the primary radio channel. This channel is used for management frames. Secondary Channel Offset: this field indicates whether the secondary radio channel has a frequency higher or lower than that of the primary channel. STA Channel Width: this field indicates the bandwidth that the access point uses in reception. RIFS mode: this field indicates whether the use of the reduced inter- frame space (RIFS) is allowed. 66 Wi-Fi Integration to the 4G Mobile Network HT Protection: this field indicates the protection mechanism to avoid interference with mobiles that are not compatible with the 802. 11n interface. Non-greenfield STA present: this field indicates if the HT_GF mode is supported by the access point. OBSS Non-HT STAs present: this field indicates that an overlapping basic service set (OBSS) contains mobiles that are not compatible with the 802.11n interface requiring protection. Dual Beacon, Dual CTS, STBC Beacon: these three modes are used when the beacon channel uses the diversity in STBC transmission. L-SIG Protection Full Support: this field indicates whether the L-SIG protection mechanism is supported. PCO Active, PCO Phase: these two fields indicate the use of the PCO mode, which makes it possible to switch a radio channel between 20 and 40 MHz. These fields are used to indicate that the PCO mode is in operation and whether the radio channel is currently 20 or 40 MHz. Basic MCS set: this field indicates the modulation and coding schemes supported by the access point. The HT Operation information element is included in the management frames BEACON, ASSOCIATION RESPONSE, REASSOCIATION RESPONSE and PROBE RESPONSE. 4.1.2. Structure of the MAC header The 802.11n interface modifies the structure of the protocol header by adding the HT Control field (Figure 4.1) after the QoS Control field. The presence of the HT Control field is indicated by the Order bit of the Frame Control field set to ONE, for QoS Data traffic frames and management frames. 802.11n Interface Bit 0-15 Bit 16-17 Bit 18-19 Bit 20-21 Bit 22-23 B