## 4 Blockchain
#### Integration of Federated Learning and Blockchain in Vehicular Networks  
IoV is capable of handling a large quantity of data transmission, storage, and real-time computations to accommodate the needs of infotainment and safety (roadside) [115]. IoV empowers Vehicle-to-Infrastructure (V2I), Vehicle-to-Vehicle (V2V), and Vehicle-to-Everything (V2X) communication, where V2I is purely dependent on VANET, and RSU is utilized to facilitate authentic and reliable information. On the other hand, V2V is an infrastructure-less communication model mainly deployed during rush hours or in emergencies. Artificial Intelligence, Machine Learning [116], and fuzzy logic [117] methods are adopted to handle vehicles direction, speed, and propagation loss, but these techniques consume large processing power, which is not feasible in the fully distributed architecture [118]. Many vehicle networks require real-time and accurate decision-making within a specified time, which is not suitable for a centralized architecture. FL is the best choice to handle such problems because of its distributed machine learning approach. Mobile  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Ref** & **Categories** & **Research Contribution** \\ \hline [93] & \multirow{2}{*}{Vehicular IoT architecture} & Author proposed SDN architecture of fog and 5C systems based on blockchain. \\ \cline{3-3} \cline{  
devices collect data, which they use for training, and generate local models. After that, local model data are supplied to the aggregator, which takes the average of local models and generates a global model. Mobile devices train each global model until the desired goal is achieved. The integration of FL and blockchain can solve the challenges in an existing centralized system, such as resource consumption limitations, and is also suitable for the sensitive applications having autonomous vehicles. The decentralized solution will optimize computing resource utilization and provide balanced workloads compared to centralized networks. Furthermore, using cryptography characteristics, data consistency is also ensured in the blockchain. The addition of cryptography characteristics to VANET will offer secure data storage and data transmission within the network. Blockchain saves details of each connected vehicle in VANET as well as traces the newly added vehicles to the network [71]. In VANET, each vehicle's identity is assured; the process allows data authentication and authorization in VANET devices. Among vehicles, blockchain ensures secure data communication [119], performing the transaction validation and saving locally trained data of different transactions for every vehicle [120]. The data breach is the most challenging issue in critical infrastructures, e.g., military data [121], connected vehicles [122], banking [123], and healthcare [124]. Blockchain provides data encryption techniques to eliminate the risk associated with data breaches and optimize VANET protocols. Furthermore, VANET transfers a large amount of real-time network data to connected vehicles. This process also secures VANET and assures an appropriate data management technique. Providing an FL solution based on the blockchain permits vehicles in VANET to communicate with one another using encrypted identities (IDs) deprived of the central server. The transaction is distributed equally after adding the transaction to the block within all devices in the network [71,125]. Ref. [126] proposed a solution for VANET infrastructures based on the blockchain-FL system. The vehicle's training model solution is managed by utilizing blockchain and FL technologies. In VANET, the structure of FL consists of the following two components:  
**Centralized Server:** In FL procedure, a centralized server plays a significant part because its main job is to communicate with end devices, such as vehicle clients, to match trained local model updates. **Vehicle Client:** They have a set of different types of sensors, such as cameras and GPS.