 the role of broadcasting beacon frames. The access point will also be connected to a distribution system which will usually be a wired network, but could also be a wireless bridge to other WLAN cells. In this case the cell supported by each access point is a BSS and if two or more such cells exist on a LAN the combined set is known as an extended service set (ESS). Distribution system connection Basic Service Set (BSS) Figure 6-3: Infrastructure Mode Topology Chapter Six In an ESS, access points (APs) will use the distribution system to transfer data from one BSS to another, and also to enable stations to move from one AP to another without any interruption in service. The transport and routing protocols that operate on the external network have no concept of mobility - of the route to a device changing rapidly - and within the 802.11 architecture the ESS provides this mobility to stations while keeping it invisible to the outside network. Prior to 802.11k, support for mobility within 802.11 networks was limited to movement of a station between BSSs within a single ESS, so-called BSS transitions. With 802.11k, which will be described further in the Section "Network Performance and Roaming (802.11k and 802.11r, p. 162)", the roaming of stations between ESSs is supported. When a station is sensed as moving out of range, an access point is able to deliver a site report that identifies alternative access points the station can connect to for uninterrupted service. The 802.11 MAC Layer The MAC layer is implemented in every 802.11 station, and enables the station to establish a network or join a pre-existing network and to transmit data passed down by Logical Link Control (LLC). These functions are delivered using two classes of services, station services and distribution system services, which are implemented by the transmission of a variety of management, control and data frames between MAC layers in communicating stations. Before these MAC services can be invoked, the MAC first needs to gain access to the wireless 