A reliable active scanning scheme for the IEEE 802.11 MAC layer handoff

In the IEEE 802.11 wireless local area networks (WLANs) with fixed access points (APs), handoff occurs when a mobile station (MS) with a communication in progress changes its associated AP from one to another. It is important to minimize the handoff latency because the active communications of a MS are interrupted before the handoff completes. Such a design needs to take into consideration the high error rate of WLANs. The IEEE 802.11 medium access control (MAC) protocol recommends an active scanning mode for the handoff process. The probe delay of the active scanning may increase greatly if the probe request is lost. However, the successful transmission of the probe request is very unpredictable due to the high collision probability and the lack of any acknowledgement. In this paper, we propose a highly reliable active scanning scheme which can be implemented with the current IEEE 802.11 channel scanning procedure. It is indicated to efficiently increase the reliability of channel scanning, especially in a noisy environment, by performing the probe request loss detection and retransmission. Furthermore, the proposed scheme reduces the overall handoff latency by providing a higher priority for management frames over data frames.