association, it can use active scanning by transmitting a Probe frame containing this SSID and waiting for a Probe Response frame to be returned by the preferred access point. A broadcast Probe frame can also be sent, requesting all access points within range to respond with a Probe Response. This will provide the new station with a full list of access points available. The process of authentication and association can then start - either with the preferred access point or with another access point selected by the new station or by the user from the response list. Station Services MAC layer station services provide functions to send and receive data units passed down by the LLC and to implement authentication and security between stations, as described in Table 6-3. Chapter Six Table 6-3: 802.11 MAC Layer Station Services Service Description Authentication This service enables a receiving station to authenticate another station prior to association. An access point can be configured for either open system or shared key authentication. Open system authentication offers minimal security and does not validate the identity of other stations - any station that attempts to authenticate will receive authentication. Shared key authentication requires both stations to have received a secret key (e.g. a passphrase) via another secure channel such as direct user input. Deauthentication Prior to disassociation, a station will deauthenticate from the station that it intends to stop communication with. Both deauthentication and authentication are achieved by the exchange of management frames between the MAC layers of the two communicating stations. Privacy This service enables data frames and shared key authentication frames to be optionally encrypted before transmission, for example using wired equivalent privacy (WEP) or Wi-Fi protected access (WPA). MAC service data A MAC service data unit (MSDU) is a unit of data passed to the unit delivery MAC layer by the logical link controller. The point at which the LLC accesses MAC s