1 Interactive WiFi Connectivity For Moving Vehicles Presented by Zhou Yinggui

2 Cellular networks tend to be expensive 1 2 Background the ubiquity of WiFi

3 Background Can WiFi deployments support common applications from moving vehicles ? Vi-Fi

4 Introduction 1 Outlines Experimental platform 2 Handoff strategies 3 ViFi design & implementation 4 Evaluation 5 Conclusion 6

5 Introduction  ViFi is a protocol that minimizes disruptions to support interactive applications.  Hard Handoff --Clients communicate with only one basestation at any given time.

6  Hard handoffs are limited by 1.gray periods --- connectivity drops sharply and unpredictably; 2.the difficulty of estimating the continuously changing channel quality.  Hard handoff methods has frequent disruptions even when clients are close to WiFi basestations. Introduction

7  Diversity--using multiple basestations simultaneously.  ViFi exploits diversity and opportunistic receptions to minimize disruptions. Introduction

8 Experiment  Experiments are performed on two vehicular mobile network testbeds: VanLAN and DieselNet.  DieselNet--Downtown of Amherst, Massachusetts trace-driven study  VanLAN--Microsoft campus in Washington. live experiments.

9 VanLAN Eleven basestations (BSes) Two vehicles equipped with Atheros 5213 chipset, omnidirectional antennae, GPS unit. The box covers 828×559 m2 area. in which at least one packet is received by vehicles from any BS. Not all pairs of BSes are within wireless range of one another. The vehicles shuttle around the campus ten times a day, with a speed of 40 Km/h. Experiment

10 The equipment of vehicles are the same with VanLAN log all beacons heard from nearby BSes traces from Channels 1 and 6 log more than 100,000 beacons in 3 days Only analysis to BSes in the core of the town and to BSes that are visible on all three days DieselNet Experiment

11 Handoff strategies Using a trace-driven evaluation on VanLAN Each BS and vehicle broadcasts a 500-byte packet at 1Mbps every 100 ms. criterions: aggregate performance--total number of packets delivered and the total time or distance… periods of uninterrupted connectivity--contiguous time intervals when the performance of an application is above a threshold.

12  hard handoff 1.RSSI:client associates to BSes with higher signal strength. 2.BRR:client associates to the BS with the highest strength. 3.Sticky:client does not disassociate from the current BS until connectivity is absent. 4.History:client associates to the BS that has historically provided the best average performance. 5.BestBS:not practical, represents an upper bound of hard handoff. AllBSes:client opportunistically uses all BSes.Ideal method that represents an upper bound of any handoff protocol  diversity methods Handoff strategies

13 Handoff strategies Aggregate Performance pick BRR as representative Average number of packets delivered per day in Van-LAN by various methods.

14 adequate (uninterrupted) connectivity: at least 50% of the packets are received in a one-second interval. black dot (gray periods) ： areas that connectivity drops sharply and unpredictably Handoff strategies Uninterrupted Connectivity go to 34

15 connectivity is often undermined by gray periods even close to BSes. gray periods tend to be short- lived, and do not severely impact aggregate performance. Handoff strategies Uninterrupted Connectivity

16 other definitions of adequate connectivity Handoff strategies Uninterrupted Connectivity

17 The insight of losses upstream direction losses are roughly independent across BSes and a packet sent by the vehicle is received by at least one BS with a high probability. the conclusion has been shown previously by S. Biswas and R. Morris. ExOR: opportunistic multi-hop routing for wireless networks. In SIGCOMM, Aug Handoff strategies

18 downstream direction gray period:even when a vehicle is associated to a BS with a low average loss rate, it can lose many packets in a small time period,hurting interactive applications. The insight of losses Handoff strategies

19 most burst losses are path dependent rather than receiver dependent. The insight of losses Handoff strategies downstream direction

20 ViFi protocol environment Diversity: A packet sent by a moving vehicle can often be heard by multiple BSes, and multiple BSes can often deliver packets to a moving vehicle. Bandwidth-limited inter-BS communication

21 Motivated by AllBSes imposes minimal additional load does not increase per packet latency can handle rapidly changing sets of BSes. ViFi protocol

