## 2 Related Works
### Security-Based Approaches  
Ref. [21] presented a new mutual authentication and key agreement protocol to improve the security, anonymity, and intractability of the VANETs with an Intelligent Transportation System (ITS). The proposed method achieves low communication and computational overheads and better security. The method can be applied to the network with low-populated areas; hence, the throughput is low.  
Ref. [22] proposed a Physical Unclonable Function to reduce overhead and network traffic and protect the network from cloning attacks. To minimize the overhead of authentication and to increase the network throughput, the network is constructed with three layers, namely roadside units, roadside unit gateways, and trusted authority. From the simulated results, it shows that this reduced greatly reduces the MAC/PHY overhead and improves security against various types of attacks. Hence, the throughput and bandwidth are low; this model becomes not suitable for densely populated areas.  
Ref. [23] constructed a multi-tiered hybrid IDS to investigate the vulnerabilities of intra-vehicle and external networks. This method incorporates signature-based IDS and an anomaly-based IDS to detect active (known) and passive (unknown) attacks. The detection accuracy achieved for active attacks is 99%, and the detection accuracy achieved for passive attacks is 98%. However, this method fails to achieve high throughput and bandwidth; hence, this model is not appropriate for highly populated areas.  
Ref. [24] presented a multi-level privacy preservation for the vehicles by introducing blockchain technology with homomorphic encryption and circle-based location verification. It is one of the decentralized location privacy-preserving models. This method achieves high security, and the system becomes efficient and feasible in practice.  
Ref. [25] designed a novel blockchain-enabled certificate-based authentication scheme to achieve low communication and computational overheads. Through this authentication process, a secure transaction is conducted, Cluster Head (CH) and the RSU as show in Figure 3. The result, this system achieves low communication and computational overheads.  
Ref. [26] presented an approach to provide security to VANETs during data transmission on both vehicles-to-vehicle as well as a vehicle to infrastructure communication. In general, a malfunction occurs in the network, so a secured AODV protocol is introduced  
Figure 3: Cluster-based VANETs.  
to protect the network from the black hole attack. Additionally, a cryptography function-based encryption and decryption is conducted to obtain better performance. This method achieves better performance in terms of packet delivery ratio as well as the drop, delay, and overhead are reduced. However, it fails to achieve high throughput.  
Ref. [27] proposed a two-level detection system to protect the VANETs from Blackhole attacks. The first level is trusting value calculation of the neighboring vehicles, and the second level is authorized Road Side Units (RSUs) creations with the help of a blockchain system. The trust score of the vehicles is improved using these levels. The network efficiency and scalability are increased, which leads to achieving overall better performance. Table 1 describes the summary of the existing methods.