Mobile stations supporting the 802.11u standard can access WLAN automatically when they are within the coverage of the network service provided by this WLAN. To achieve this goal, the stations need to keep “On” states including idle and active all the time. However, studies have noted that the idleness of stations often lead to considerable power consumption. Although the conventional Power Saving Mode (PSM) can provide energy saving effect to some extent, its own disadvantage leads to lower energy efficiency when the number of stations accessing the target WLAN.