Congestion Control in VANETs

Participants : Paul Muhlethaler, Anis Laouiti.

We have reviewed the schemes of Congestion Control in VANETs for safety messages. The solutions proposed are: to adapt the generation rate, to adapt the transmission power or to adapt the carrier sense threshold. Some mechanisms employ different states depending on the channel load. Some other schemes use recursive adaptation of their parameters (e.g. LIMERIC). According to a few studies the recursive adaptation system provide a better adaptation of the VANET to the channel load. We will study how the transmission rate and the carrier sense threshold (or transmission power) can be best adapted in order to send CAM: Car Awareness Messages with the highest rate and to the furthest vehicles while maintaining the total load below a given threshold. We will also study the better combination of transmission rate and the carrier sense threshold for the CAM.

TDMA schemes for VANETs

Participants : Mohamed Hadded, Paul Muhlethaler, Anis Laouiti.

This is a joint work with Leila Saidane and Rachid Zabrouba from ENSI (Tunisia).

Vehicular Ad-hoc NETworks (VANETs) help improving traffic safety and efficiency. Each vehicle can exchange information to inform other vehicles about the current status of the traffic flow or a dangerous situation such as an accident. Road safety and traffic management applications require a reliable communication scheme with minimal transmission collisions, which thus increases the need for an efficient Medium Access Control (MAC) protocol. However, the MAC in a vehicular network is a challenging task due to the high speed of the nodes, frequent changes in topology, the lack of an infrastructure, and various QoS requirements. Recently several Time Division Multiple Access (TDMA)-based medium access control protocols have been proposed for vehicular ad hoc networks in an attempt to ensure that all the vehicles have enough time to send safety messages without collisions and reducing end-to-end delay and packet loss rate. We have identified the reasons for using the collision-free medium access control paradigm in VANETs. We have then presented a novel topology-based classification and we provide an overview of TDMA-based MAC protocols that have recently been proposed for VANETs. We have focus on the characteristics of these protocols as well as their benefits and limitations. Finally we have given a qualitative comparison, and we have discussed some open issues that need to be tackled in future studies to improve the performance of TDMA-based MAC protocols for vehicle to vehicle V2V communication.