ervices (at the "top" of the MAC layer) is termed the MAC service access point or SAP. This service ensures the delivery of MSDUs between these service access points. Control frames such as RTS, CTS and ACK may be used to control the flow of frames between stations, for example in 802.11b/g mixed-mode operation. Distribution System Services The functionality provided by MAC distribution system services is distinct from station services in that these services extend across the distribution system rather than just between sending and receiving stations at either end of the air interface. The 802.11 distribution system services are described in Table 6-4. 802.11 PHY Layer The initial 802.11 standard, as ratified in 1997, supported three alternative PHY layers; frequency hopping and direct sequence spread Wireless LAN Standards Table 6-4: 802.11 MAC Layer Distribution System Services Service Description Association This service enables a logical connection to be made between a station and an access point. An access point cannot receive or deliver any data until a station has associated, since association provides the distribution system with the information necessary for delivery of data. Disassociation A station disassociates before leaving a network, for example when a wireless link is disabled, the network interface controller is manually disconnected or its host PC is powered down. Reassociation The reassociation service allows a station to change the attributes (such as supported data rates) of an existing association or to change its association from one BSS to another within an extended BSS. For example, a roaming station may change its association when senses another access point transmitting a stronger beacon frame. Distribution The distribution service is used by a station to send frames to another station within the same BSS, or across the distribution system to a station in another BSS. Integration Integration is an extension of distribution when the access point is a portal to a non-802.11 network and th