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On the Evaluation of Caching in Vehicular Information Systems

On the Evaluation of Caching in Vehicular Information Systems. N. Loulloudes, G. Pallis, M. D. Dikaiakos Department of Computer Science, University of Cyprus. Introduction. Traffic Accidents (EU only) 39.200 deaths 3.3 million casualties

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On the Evaluation of Caching in Vehicular Information Systems

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  1. On the Evaluation of Caching in Vehicular Information Systems N. Loulloudes, G. Pallis, M. D. Dikaiakos Department of Computer Science, University of Cyprus 9th Hellenic Data Management Symposium

  2. Introduction • Traffic Accidents (EU only) • 39.200deaths • 3.3 millioncasualties • €180 billion in material losses (2x the total EU budget for all activity) • Search for free parking place (EU only) • 44%of entire traffic is searching for a free parking place • €3.5 million for gasoline and diesel are spend annually • 150.000 hours of waiting time annually • Vehicular Information Systems can: • Provide traffic conditions monitoring and hazard warnings • Discover the availability of road-side facilities (parking, gas stations) N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  3. Vehicular Ad Hoc Networks (VANETs) • Sub-class of MANETs • Characteristics • Very high mobility • Frequent topology changes and network fragmentation • Ample power, process and storage capabilities Cellular Networks Vehicular Ad-Hoc Networks N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  4. Vehicular Information Systems (VIS) Impose a high network overhead in order to obtain and maintain global or partial view of the vehicular environment conditions Saturation of limited network resources Low response times, Low information quality Robust mechanisms for efficient information dissemination N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  5. Research Motivation • Key Question • “Does the co-existence of a pro-active, location-aware, communication protocol and caching maintain acceptable levels of information quality while sustaining network performance? “ • Vehicular Information Transfer Protocol (VITP) • A pro-active, location-aware, application layer communication protocol for Vehicular Computing • VITP specifies the syntax and semantics of messages exchanged between VITP peers. [Dikaiakos et al. JSAC’07] • VITP Transaction Phases: • Dispatch-Query Phase • VAHS-Computation Phase • Dispatch-Reply Phase • Reply-Delivery Phase N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  6. Dispatch-Query Phase N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  7. Incoming Query Cache Reply Forward Incoming Query Dispatch Query Phase Modification • When a peer receives a query checks to see if it can be served from the cache. • If there is a CACHE HIT update cached object’s “last_accessed” parameter  Generate reply containing the cached data and send to source node. • Else if there is a CACHE MISS  forward query towards destination. HIT Lookup MISS N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  8. VAHS Computation Phase S N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  9. Dispatch-Reply Phase N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  10. Incoming Reply Cache Forward Incoming Reply Dispatch-Reply Phase Modification • When a peer receives a reply checks whether the message is already cached locally. • If there is a CACHE HIT Update cached object with the new one • Else if there is a CACHE MISS  Cache new message • Forward Reply to Source Lookup N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  11. Reply-Delivery Phase N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  12. Enabling Caching Support in VIT • VITP message syntax extended to support cache-based location aware service - Introduce Cache – Control Headers • Headers act as directives to VITP peer caching decisions. N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  13. Problem Statement Can the cached enabled-VITP maintain acceptable levels of information quality while sustaining network performance? N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  14. Simulation Test bed Setup Vehicular Mobility Generation TrafficModeller SUMO • Simulated traffic in two real cities with different topological layouts • 970 and 875 mobility traces respectively for the above cities were generated using TrafficModeller and SUMO N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  15. Simulation Test bed Setup • Network Simulation Setup • Performed using ns-2.33 • Vehicles are equipped with IEEE 802.11 communication hardware (200m coverage) and are VITP enabled • Simulation duration set to 1000s • 200s warm up phase to allow caches to reach a level of stability N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  16. Evaluation Scenarios and Query Generation Scenario 1: Vehicles are aware of the road topology. Use a “forward scan” query system to identify road conditions from current position to destination • Objective: • Can cached-based VITP maintain an acceptable level of traffic information quality and minimize network overhead caused by traffic queries ? N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  17. Evaluation Scenarios and Query Generation Scenario 2: Same as before. Unscheduled events (i.e. accidents) take place in the candidate road-paths to destination • Objective: • Can cache-based VITP accurately capture the traffic conditions and the existence of un-scheduled events? N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems On the Evaluation of Caching in Vehicular Information Systems HDMS, July 2nd 2010 19

