Make-Before-Break MAC Layer Handoff in 802.11 Wireless Networks

To support real-time applications such as Voice-over-IP within a 802.11 Wireless LAN, efficient handoff mechanisms are required when a mobile client moves from one Access Point to another. In this paper, we present the design, implementation and performance results of MAC layer (layer-2) handoff algorithms which implement make-before-break mechanisms at the MAC layer unlike current algorithms that are based on break-before-make. As a baseline, we first present an algorithm that uses a single radio card on the client (as is done traditionally), but optimizes MAC layer handoff by using periodic scanning in the background for Access Points on other channels even when it is already connected and sending and receiving data to/from an Access Point. We then present two other algorithms that use two radio cards on the client so that when one card is actively receiving data, the other card can scan for neighboring Access Points.