ABIDING GEOCAST FOR WARNING MESSAGE DISSEMINATION IN VEHICULAR AD HOC NETWORKS 指導教授：許子衡 教授 報告學生：翁偉傑 Qiangyuan Yu , Geert Heijenk IEEE Communications Society in the ICC 2008 workshop proceedings

OUTLINE Abstract Introduction Proposed system Experiments Conclusion

ABSTRACT The objective is that as much as possible all relevant vehicles receive the warning before they reach the warning line. Opposite vehicles are preferred as relays to reduce broadcast overhead and help message delivery upstream. Wait time of individual relay vehicles are set dynamically for the next broadcast.

INTRODUCTION (1/2) Vehicular ad hoc networks (VANETs)是短距離無 線通信（例如，IEEE802.11）在車輛之間。 multihop ad-hoc network traffic monitoring message dissemination RBM (Role-Based Multicast)[1]， 要求每個節點維 護一個列表(neighbors)，維護將產生額外的開銷。 TRADE和DDT[2]，只能通過車輛行駛方向一致， 而不能在相反方向的交通通道。

INTRODUCTION (2/2) How to ensure that a traffic warning message is disseminated with high probability and low overhead to all the potentially affected vehicles? We emphasis two points: Abiding geocast Disseminate in time

PROPOSED SYSTEM (1/8) 車輛都配備了全向天線的無線電，傳輸範圍為 R 。 車輛之間的通訊應該是雙向的。 We consider the system with following assumptions: Given a bidirectional stretch of road. Vehicles move throughout the road with constant speed randomly chosen within an allowable range. Only one initiator of disseminating, other vehicles act as relays. Only one active warning message to be disseminated.

PROPOSED SYSTEM (2/8) Dissemination strategies effect distance is the distance between safety line and effect line. We use the notion of effect line to indicate beyond which point vehicles will become inactive and not broadcast any more.

PROPOSED SYSTEM (3/8) traffic volume λ （輛 /小時） the headways between consecutive vehicles are distributed exponentially with mean 3600/ λ. [Smean-ε，Smean+ε]，這裡 Smean是速度均值和ε是 速度的變化 effect distance factor γ

PROPOSED SYSTEM (4/8) 當車輛超出傳輸範圍，稱 fragmentation。 a relay vehicle is only responsible for delivering the message to vehicles travelling in the opposite direction. How to set the wait time depends on some conditions: 移動方向和發送者、接收者的速度 發送者和接收者的位置 the safety line and effect line的距離

PROPOSED SYSTEM (5/8) The wait time can be set for vehicles leaving and approaching the event using 當前位置 傳輸範圍 最大速度 速度

PROPOSED SYSTEM (6/8) Updating the wait time dynamically one active vehicle can wait for some time without periodically broadcasting if it knows another vehicle moving in the same direction is active now and broadcasting periodically.

PROPOSED SYSTEM (7/8) Message form In the algorithm we use a timer to control the next broadcast of active vehicles,After broadcasting, the vehicle will set its timer for the next broadcast.

PROPOSED SYSTEM (8/8) Algorithm

Experiments (1/3) OMNET++ simulator The location of the warning event is at 0 meters simulation time is 4000s the start time of the warning event is at 400s

EXPERIMENTS (2/3) 開銷少取決於交通量 在dense和sparse的網路

EXPERIMENTS (3/3) 大約 10分鐘後，廣播開銷穩定的狀態 在到達警告事件，有足夠的時間來做出決定。

CONCLUSION vehicles traveling in opposite direction are used as preferred relays to overcome fragmentation and reduce broadcasting. Dynamic wait time setting is adopted to save unnecessary broadcasting while keeping the warning message in the effect area. The record of last-opposite vehicle is the key for one vehicle to get information of other vehicles moving in the same direction, especially in sparse networks.