Fast MAC-Layer Scanning in IEEE 802.11 Fixed Relay Radio Access Networks

Mobile real-time services require wireless networks with low handoff latencies. This paper focuses on the handoffrelated MAC-Layer scanning latencies in IEEE 802.11- based Fixed Relay Radio Access Networks (FRRAN). The IEEE 802.11 scanning procedure is known to have latencies of 300 to 550 ms, which exceeds the requirements of real-time applications like Voice over IP (VoIP) by far. Previous solutions for this problem have only focused on the IEEE 802.11 infrastructure mode. However, the IEEE 802.11 ad-hoc mode is a better basis for FRRANs because it enables mobile nodes to use all relays in range, e.g. for load balancing purposes, instead of being associated to a single access point only as in infrastructure mode. Therefore, we present a novel fast-scan concept for IEEE 802.11 ad-hoc-mode-based FRRANs. The key idea is to answer MAC-layer probe requests by IP-based probe responses, which are sent back to a scanning node via the FRRAN infrastructure. The benefit of this cross-layer concept is that scanning nodes can return to their current channel quickly and can continue to send and receive normal data packets while awaiting the IP-based probe responses. We implemented our solution for commodity IEEE 802.11 hardware and could reduce scanning latencies to 70 to 80 ms.