on the evaluation of caching in vehicular information systems
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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 - PowerPoint PPT PresentationTRANSCRIPT
On the Evaluation of Caching in Vehicular Information Systems
N. Loulloudes, G. Pallis, M. D. DikaiakosDepartment of Computer Science,
University of Cyprus
July 2nd 20109th Hellenic Data Management Symposium
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Introduction
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
Traffic Accidents (EU only) 39.200 deaths 3.3 million casualties €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:1. Provide traffic conditions monitoring and hazard warnings2. Discover the availability of road-side facilities (parking, gas stations)
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Cellular Networks
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
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
Vehicular Ad-Hoc Networks
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Vehicular Information Systems (VIS)
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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
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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:
1. Dispatch-Query Phase 2. VAHS-Computation Phase3. Dispatch-Reply Phase4. Reply-Delivery Phase
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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Dispatch-Query Phase
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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Dispatch Query Phase ModificationWhen 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.
Incoming Query
Reply Forward Incoming Query
HIT
MISS
Lookup
Cache
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VAHS Computation Phase
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
S
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Dispatch-Reply Phase
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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Dispatch-Reply Phase Modification
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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 oneElse if there is a CACHE MISS Cache new messageForward Reply to Source
Incoming Reply
Forward Incoming Reply
Lookup
Cache
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Reply-Delivery Phase
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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.
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Directive Value Description
Cacheable Boolean The reply can be cached
Expires Time-stamp The time after the reply is considered expired (if cached)
Private Boolean The cached reply can be reused only from the original peer that requested it
Public Boolean The cached reply can be reused by any peer
Validate-after Seconds A peer must validate the cached reply with the target area after “n” seconds since the reply generation time
P-validate-after Boolean A peer must validate the cached reply with the neighbour peers after “n” seconds since the reply generation time
P-validate Boolean A peer must validate the cached reply with the neighbour before servicing a request
Retransmit Boolean Serve incoming request first from cache and retransmit it to target area also
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Problem Statement
Can the cached enabled-VITP maintain acceptable levels of information quality while sustaining
network performance?
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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Simulation Test bed Setup
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
• 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
TrafficModeller SUMO
Vehicular Mobility Generation
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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
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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Evaluation Scenarios and Query Generation
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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 ?
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Evaluation Scenarios and Query Generation
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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?
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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.
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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Evaluation – Querying Road Traffic Conditions
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
• 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.
Round-TripTime
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Evaluation – Querying Road Traffic Conditions
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
• 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.
Query Recall
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Evaluation – Querying Road Traffic Conditions
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
• 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
Information Accuracy
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Evaluation – Querying Road Traffic Conditions
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
• 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
ExchangedMessages
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Evaluation – Querying Road Traffic Conditions
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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Evaluation Scenarios and Query Generation
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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?
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Evaluation – Querying Road-Side Facilities
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
• 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.
Query Recall
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Evaluation – Querying Road-Side Facilities
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
• 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
RSUInformationAccuracy
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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%.
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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The Dynamics of Vehicular Networks in Urban Environments
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
G(t) = undirected graph of VANET at time t
V = {Ui) - vehicles.E = {Eij} – communication links among vehicles
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Data – Knowledge Perspective
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
• 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
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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?”
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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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
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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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
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
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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)
July 2nd 2010 N. Loulloudes - On the Evaluation of Caching in Vehicular Information Systems
Thank You !!!
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