1. Urban Multi-Hop Broadcast Protocol for Inter-Vehicle Communication Systems Δημόκας Νικόλαος Data Engineering Laboratory, Aristotle University of Thessaloniki

2. VANET Key features: Mobility rate is high Movement direction and speeds are predictable Vehicle enter and leave the network frequently Broadcast is a frequently used method VANET applications relying on broadcast: Traffic, Accident warnings Weather (warning packets generated when the road is slippery) Delivery of advertisements and announcements from hotels, restaurants etc

3. Broadcast Disadvantages of multi-hop broadcast: Packet collisions, hidden nodes, interference It is difficult to disseminate the packets to different road segments due to tall buildings around intersections IEEE 802.11: RTS/CTS handshake and acknowledgement mechanisms decreases the hidden terminal problem and makes the protocol reliable May cause packet storms around the source UMB is designed to address: Broadcast storm Hidden node Reliability problems

4. UMB Key Idea: Directional broadcast. Sender node try to select the furthest node in the broadcast direction to assign the duty of forwarding without any apriori topology information Intersection broadcast. Repeaters at the intersections, forward the packet to all road segments. Assumption: Each node knows the location of itself, intersections and repeaters Goals: Avoiding collisions due to hidden nodes. Using the channel efficiently Making the broadcast communication as reliable as possible Disseminating messages in all directions at an intersection

5. Directional Broadcast (1/3) Divide the road portion inside the transmission range into segments If there is more than one node in the furthest segment, then it is divided If segment based iterations are not sufficient to pick only one node, then they enter to a random phase UMB uses RTB and CTB An RTB packet includes source node position and broadcast direction A node receiving RTB packet Compute the distance to source node Based on the distance, it sends a channel jamming signal, called black-burst Sends its black-burst in the shortest possible time Listens to the channel. If it is empty, then its black-burst was the longest and replies with CTB after CTBTIME.

6. Directional Broadcast (2/3) Receivers send black-burst signals proportional to their distance to the source When there are more than one vehicle in the furthest non empty segment They all find the channel empty and continue to send CTB packets Source node detects the collisions and repeat RTB

7. Directional Broadcast (3/3) The furthest segment is divided into N max sub-segments Only nodes that sent the longest black-burst in the previous iteration can join to the current iteration Random collision resolution phase after the D max iteration Source node goes back to the first segment iteration after a random amount of time

8. Intersection Broadcast (1/2) If the forward node is inside the transmission range of a repeater, the node sends the packet to the repeater using point-to-point IEEE802.11 Packet Loops All cars in the network record the packet IDs when they hear packets Repeaters record the packet IDs

9. Intersection Broadcast (2/2) RTB/CTB/DATA/ACK handshake is repeated several times in intersections Disadvantage Waste bandwidth Degrade the overall performance of the network, since packets from all directions will wait the for the repeater to be idle Solution Repeaters do not repeat the information in the DATA packet if the forward node has already received the message