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

To support real-time applications such as Voice-over-IP within an 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 periodically probing in the background for Access Points on other channels even when it is already associated with an Access Point and actively sending and receiving data. We then present two novel algorithms that use two radio cards on the client so that when one card is involved in communicating data, the other card can probe for neighboring Access Points. Of these multiple radio algorithms, the first one uses dedicated data and control cards at the client thereby implementing a soft version of make-before-break. The second algorithm uses two cards which can both perform control and data forwarding functions thereby implementing a strict version of make-before-break. Experimental results from a prototype implementation show that the make-before-brake algorithms lead to a significant reduction in MAC layer handoff overhead; the algorithm for the strict version of the make-before-break mechanism leads to sub 10 millisecond hand-off latency.