# Blockchain and IoT Convergence--A Systematic Survey on Technologies, Protocols and Security
## 5 Distributed Ledgers
### Consensus
#### 5.2.5 Others  
1. Proof of Burn [31]: the probability for a specific node to success in mining new block is directly proportional to the number of coins burned by itself. The burning process consists of sending tokens to a specific address that cannot send tokens. These tokens are thus 'burned' in the sense that they are no longer in circulation and therefore inaccessible. This is a similar idea to PoW but without wasting real world energy. Proof of burn has many of the same criticisms as PoS; the consensus is determined by the richest nodes in the network.
2. Proof of capacity [32; 33]: Similar to Proof of Work, but uses storage instead of computation. Cryptographically signed data is written to the local storage according the following rules: * A very slow hash function is computed and stored. In the process, the hard drives are filled with groups of the precomputed hashes, and each group contains 4096 pairs. * While mining, a deadline time from a specific pair for each group is calculated. This deadline time represents the time to wait for mining another block after the last one. * The right to mine the next block is granted to the node who have the shortest deadline.
3. The Tangle [34; 35]: the most famous alternative to standard Blockchain structures, 'The Tangle' use a DAG (Direct Acyclic Graph) to store transactions. Better explanation of this method used to achieve a different kind of consensus is reviewed in Section 8.1.
4. Hashgraph Consensus [36]: Similar to IOTA's Tangle, a DAG is created but following different rules. The new data structure named Hashgraph uses a gossip protocol to spread information throughout the network, and a virtual voting mechanism to achieve consensus involving random sync and validation between nodes. Any node can view the world from the perspective of any other node since last sync. In this way each node can determine if a given set of information or transactions is valid by checking if at least two thirds of the network's nodes have witnessed specific transaction.  
Table 3 shows the different consensus versions used in major blockchain projects  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**Project** & **PoW** & **\textasci{-PoW}** & **PoS** & **\textasci{-PoS}** & **DPoS** & **\textasci{-DPoS}** & **pBFT** & **\textasci{-pBFT}** \\ \hline Bitcoin [21] & SHA256 & & & & & & \\ Ethereum [37] & Ethash & & & & & & \\ Litecoin [38] & Scrypt & & & & & & \\ Monero [39] & CryptoNight & & & & & & \\ HDAC [40] & EPoW & & & & & & \\ Lynx [41] & HPoW & & & & & & \\ Komodo [42] & dPoW & & & & & & \\ Purple [43] & SSPoW & & & & & & \\ Dash [44] & & & X & & & & \\ Stellar [45] & & & X & & & & \\ Cosmos [46] & & & X & & & & \\ Vericoin [47] & & & PoST & & & & \\ Shield [48] & & & & PoS Boo & & & \\ XSN [49] & & & TPoS & & & & \\ Reddcoin [50] & & PoSV & & & & & \\ Cardano [51] & & Ouroboros CA & & & & & \\ Tron [52] & & & X & & & & \\ Bitshares [53] & & & X & & & & \\ Steem [54] & & & X & & & & \\ Ark [55] & & & X & & & & \\ Lisk [56] & & & X & & & & \\ EOS [57] & & & BFT DPoS & & BFT DPoS & \\ Zilliqa [58] & & & & X & & & \\ Sawtooth [59] & & & & & & & \\ Neo [60] & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Popular Blockchain consensus mechanisms.