## 2 Related Work
### Performance Comparison  
As blockchain is making progress with every day, new protocols are coming into existence. The scalability issue is also addressed in almost every concurrency. Many new consensus protocols are used in cryptocurrencies. Performance of these consensus algorithms is compared [21] in a delegated way by combining different studies discussed in Table 1. Complete comparison is available in the study, where some protocols are considered as a reference. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Consensus** & **Decentralization** & **Accessibility** & **Scalability** & **Comp.** & **Storage** & **Network** & **Latency** & **Throughput** & **Suitability** \\ \hline PoW & High & Public, PL. & High & High & High & Low & High & Low & No \\ \hline dPBT & Medium & Private, P. & High & Low & High & High & Medium & High & Partially \\ \hline Stellar & High & Public, PL. & High & Low & High & Medium & High & Partially \\ \hline PoI & High & Public, PL. & High & Low & High & Low & Medium & High & Partially \\ \hline Ripple & High & Public, PL. & High & Low & High & Medium & High & Partially \\ \hline Raft & Medium & Private, P. & High & Low & High & N/A & Low & High & Partially \\ \hline Omniledger & High & Public, PL. & High & Medium & Low & Medium & High & Partially \\ \hline RapidChain & High & Public, PL. & High & Medium & Low & Low & Medium & High & Partially \\ \hline DPoS & Medium & Public, PL. & High & Medium & High & N/A & Medium & High & Partially \\ \hline PoS & High & Private, P or PL & High & Medium & High & Low & Medium & Low & Partially \\ \hline PoET & Medium & Private, P or PL & High & Low & High & Low & Low & High & Full \\ \hline PBFT & Medium & Private, P. & Low & Low & High & High & Low & High & Full \\ \hline Tangle & Medium & Public, PL. & High & Low & Low & Low & Low & High & Full \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Performance comparison for different consensus algorithms [21]. A cryptocurrency IoTA introduced a very new technology for a distributed ledger called Tangle. It is very highly scalable, with low computing and storage overhead. Moreover, it is very lightweight and specially designed for IoT devices with no transaction fee. Tangle uses DAG (Directed Acyclic Graph) in which each transaction is linked to the previous two transactions approved by it to add on the ledger through PoW. Tangle uses DAG for validation, so that is why it is not considered in the blockchain list and promises to overcome the existing barrier of decentralization for IoT resource-constrained devices. There is no mechanism in Tangle for the selection of the older two nodes to validate in IoT scenario; however, in IoTA cryptocurrency, Tangle runs an algorithm called tip selection for transactions [22]. Another problem is highlighted in [19], if a hacker acquires 33% (one third) hashing power of the total, then it can make it vulnerable and insecure. As the most trusted platform for computing SGX, Intel proposed a new consensus mechanism proof of elapsed time (PoET) that is primarily based on Byzantine Fault Tolerance (BFT), which focuses on reducing the energy requirement. This protocol is lightweight and perfectly suitable for IoT in public and private ledger domains [23]. The suitability of this protocol can also be checked from the performance comparison table. PBFT (Practical Byzantine Fault Tolerance) is a Byzantine Fault Tolerance protocol, which is highly practical with low algorithm complexity in distributed systems [24] and contains five phases (1) request, (2) pre-prepare, (3) prepare, (4) commit and (5) reply. In PBFT, a client sends a message to the primary node which forwards it to other nodes to reach consensus. The message goes through five phases to complete the round of consensus among nodes and, finally, these nodes reply to the client. Nodes maintain common state to take consistent action in PBFT in each consensus round. PBFT creates and validates the block in DPoS to reduce the time in the consensus round [25].