owledge the received frame. This can be a traffic frame, a management frame or the PS- Poll control frame. The RA field copies the MAC address contained in the Address 2 field of the received frame (Figure 2.2). The Subtype subfield is set to 1101. The PS-POLL frame is sent by the station to warn the access point that it has left sleep mode. The BSSID field contains the MAC address of the access point and the TA field of the station. The AID field is an identifier assigned to the station during the association phase (Figure 2.2). The Subtype subfield is set to 1010. 2.1.3. Structure of management frames The Type subfield is set to 00 for management frames. The BEACON management frame is a beacon channel that broadcasts information on the network. It contains mandatory fields and optional fields (Figure 2.3). The Subtype subfield is set to 1000. MAC Layer Short Short Channel Privacy Pollable Preamble Agility mandatory optional Beacon Supported Timestamp Interval Frame Destination Source Duration BSSID Message Control Address Address Beacon Figure 2.3. Structure of the BEACON management frame Timestamp: this field is coded on 64 bits. It contains the timestamp of the frame. Beacon Interval: this field is coded on 16 bits. It indicates the frequency of emission of the beacon channel. Capability Information: this field is coded on 16 bits. It contains the characteristics of the access point: - the type of network architecture (ESS, IBSS); - the implementation of the security (Privacy); - the use of a short preamble for the 802.11g radio interface; - the use of a short slot time for the 802.11g radio interface; - the use of the DSSS-OFDM physical layer for the 802.11g radio interface. SSID (Service Set Identifier): this field has a variable length less than or equal to 34 bytes. It provides the identifier of the ESS (Extended Service Set) network. Supported Rates: this field is composed of several information elements. Each element has a variable length less than or equal to 10 bytes and specifies the rates supported 