Urban Multi-Hop Broadcast Protocol for Inter-Vehicle Communication Systems

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

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 Nmax 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 Dmax 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