# Systematic Literature Review of Challenges in Blockchain Scalability
## 8 RQ3: How Researchers Address the Scalability Issue in Blockchain?
### On-Chain Solution
#### 8.1.4 Consensus-Protocol-Related Approaches
Some practical variants of PBFT have been proposed, such as the Steller consensus model [34]. There is not much difference between PBFT and Steller. In PBFT, every node requires one to inquire about other nodes, whereas in Stellar Consensus Model other nodes can choose which set of participants to rely on. Proof of authority/proof of identity consensus model  
Proof of identity/proof of authority consensus model [8] rely on the complete publishing nodes. This protocol only involves nodes whose identity is linked to real-life information. In the PoA/PoID model, nodes need to reveal their real-life identities to gain a chance to publish a new Block. The defined identity should be identifiable and confirmable in the blockchain network. Furthermore, the nodes are required to put their real-life identity at stake along with their reputation to earn the chance to become a publishing node. However, the reputation of these nodes is a concern based on their actions and activities performed in the network, which means any malicious activity in the publishing can ruin their reputation in the public blockchain networks. However, their reputation would improve if the node performed in the way that other nodes agreed on. There is much less of a chance for a node with a poor reputation to become a publishing node. It is therefore essential for publishing to retain high status. It is not preferred to implement this model in permissionless blockchain because it requires trust between nodes. Proof of elapsed time  
The PoET consensus protocol randomly chooses the leader by running an election protocol [18]. It was often named as chance-based SGX-based election model. In this model, the leader is chosen to be responsible for adding the next block to the blockchain. In the processing of random leader votes, this model would concern thousands of entrusted nodes and free participation. Therefore, for experiencing the efficiency of this model, it is essential that the election of leader should be distributed between the most available nodes, while the remaining nodes would be kept responsible for ensuring transparency in the selection process and confirm that there is no manipulation in the whole leader selection process. The need for a transparent mechanism in the selection of the leader could be fulfilled by the trustworthy execution environment (TEE), in which security would remain constant during the process. Intel SGX and TEE are supposed to execute the validation and mining process. Every miner needs permission to execute the code within the TEE for waiting time, and miner with the shortest wait-time becomes leader node. Any internal and external tampering could be avoided by the TEE function. This model requires special equipment or hardware, which is considered a drawback. Bitcoin-NG  
Bitcoin-NG [127] is new generation consensus model derived from PoW Consensus model [2]. It splits time into different epochs, and a leader is responsible for transaction sterilization in each epoch. This model introduces two new types of Block: the critical block and the microblock. The critical block does not carry transaction data, and it is generated by the miner (nodes) via PoW model and is only used for the leader selection. In contrast, the leader is responsible for creating a microblock containing transaction data. The transaction can therefore be processed continuously unless the next leader ensures to decrease transaction confirmation time which improved the throughput. Proof of reputation consensus mechanism In PoR [56] protocol, a node's reputation is made on the basis of its assets, transactional activities, and participation in the consensus process. A leader who possesses the best reputation will be able to generate a new Block. Voting process will be initiated for new block validation. There is another scalable protocol on the similar idea of reputation, known as delegated proof of reputation [99]. The protocol's novelty is that it replaces coin staking with a reputation-based method.