Vehicular Network Applications

1. Vehicular Network Applications • VoIP • Web • Email • Cab scheduling • Congestion detection • Vehicle platooning • Road hazard warning • Collision alert • Stoplight assistant • Toll collection • Deceleration warning • Emergency vehicle warning • Border clearance • Traction updates • Flat tire warning • Merge assistance

2. Congestion Detection • Vehicles detect congestion when: • # Vehicles > Threshold 1 • Speed < Threshold 2 • Relay congestion information • Hop-by-hop message forwarding • Other vehicles can choose alternate routes

3. Deceleration Warning • Prevent pile-ups when a vehicle decelerates rapidly

4. Wireless Technologies for Vehicular Networks • Cellular networks • High coverage, low bandwidth, expensive • WiFi networks • Moderate coverage, high bandwidth, free • Combine all of them to achieve low cost, high bandwidth, and high coverage

5. InteractiveWiFi Connectivity from Moving Vehicles Aruna Balasubramanian, Ratul Mahajan Arun Venkataramani, Brian N Levine, John Zahorjan University of Massachusetts Amherst Microsoft Research University of Washington

6. Target Scenarios • A car is within the range of multiple APs • How common? • Low data rate but low delay • Alternatives?

7. Overview • Given enough coverage, can WiFi technology be used to access mainstream applications from vehicles? • Existing work shows • the feasibility of WiFi access at vehicular speeds • focus on non-interactive applications. e.g., road monitoring Internet

8. Outline • Can popular applications be supported using vehicular WiFi today? • Performance is poor due to frequent disruptions • How can we improve application performance? • ViFi, a new handoff protocol that significantly reduces disruptions • Does ViFi really improve application performance? • VoIP, short TCP transfers

9. VanLAN: Vehicular Testbed Uses MS campus vans Base stations(BSes) are deployed on roadside buildings Currently 2 vans, 11 BSes

10. Measurement study • Study application performance in vehicular WiFi setting • Focus on basic connectivity • Study performance of different handoff policies • Trace-driven analysis • Nodes send periodic packets and log receptions

11. Handoff policies studied • Practical hard handoff • Associate with one BS • Current 802.11 • Ideal hard handoff • Use future knowledge • Impractical

12. Handoff policies studied • Practical hard handoff • Associate with one BS • Current 802.11 • Ideal hard handoff • Use future knowledge • Impractical • Ideal soft handoff • Use all BSes in range • Performance upper bound

13. Comparison of handoff policies Disruption • Summary • Performance of interactive applications poor when using existing handoff policies • Soft handoff policy can decrease disruptions and improve performance of interactive applications Practical hard handoff Ideal hard handoff Ideal soft handoff

14. Outline • Can popular applications be accessed using vehicular WiFi? • How can we improve application performance? • ViFi, a practical diversity-based handoff protocol • Does ViFi really improve application performance? • VoIP, short TCP transfers

15. Design a practical soft handoff policy • Goal: Leverage multiple BSes in range • How often do we have multiple BSes? • Not straightforward • Constraints in Vehicular WiFi • 1. Inter-BS backplane often bandwidth-constrained • 2. Interactive applications require timely delivery • 3. Fine-grained scheduling of packets difficult Internet

16. Why are existing solutions inadequate? • Opportunistic protocols for WiFi mesh (ExOR, MORE) • Uses batching: Not suitable for interactive applications • Path diversity protocols for enterprise WLANs (Divert) • Assumes BSes are connected through a high speed back plane • Soft handoff protocols for cellular (CDMA-based) • Packet scheduling at fine time scales • Signals can be combined

17. ViFi protocol set up • Vehicle chooses anchor BS • Anchor responsible for vehicle’s packets • Vehicle chooses a set of BSes in range to be auxiliaries • e.g., B, C and D can be chosen as auxiliaries • ViFi leverages packets overheard by the auxiliary A Internet B C D

18. ViFi protocol • Source transmits a packet • If destination receives, it transmits an ack • If auxiliary overhears packet but not ack, it probabilistically relays to destination • If destination received relay, it transmits an ack • If no ack within retransmission interval, source retransmits Source Dest Downstream: Anchor to vehicle A A Dest B B D C D Source C Upstream: Vehicle to anchor

19. Why relaying is effective?

20. Why relaying is effective? • Losses are bursty • Independence • Losses from different senders independent • Losses at different receivers independent A A Upstream B B Downstream D C C D 20

21. Guidelines for probability computation • 1. Make a collective relaying decision and limit the total number of relays • 2. Give preference to auxiliary with good connectivity with destination • How to make a collective decision without per-packet coordination overhead?

22. Determine the relaying probability • Goal: Compute relaying probability RB of auxiliary B • Step 1: The probability that auxiliary B is considering relaying • CB = P(B heard the packet) . P(B did not hear ack) • Step 2: The expected number of relays by B is • E(B) = CB¢RB • Step 3: Formulate ViFi probability equation,  E(x) = 1 • to solve uniquely, set RB proportional to P(destination hears B) Step 4: B estimates P(auxiliary considering relaying) and P(destination heard auxiliary) for each auxiliary • ViFi: Practical soft handoff protocol uses probabilistic relaying for coordination without per-packet coordination cost

23. ViFi Implementation • Implemented ViFi in windows operating system • Use broadcast transmission at the MAC layer • No rate adaptation • Deployed ViFi on VanLAN BSes and vehicles

24. Outline • Can popular applications be accessed using vehicular WiFi? • Due to frequent disruptions, performance is poor • How can we improve application performance? • ViFi, a practical diversity-based soft handoff protocol • Does ViFi really improve application performance?

