SyncScan: practical fast handoff for 802.11 infrastructure networks

Abstract

ó Wireless access networks scale by replicat- ing base stations geographically and then allowing mobile clients to seamlessly ìhand offî from one station to the next as they traverse the network. However, providing the illu- sion of continuous connectivity requires selecting the right moment to handoff and the right base station to transfer to. Unfortunately, 802.11-based networks only attempt a hand- off when a client's service degrades to a point where con- nectivity is threatened. Worse, the overhead of scanning for nearby base stations is routinely over 250ms ñ during which incoming packets are dropped ñ far longer than what can be tolerated by highly interactive applications such as voice telephony. In this paper we describe SyncScan, a low-cost technique for continuously tracking nearby base stations by synchronizing short listening periods at the client with peri- odic transmissions from each base station. We have imple- mented this SyncScan algorithm using commodity 802.11 hardware and we demonstrate that it allows better handoff decisions and over an order of magnitude improvement in handoff delay. Finally, our approach only requires trivial implementation changes, is incrementally deployable and is completely backward compatible with existing 802.11 stan- dards.