indicates the time during which the radio resource is immobilized; MAC Layer - AID (Association Identifier) indicates the name of an association identifier in the case of the transmission of a PS (Power Save)-POLL control frame. Address: there are four address fields, each of which is six bytes long. The construction rule is identical to that of an Ethernet MAC address. These fields indicate the basic service set identifier (BSSID), source address (SA), destination address (DA), transmitter address (TA) and receiver address (RA). To DS From DS Address 1 Address 2 Address 3 Address 4 BSSID BSSID BSSID Table 2.2. Meaning of Address fields Sequence Control: this field contains two subfields: - Sequence Number: this subfield is coded on 12 bits. It indicates the number of the frame modulo-4096; - Fragment Number: this subfield is coded on four bits. It indicates the number of the fragment in the frame. The value is equal to ZERO for the first fragment. All fragments of the same frame have the same value of the frame number. FCS (Frame Check Sequence): this field is coded on 32 bits. It contains the cyclic redundancy code for error detection. 2.1.2. Structure of control frames The Type subfield is set to 01 for control frames. The RTS (Request To Send) frame is transmitted by the station to request the access point to access to the radio resource. The RA field contains the MAC address of the access point and the TA field of the station (Figure 2.2). The Subtype subfield is set to 1011. Wi-Fi Integration to the 4G Mobile Network The CTS (Clear To Send) frame is transmitted by the access point to allow the station to access the radio resource. The RA field contains the MAC address of the station (Figure 2.2). The Subtype subfield is set to 1100. RTS frame Frame Duration Control CTS frame Frame Duration Control Frame ACK frame Duration Control PS-Poll frame Frame BSSID Control Figure 2.2. Structure of control frames The ACK (Acknowledgment) frame is transmitted to ackn