25. Evaluation • Evaluation based on VanLAN deployment • ViFi reduces disruptions • ViFi improves application performance • ViFi’s probabilistic relaying is efficient • Also in the paper: Trace-driven evaluation on DieselNet testbed at UMass, Amherst • Results qualitatively consistent

26. ViFi reduces disruptions in our deployment ViFi Practical hard handoff

27. ViFi improves VoIP performance • Use G.729 codec > 100% ViFi seconds Practical hard handoff Length of voice call before disruption Disruption = When mean opinion score (mos) is lower than a threshold

28. ViFi improves performance of short TCP transfers • Workload: repeatedly download/upload 10KB files > 50% > 100% ViFi Practical hard handoff Number of transfers before disruption Median transfer time (sec) Disruption = lack of progress for 10 seconds

29. ViFi uses medium efficiently • Efficiency: • Number of unique packets delivered/ Number of packets sent • It’s efficient for their testbed, but may not be the case in general. Why? efficiency ViFi Practical hard handoff

30. Conclusions • Improves performance of interactive applications for vehicular WiFi networks • Interactive applications perform poorly in vehicular settings due to frequent disruptions • ViFi, a diversity-based handoff protocol significantly reduces disruptions • Experiments on VanLAN shows that ViFi significantly improves performance of VoIP and short TCP transfers

31. Comments • Interesting problem domain • Target low-bandwidth applications, for which cellular networks are sufficient • Have multiple APs within range tuned into the same channel • May not be common and lose spatial diversity • Use the lowest data rate • Common to have multiple or fewer than 1 relay(s) for each tx • Relay is not compelling • Uplink: sufficient to relay data to one AP • Downlink: if best AP is selected, the need for relay is low • If relay has to be used, MORE like opportunistic routing may be more efficient • They dismissed opportunistic routing due to its potential large delay due to batch • But their delay can be high since retx timeout is generally large in order to account for variable contention delay

33. Motivation • People want to communicate while on the move • Average one way commute (2005): • US: 24.3min, World: 40min • Passengers want to watch videos, listen to songs, etc. • Why not just use cellular networks? • Expensive: $30-$60/month • 5GB/month -> 2Kbps! • 40% 3G capable devices have no 3G plan • iPod Touch sales ~ iPhone sales • Bandwidth and backhaul limitations • Limited video quality (96-128kbps, < 10min long) • Carriers interested in WiFi offloading • Arms race between • Increase in cellular bandwidth • Higher resolution screens and videos • Goal: Enable high bandwidth applications (e.g., video) in vehicular networks via WiFi ✗ ✔

34. Opportunistic WiFi connectivity • Gas stations and local shops deploy APs Internet • Devices in vehicles contact roadside APs • Passengers watch videos, download files • Compelling usage scenario • Taxi companies provide value-added services to passengers • Previous work: low-bandwidth applications • We focus on delivering high-bandwidth content • e.g. video streaming

35. Synergy among connections High b/w, low coverage High b/w, short-lived Mesh Network Wireless LAN VCD High b/w, persistent Internet Access Vehicle Relay Low b/w, persistent High b/w, high delay 35

36. Contributions New techniques for replication optimization Goal: Fully utilize wireless bandwidth during contact Optimized wireline replication to Internet-connected APs Replication using vehicular relays to unconnected APs Use mesh for replication and caching New algorithm for mobility prediction Predict set of APs that will be visited by vehicle Critical for success of replication techniques Algorithm: voting among K nearest trajectories

37. Trace-driven simulation and emulation San Francisco cabs, Seattle buses, Shanghai cabs Two testbeds on UT campus 802.11b: 14 APs deployed inside 8 campus buildings, 20-60ft from the road 802.11n: 4 APs outdoor, 1-5ft from the road Smartphone and laptop clients HP iPAQ and HTC Tilt Stream H.264 videos at 64Kbps Evaluation Methodology 13, 14 3 4 5 6 7,8,9,10, 11,12 2 1

38. Summary: Vehicular Content Distribution • KNT: A new mobility prediction algorithm • Based on voting among K nearest trajectories • 25-94% more accurate than 1st and 2nd order Markov models • A series of novel replication schemes • Optimized wireline replication and mesh replication • Opportunistic vehicular relay based replication • Extensive evaluation: simulation + testbed + emulation • Simulation using San Francisco taxi and Seattle bus traces • 3-6x of no replication, 2-4x of wireline or vehicular alone • Full-fledged prototype deployed on two real testbeds • 14-node 802.11b testbed and 4-node 802.11n testbed • 4.2-7.8x gain over no replication • Emulab emulation with real AP/controller and emulated vehicles • Show system works at scale and is efficient • Validate our trace-driven simulator