22 the vehicle designates one of the nearby BSes as the anchor(by BRR). The vehicle designates other nearby BSes as auxiliary. The vehicle embeds the identity of the current anchor and auxiliary into the beacons which are broadcasted periodically. The vehicle also embeds the identity of the previous anchor for salvaging. Protocol overview ViFi protocol

23 1. src transmits the packet P. 2. If dst receives P, it broadcasts an ACK. 3. If an auxiliary overhears P, but within a small window has not heard an ACK, it probabilistically relays P. 4. If dst receives relayed P and has not already sent an ACK, it broadcasts an ACK. 5. If src does not receive an ACK within a retransmission interval, it retransmits P. Protocol overview ViFi protocol

24 1. src transmits the packet P. 2. If dst receives P, it broadcasts an ACK. 3. If an auxiliary overhears P, but within a small window has not heard an ACK, it probabilistically relays P. 4. If dst receives relayed P and has not already sent an ACK, it broadcasts an ACK. 5. If src does not receive an ACK within a retransmission interval, it retransmits P. Upstream packets are relayed on the inter-BS backplane and downstream packets on the vehicle-BS channel why relaying is better than a retransmission ? Protocol overview ViFi protocol

25 Computing relaying probability balance the trade-off between false negative(no diversity) and false positive (excessive load) The guidelines of probability computation G1: Account for relaying decisions made by other potentially relaying auxiliaries. G2: Prefer auxiliaries with better connectivity to the destination. G3: Limit the expected number of relayed transmissions. ViFi protocol

26 The overall strategy is to compute relaying probabilities locally and the expected number of packets is equal to 1. C i is the B i’s probability that B i has heard the packet but not an acknowledgment r i is B i’s relay times （ relay probability ） which is less than one. Computing relaying probability ViFi protocol

27 P ab represents the probability that b correctly receives a transmission from a Computing relaying probability ViFi protocol

28 C i = P src,Bi (1 − P src,dst P dst,Bi ) Ci is the Bi’s probability that Bi has heard the packet but not an acknowledgment(assumed independent) Computing relaying probability ViFi protocol

29 Opportunistic receptions provide a low-overhead but unreliable means. With probabilistic relaying, each BS relays based on an independently computed relaying probability, which avoids the need for explicit coordination messages between BSes. The resulting protocol is lightweight, decentralized,simple and works well. ViFi protocol

30 Salvaging Sometimes a vehicle moves out of range before the anchor BS can deliver packets from the Internet. newly designated anchors salvage packets by contacting the previous anchor over the backplane. the old anchor transfers any unacknowledged packets within a certain time threshold(based on the minimum TCP retransmission timeout,3s). ViFi protocol

31 current incoming reception probability packet reception probability from them to other nodes(learn from other nodes) Estimating packet reception probabilities using beacons P ab:the number of beacons received in a given time interval divided by the number that must have been sent. Beacons ViFi protocol

32 close to ideal two-fold compared to current methods low false positive and false negative rates Evaluation link- layer interactive application coordination mechanism

33 Link-layer performance Evaluation

34 Link-layer performance Evaluation back to 14

35 most of ViFi’s gain is a result from diversity. Given that only 1.2% of the packets are salvaged, this benefit of salvaging is disproportionate （ 10% ）. application performance Evaluation TCP

36 VoIP VoIP is more challenging than TCP because quality is sensitive to both loss and delay. The standard for evaluating a voice call is the Mean Opinion Score (MoS) application performance Evaluation

37 interruption:the MoS value drops below 2 for a three-second period. ViFi average MoS is 3.4 compared with 3.0 of BRR. VoIP application performance Evaluation

38 Effectiveness of coordination Evaluation

39 Conclusion 1.current WiFi is unsuitable for vehicular client 2.firstly studyed basestation diversity ， then designed ViFi 3.key to its effectiveness is a decentralized probabilistic algorithm 4.excellent link-layer performance 5.doubled the number of successful TCP transfers and the length of disruption-free VoIP calls

40 Appendix Extent of diversity Efficiency of medium usage Limitations Deployment