Most of consumer electronic devices, including HDTV/IPTV, multimedia teleconference systems, on-line game stations, and HiFi-audio systems, are expected to be connected through WiFi WLANs in home/office networks. Especially, the realtime multimedia applications are requiring guaranteed bandwidth to provide guaranteed QoS/QoE. As a consequence, correct estimation of the available bandwidth in overlapped WiFi WLANs in home/office network environments is one of the essential functions for efficient provisioning of QoS-guaranteed realtime multimedia applications and optimized network resource utilizations in future Internet. In this paper, we propose a cognitive passive estimation of the available bandwidth (cPEAB) by correct measurements of i) the proportion of waiting and backoff delay, ii) packet collision and packet error probability, iii) acknowledgement delay, iv) channel idle time compared to measurement period, and v) traffic from hidden/exposed terminals. Using a series of experiments, we found that the proposed scheme estimates the available bandwidth more accurately compared with other schemes.