Interactive WiFi Connectivity For Moving Vehicles

A. Balasubramanian, R. Mahajan, A. Venkataramani, B. N. Levine, J. Zahorjan, "Interactive WiFi Connectivity For Moving Vehicles", ACM SIGCOMM Conference, (August 2008).

One line summary: This paper describes a protocol for providing WiFi connectivity to moving vehicles called ViFi.

Summary

This paper introduces ViFi, a protocol for providing WiFi connectivity to moving vehicles with minimal disruptions. ViFi achieves this by allowing a vehicle to use several base stations (BSes) simultaneously so that handoffs are not hard handoffs. The paper begins by evaluating six different types types of handoff policies on their VanLAN testbed at the Microsoft Redmond campus: (1) RSSI—the client associates with the base station with the highest received signal strength indicator, (2) BRR—the client associate with the base station with the highest beacon reception ratio, (3) Sticky—the client doesn’t disassociate with a base station until connectivity is absent for some defined period of time, then chooses the one with the highest signal strength, (4) History—the client associates with the base station that has historically provided the best performance at that location, (5) BestBS—the client associates, in a given second, with the base station that will provide the best performance in the next second, and (6) AllBSes—the client opportunistically uses all base stations in the vicinity. The first five policies use hard handoffs. The last is an idealized method that represents an upper bound. The metrics used to judge these policies are aggregate performance and periods of uninterrupted connectivity. Sticky performs the worst, and RSSI, BRR, and History perform similarly. AllBSes is by far the best. They also observe gray periods in which connection quality drops sharply and unpredictably.

The authors explain that the effectiveness of AllBSes stems from the fact that the vehicle is often in the range of multiple BSes and packet losses are bursty and roughly independent across senders and receivers. They design ViFi as a practical version of AllBSes. In ViFi, there is an anchor base station and a set of auxiliary base stations. The identities of these are embedded in the beacons that the vehicle broadcasts periodically, along with the identity of the previous anchor. An auxiliary base station attempts to relay packets that have not been received by the destination. This is done with according to an estimated probability that is computed according to the following guidelines: (1) relaying decisions made by other potentially relaying auxiliaries must be accounted for, (2) auxiliaries with better connectivity to the base station should be preferred, and (3) the expected number of relayed transmissions should be limited. ViFi also allows for salvaging, in which new anchors contact the previous anchor for packets that were supposed to be delivered to the vehicle but weren’t able to be delivered before the vehicle moved out of range. ViFi uses broadcasts and implements its own retransmits. It doesn’t implement exponential backoff because it assumes collision will not be a problem. To evaluate ViFi the authors deploy it on VanLAN and do a trace-driven simulation using measurements from another testbed called DieselNet. The most important findings according to the authors are: (1) the link layer performance of ViFi is close to ideal, (2) ViFi improves application performance twofold over current handoff methods, (3) ViFi places little additional load on the vehicle-base station medium, and (4) ViFi’s mechanism for coordinating relays has low false positive and false negative rates.

Critique

I didn’t find this paper very exciting but I still thought there were a lot of good things about it. The idea behind ViFi isn’t all that novel since it has been used in cellular networks. I liked that in this paper the authors first examined a variety of techniques and gave results for those. One thing I am confused about is how they implemented AllBSes to test it considering they argued that it is impractical to implement and represents the ideal. In general, the paper seemed to have a lot of evaluation in it, which I liked. Although ViFi did perform well in the settings in which it was tested, overall, I’m not sure how practically useful it is. For example, they assume that collisions are not an issue. If ViFi were to become successful and widely deployed it’s possible that collisions might become a problem, and they don’t even attempt to consider situations in which there might be collisions. Also, I can only imagine people wanting to connect to the Internet if they are going fairly long distances but as the authors mention in the paper, WiFi deployments may not be broad enough and lack of coverage between islands may make it unusable. There are several other deployment challenges too, and not just those mentioned in the paper. I just can’t imagine it being desirable in many settings. I don’t see cities wanting to invest in this. I’m not sure you could get a lot of other people besides the city government to freely provide access. It could be useful to deploy many ViFi base stations along a commuter rail line though, but in that case, since you know the speed, location, and schedule of the moving vehicles you could probably do even better than ViFi.