  18. Metrics • Query Recall: the number of replies received while issuing queries towards a specific location of interest over the number of replies that should have been received • Response Time: average Round Trip Time in seconds of a successful VITP transaction • Information Accuracy: how close the received value describing some information at a location of interest is to the actual value • Number of Exchanged Messages: total number of exchanged messages, including geographic routing message and VITP query resolution messages. N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  19. Evaluation – Querying Road Traffic Conditions Round-Trip Time • Longer TTL  better information diffusion  increased probability that information is found from vehicle’s cache. • Due to geographic routing, queries traverse a greater number of hops to overcome break-downs and reach target location. • RTT improves up to 31% for Region 1 and up to 27%for Region 2. N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  20. Evaluation – Querying Road Traffic Conditions Query Recall • As TTL increases  replicated information in the network increases  queries are served from information in vehicle’s cache. • Increased probability that otherwise unresolved queries will be resolved from vehicle’s cache. N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  21. Evaluation – Querying Road Traffic Conditions Information Accuracy • As TTL increases  queries resolved from cache  accuracy drops  Cached items do not reflect accurately road conditions • Accuracy is heavily influenced by un-scheduled events and road topology. • Cache-enabled VITP can maintain high level of accuracy: up to 83% for Region 1 and up to 33%for Region 2 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  22. Evaluation – Querying Road Traffic Conditions Exchanged Messages • Cached-based VITP reduces network overhead • Up to 21% for Region 1 • Up to 27% for Region 2 • Fewer exchanged messages  fewer network failures  increased network reliability N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  23. Evaluation – Querying Road Traffic Conditions N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  24. Evaluation Scenarios and Query Generation Scenario 3: Vehicles issue queries to discover road-side facilities (RSUs) such as gas-stations. RSUs broadcast information at fixed time intervals • Objective: • Can cached-based VITP locate and aid in the dissemination of information broadcasted by RSUs? N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems On the Evaluation of Caching in Vehicular Information Systems HDMS, July 2nd 2010 26

  25. Evaluation – Querying Road-Side Facilities Query Recall • Lack of a diffusion mechanism  very low probability to retrieved information broadcasted by RSU (i.e.. T=0s) • As TTL increases  query recall increases  caching can diffuse RSU information. • Increases probability that RSU related queries will be resolved. N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  26. Evaluation – Querying Road-Side Facilities RSU Information Accuracy • Even for low TTL (50s), query recall up to 52% is achieved with information accuracy up to 74% • Increasing TTL  decreases RSU information accuracy • While TTL = 200s gives recall up to 72% for Scenario 2, it only manages to reflect 53% of the information accuracy N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  27. Conclusions • With cache-based VITP: • Acceptable levels of information quality are maintained • Accuracy up to 65%. • Network overhead is reduced • Under normal traffic conditions where vehicle mobility presents periodicity • In the presence of un-scheduled traffic events (no periodicity) • Dissemination of RSU information without any diffusion mechanism is infeasible. • Cache-based VITP allows the diffusion of such information • Even for low TTL, it achieved a query recall up to 52% with information accuracy of up to 74%. N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  28. The Dynamics of Vehicular Networks in Urban Environments G(t) = undirected graph of VANET at time t V = {Ui) - vehicles. E = {Eij} – communication links among vehicles N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  29. Data – Knowledge Perspective • Which effects does mobility and road topology impose on the VANET ? • How can these be exploited to improve inter-vehicle communication? • Study the spatio-temporal evolution of the VANET communication graph N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  30. Engineering Perspective • Routing Protocol Design • “Which are the highest-quality vehicles to carry out the forwarding process?” • “Which are the bridge nodes so as to deliver messages when the network is fragmented?” • Geo-casting (location multi-casting) • “How can we spread the message with the minimal number of rebroadcasts so as to reduce collisions and latency?” • Road-Side Unit Placement • “What is the distribution of the position of vehicles?” N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  31. Graph Metrics Examined • Network Oriented metrics • Degree, Diameter, Graph Density • Centrality Metrics • Betweeness Centrality, Lobby Index • Clustering Metrics • Clusters, Clustering Co-efficient, Communities, Conductance • Link-Level Metrics • Link Duration, Connected Periods, Re-Healing Periods N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  32. Traffic Data Studied • Real GPS traces [CabSpotting Project] • Realistic Vehicular Traces • Use of tools that generate realistic traces from highly accepted mobility models [VanetMobiSim] • Mobility traces on different road topologies • Simulation of different wireless communication standards • IEEE 802.11a (75m) - 802.11p (300m) • Market Penetration (20% - 100%) • Presence of RSUs • Recorded and studied 742000 snapshots of the VANET graph N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  33. General Observations • The VANET communication graph exhibits small world properties • At 300m  average degree of separation: ~8 hops • At 300m transmission ranges, the VANET includes a giant cluster occupying a large portion of the geographic space • Existence of communities – “Data islands” • At 75m transmission ranges 3 or more RSUs per Km2 are required to maintain stable cluster connectivity • Road topology affects the existence of “central” nodes • Lobby index can identify “central” nodes • Large link durations (~34s at 75m - ~117s at 300m) N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

  34. Thank You !!! N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems

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