A framework for fast handoff in IEEE 802.11 based systems

In standard IEEE 802.11 based systems, when the wireless client migrates away from the radio range of the currently associated access point (AP), network applications temporarily loose connectivity till the client is able to re-associate itself with a new AP. The delay that occurs during the break-off interval can vary from a few hundreds of microseconds to a few seconds. However, delay sensitive applications such as Voice over IP (VoIP) or streaming multimedia applications usually are unable to tolerate such long connectivity delays that fall beyond the range of 50 - 200 ms. This results in dropped calls or frozen video frames. In this paper we describe the design, implementation, and evaluation of a software based framework that facilitates seamless and transparent handoff between different APs in standard IEEE 802.11 based wireless local area networks (WLANs). Although different solutions are available in the literature that seek to address the handoff latency, most of them propose changes that are outside the purview of the current 802.11 standards. We have specifically kept such compatibility restrictions in mind and have devised a software based client side solution that is capable of reducing handoff delays to an average value of 20 ms. It is available as a driver update to the client and requires no additional support from the network. As part of our solution, we have successfully implemented and tested our proposed solution framework on Atheros AR5212 chipsets using the open source MadWifi driver.