1. Data Engineering Laboratory, Aristotle University of Thessaloniki Data Pouring and Buffering on the Road - A New Data Dissemination Paradigm for Vehicular Ad Hoc Networks Δημόκας Νικόλαος

2. VANETs New research challenges • Due to fast vehicle movement, the link topology changes rapidly => well studied structures such as tree, clustering, grid are hard to set up and maintain. • The conventional broadcast mechanism may lead to broadcast storm • The vehicle mobility is partially predictable since it is limited by the traffic pattern and the road layout.

3. Push-based Data Dissemination in VANET • Task • Deliver the data to all the vehicles within a given area • Applications • Transportation control • E-advertisement • Emergency announcement • Metrics • Dissemination capacity • The maximum number of data items can be disseminated by the system • Delivery ratio • The percentage of the data items can be received by the vehicles • Network traffic

4. Opportunistic Dissemination (OD) • Idea: opportunistic data exchange • Vehicles store and carry the data • Propagate data to the encountered vehicles and obtain new data in exchange • Dissemination capacity is low. Performance suffers when vehicle density is high • Excessive interference • Hard to schedule the transmission • Too many redundant exchanges

5. Crossing Road C-Road Primary Road P-Road Data Pouring Scheme (DP) • Basic idea: explore road layout and partially predictable vehicle moving pattern • Pour the data along several selected Primary Road (P-Road) by periodic broadcasting • All the roads intersected with the P-Roads are called Crossing Roads(C-Roads) • Vehicles on the C-Road passively receive the data when moving through the intersections on the P-Roads. • Techniques to make it reliable • Invalidation • High overhead

6. Reliable DP • RTS/CTS handshake to reduce collisions and hidden node problem • Sender node rebroadcast the data if it does not hear the rebroadcast from the next forward node • Advantage • Improve data delivery ratio • Disadvantage • Complicates the transmission with more control messages • A lot of bandwidth will be wasted

7. DP with Intersection Buffering (DP-IB) • Basic idea of DP-IB • Each intersection buffers the data, and rebroadcasts periodically • Data center only broadcasts for data invalidation or refresh the lost data copies. • Objective of DP-IB • Reduce the amount of data poured from the source • Increase the broadcast throughput • Find the broadcast cycle time through analysis

8. DP with Intersection Buffering (DP-IB) • Intersection data buffering and rebroadcasting • In the first period the IBer broadcast its buffered data while the forwarding nodes hold their data (busy period) • In the second period, the forwarding nodes forward pending packets (idle period) • Intersection contention avoidance protocol • All nodes switch between two modes: active and inactive forward mode • A node switches to inactive after receiving a broadcast data packet from an IBer • Node switches back to active if it does not receive broadcast packet from the IBer for a time period

9. DP with Intersection Buffering (DP-IB) • n – number of data items • Davg – average data items • m – number of data centers which have stored data at the IBer • Ii – receives the data packet from the ith data center every Ii time interval • T – broadcast cycle of IBer