A Survey of Security Services, Attacks, and Applications for Vehicular Ad Hoc Networks (VANETs) <s> Research Work on Availability <s> In this paper, we propose a method which efficiently collects, retains and propagates traffic information using inter- vehicle communication with "message ferrying" technique used in vehicular ad hoc network (VANET). In the proposed method, we use buses as message ferries which travel along regular routes. In order to improve information propagation efficiency in low- density areas, buses collect as much traffic information as possible from cars in their proximity, and periodically disseminate the collected information to neighboring cars. We have implemented the proposed system on the traffic simulator NETSTREAM and compared information propagation efficiency between our proposed method and a method which uses inter-vehicle communication among only ordinary cars. In the simulation, the proposed method improved the efficiency up to 50%. <s> BIB001 </s> A Survey of Security Services, Attacks, and Applications for Vehicular Ad Hoc Networks (VANETs) <s> Research Work on Availability <s> It is useful to share location-dependent data items generated by vehicles such as a picture image of a traffic accident and the traffic condition around an intersection in vehicular ad hoc networks (VANETs) for improving the driver safety and comfort. In VANETs, due to the mobility of vehicles, it is difficult to reliably access data on other nodes. To improve the accessibility of data from a vehicle to a vehicle, it will be effective to distribute replicas of location-dependent data in a certain areas such as the area around the birthplace of the data item so that a request message geocasted to a location of interest can reache a vehicle that has the replica of the requested data item. In this paper, we propose an information-sharing scheme for location-dependent data items generated by vehicles using the push and pull approaches to balance the reliable message delivery and the traffic for the data dissemination called “assigning populated area as message storage area (APAM) scheme”. In the APAM scheme, request messages for location-dependent data are sent from each vehicle to locations along routes where there are many vehicles. Additionally, replicas of the data items are distributed around the birthplace and on the main streets where the vehicle density is high. These operations enable the request messages to encounter the vehicles that have the replica of the corresponding data item on the main streets. Simulation results showed that the proposed scheme improved the access success rate compared with the existing strategies that distribute replicas to vehicles near the birthplace of the original data items. <s> BIB002 </s> A Survey of Security Services, Attacks, and Applications for Vehicular Ad Hoc Networks (VANETs) <s> Research Work on Availability <s> Vehicle Ad-hoc Network (VANET) has recently received a lot of attention as the fundamental technology of Intelligent Transportation System. Among other characteristics of VANET, the rapid mobility particularly causes the problem of time constraint in delivery of data, thereby resulting in the low data accessibility. Therefore, the requested service is likely to suffer from poor performance. This paper proposes an efficient method to improve data accessibility in VANET by utilizing the data replica of Roadside Unit (RSU). Both data access pattern and driving pattern are utilized to decide which data item should be replicated in RSU. The conducted simulations clearly reveal that the proposed methodology outperforms other schemes in a variety of environments. <s> BIB003
In recent years, an intense amount of research work has been done, which enhanced the performance of availability services by introducing new protocols. In this section, we will explain the robust existing methods, which have been used to enhance the performance of availability in VANETs. Kitani et al. BIB001 presented a new method named as a message ferrying, which is used to improve the message circulation in less populated areas. This method utilized buses to obtain maximum traffic information from vehicles such as location, fuel, and acceleration in their vicinity and then gathered information and forwarded the collected information to the neighboring vehicles. The proposed method was implemented on the NETSTREAM traffic simulator and then compared the information propagation efficiency with other competent methods. In the proposed method, the author did not mention the detail performance parameters, which were involved, and the study was limited to a low density area. Okamoto and Ishihara BIB002 introduced a method of information sharing for location-dependent data, which are generated by vehicles using a pull and push method to balance the message delivery and the traffic data assigning the populated area as a message storage area (APAM) scheme. This  In recent years, an intense amount of research work has been done, which enhanced the performance of availability services by introducing new protocols. In this section, we will explain the robust existing methods, which have been used to enhance the performance of availability in VANETs. Kitani et al. BIB001 presented a new method named as a message ferrying, which is used to improve the message circulation in less populated areas. This method utilized buses to obtain maximum traffic information from vehicles such as location, fuel, and acceleration in their vicinity and then gathered information and forwarded the collected information to the neighboring vehicles. The proposed method was implemented on the NETSTREAM traffic simulator and then compared the information propagation efficiency with other competent methods. In the proposed method, the author did not mention the detail performance parameters, which were involved, and the study was limited to a low density area. Okamoto and Ishihara BIB002 introduced a method of information sharing for location-dependent data, which are generated by vehicles using a pull and push method to balance the message delivery and the traffic data assigning the populated area as a message storage area (APAM) scheme. This method is limited to deliver the reliable information based on the pull and push method, but may incur a large amount of computational charges. Akila and Iswarya  introduced an effective data replication technique to manage data access application in VANETs such as location, fuel, and acceleration. Due to a large number of vehicle mobility and change in network topology, which lead to an increase in the parameter changes and cause frequent disconnections. If disconnections happen frequently, then vehicles will not be able to communicate and share data with each other. Specifically, the data replication is used to improve the performance of data access in a distributed system. Since the vehicles contain less storage, they cannot replicate heavy mp4 files or some short duration video clips. This problem is significantly improved by generating the request to the vehicles in a platoon to give some part of their buffers to reproduce data while sharing the same platoon and data among other vehicles. In case a vehicle wants to leave a platoon, it transfers its buffered data to other vehicles prior to leaving a platoon. Therefore, the other vehicles have an access to the data after it leaves. This method has limitations because vehicles frequently leaving and entering a platoon may require a large amount of computational time and incur computational charges. Park and Lee BIB003 introduced an effective method to enhance the data accessibility in VANETs by utilizing the data replica of the RSU. In this approach, selection of the data item is made by using the data access pattern and driving pattern, which must be reproduced in the RSUs. Then, the reproduced data are sent directly to the surrounding vehicle without involving communication with RSUs. The main drawback of this approach is, if the data size is larger, then the data replication process may require a large amount of time to handle the replication process.