## 4 Blockchain  
Blockchain has recently piqued the interest of both academics and industry. It is extremely effective to handle data transferring constraints between heterogeneous devices On the Internet of Vehicles (IOV) while ensuring privacy and security [76]. Blockchain was developed to service banking and digital money (Bitcoin); later on, it was implemented in other spheres of life. ITS has minimized the urbanization transportation problems. The development of wireless sensors in vehicles revolutionizes communication between vehicles, thereby creating Vehicular Ad-hoc Networks (VANETs) [77; 78]. VANET improves network traffic flow to provide timely information and facilitate intelligent transportation services. VANET is employed because it has the capability of self-organizing data transmission to on-road vehicles and to allow applications, e.g., safety warnings and assistance in-vehicle driving [79; 80]. Instead of cellular vehicular-to-everything (C-V2X), the European Parliament has adopted DSRCs (dedicated short-range communications) [81], because soon, VANET will have broader applications, and it is also 'infrastructure-less'.  
Without any central medium, blockchain is a novel technology that permits transactions to different peers in the network and also influences the distributed ledger [82]. To promote data credibility and accountability, several VANET applications have recently been identified as the potential value of this new technology [83]. Because of the autonomous infrastructure of the vehicles, they need to exchange data with more 'trust' in different intelligent transportation situations, e.g., smart contracts, which already benefit from blockchain technology [84].  
In blockchain technology, nodes perform a validation process because they do not trust each other whenever a new block is created in the system [82]. After the validation process, the existing nodes trust the newly created block. The process outlined above is called Proof-of-Work (PoW); certain transactions are valid within the network because of the above process. It is perfect if the newly created block is verified by Full Nodes (FNs) within the network [85], which requires a broadcast of the newly created block to all FNs within the whole network. This process takes several more significant factors from others in blockchain performance in a network, such as total broadcast delay and the number of nodes. The critical factors in blockchain networks include: security and credibility of consensus and blockchain network energy consumption [86].  
The main focus of a VANET is to improve credibility and security in data exchange. To improve the security of the VANET data, [87] proposed a three-layer framework. To break the relationship between public keys and real identities from the privacy perspective, [88] proposed an anonymous reputation system based on blockchain. The blockchain was used for modeling data communication between flying drones securely in a 5G network in [89]. For secured storage system and data sharing in VANET, [90] studied a consortium blockchain technique. Another research work also used smart contract technologies and consortium blockchain to accomplish secured data sharing and to store in VANET [91]. The study assumes that edge computing servers in the vehicular network included RSUs suffering from severe privacy and security issues and are not fully trusted. Finally, in electric vehicles, blockchain technology is used to improve the edge computing [92]. According to the perspectives of energy and information interactions between vehicles, context-aware applications are recognized, in which energy involvement amount and data involvement frequency are used to achieve PoW.  
Table 1 analyzes and describes the role of blockchain in the Vehicular Internet of Things (VIoT) environment from different features perspectives. Table 1 also illustrates and categorizes existing proposed scenarios.