Interactive WiFi Connectivity For Moving Vehicles

Summary

The key idea of this paper is to exploit opportunistic reception of packets by all base stations within range of the source to improve connectivity of WiFi for moving vehicles. The paper motivates this work by noting that currently 802.11b connections can only connect to one base station at a time. The authors compare the performance and connectivity of several hand off mechanisms:

• RSSI - picks the BS with highest signal strength
• BRR - picks the BS with the highest beacon reception ratio
• Sticky - keeps the current BS until it is absent for some time
• History - picks the historically best BS at this location
• BestBS - picks the single best BS using an ideal model
• AllBSes - opportunistically uses all BSes in the vicinity

They find that AllBSes performs best by about 25%, RSSI, BRR, and History all perform similarly. Sticky performs worst. Motivated by the results of this study the authors design a system that uses all BSes in the area to improve connectivity.

The design uses beacons to designate an anchor or primary base station and all other base stations are designated as auxiliary. When the source broadcasts a packet, if the anchor hears it then it broadcasts an ACK. All the auxiliary nodes listen for packets. If an auxiliary node hears a packet but does not hear an ACK shortly after, it probabilistically relays the overheard packet to the anchor. Also, when switching anchors the new anchor will "salvage" unsent packets from the old anchor.

The authors show that their proposed algorithm significantly outperforms BRR in all cases and approaches the ideal performance.

Thoughts

The idea proposed in this paper seems very similar to ExOR idea of exploiting overhearing only applied to a set of access points with a "back-plane" instead of a full mesh network.