eport on the measured channel activity to the access point. When necessary, channel switching is initiated by the access point, which sends a channel switch announcement management frame to all associated stations. This announcement identifies the new channel, specifies the number of beacon periods until the channel switch takes effect, and also specifies whether or not further transmissions are allowed on the current channel. The access point can use the short interframe Chapter Six spacing (SIFS - see the Section "Wireless Media Access, p. 144") to gain priority access to the wireless medium in order to broadcast a channel switch announcement. Dynamic frequency selection is more complicated in an IBSS (ad-hoc mode) as there is no association process during which supported channel information can be exchanged, and no access point to coordinate channel measurement or switching. A separate DFS owner service is defined in 802.11h to address these complications, although channel switching remains inherently less robust in an IBSS than in an infrastructure mode BSS. Network Performance and Roaming (802.11k and 802.11r) A client station may need to make a transition between WLAN access points for one of three reasons, as described in Table 6-12. The 802.11 Task Groups TGk and TGr are addressing issues relating to handoffs or transitions between access points that need to be fast and reliable for applications such as VoWLAN. TGk will standardise radio measurements and reports that will enable location-based services, such as a roaming station's choice of a new access point to connect to, while TGr aims to minimise the delay and maintain QoS during these transitions. 802.11k; Radio Resource Measurement Enhancements The 802.11 Task Group TGk, subtitled Radio Resource Measurement Enhancements, began meeting in early 2003 with the objective of Table 6-12: Reasons for Roaming in a WLAN Roaming need Description Mobile client station A mobile client station may move out of range of its current access point and need to transit