This paper describes ViFi, a protocol for minimizing interruptions in connectivity of WiFi in moving vehicles. The authors describe six policies for handoffs between base stations: RSSI (highest signal strength BS), BRR (highest beacon reception ratio), Sticky (stay with BS until connectivity drops too low), History (BS with historically best performance), BestBS (BS that will be best in future), and AllBSes (utilize all BSes in range). AllBSes represents an upper bound to performance; if packets are lost for one BS, there will likely be another that received them, particularly in situations involving bursty loss.
ViFi works by having the vehicle designate a BS as its anchor, acting as the vehicle's gateway to the Internet, and other BSes in range of the vehicle are auxiliaries. The vehicle periodically broadcasts beacons that contain info about the current anchor and auxiliaries as well as the previous anchor. ViFi is actually deployed in VanLAN, and the authors use traces collected from DieselNet for simulations as well.
ViFi employs the techniques of diversity and salvaging to improve session connections; diversity is the use of multiple basestations to deliver packets, and salvaging is the transmitting of unacknowledged data from the Internet held in previous anchors. Some of the difficulties in ViFi are similar to those in the ExOR system, including coordination between nodes.
The algorithm for transmission works as follows: if the intended destination receives a packet, it sends an ACK. Any auxiliaries that overhear the packet and don't hear the ACK within a certain period probabilistically relay the packet. These relaying auxiliaries must take into account other auxiliaries that might be relaying as well as quality of connectivity, all while trying to keep the expected number of relays to a minimum. Packet reception rates are based on the beacons the vehicle sends.
ViFi doesn't do any packet reordering, as the authors claim it's not necessary for good performance, but it might have been nice to see some numbers about how frequent reordering is. The graphs involving session length took me a little longer to understand than I would have liked; I think there may be more intuitive ways of showing how good the connections were.
On the whole, ViFi seems like a pretty good idea and gets good results for short TCP transfers and VoIP sessions. I wonder how representative of a typical vehicle workload these tests are, though. Certainly short TCP transfers as from web surfing makes sense, but how often are people Skyping on a Microsoft shuttle bus? I liked that the paper talked about the limitations of ViFi; unfortunately, it sounds like large-scale deployment would be very difficult, and if ViFi were only implemented in large islands, maintaining connectivity could be complex.
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