Not all cryptocurrency or digital currency are based on Bitcoin. In fact, some of them don’t even use a blockchain. They are graph-based networks (e.g. DAG, Hashgraph) which arrive at consensus much differently. The notion of a blockchain has become the most synonymous with cryptocurrency, but that is not applicable to all. IoTA, Hedera, Nano and Byteball are examples of graph-based networks. The most common type used is a DAG (Directed Acyclic Graph), which is more scalable network solution than blockchain based distributed systems. A DAG is not a blockchain but both use decentralized cryptographic databases in a sense that a Ferrari is not a lamborghini but are both cars.
A blockchain connects blocks by hashes which can be traced back to a primordial block or “Genesis Block” which is the root of all hashes. It uses a tree topology of nodes called a Merkle Tree, which has leaf nodes that contain the cryptographic hash from child nodes. When these hashes are concatenated, they can be traced back to the Genesis Block in the network. Blockchains use a consensus mechanism to validate the blocks, with PoW (Proof-of-Work) being one example that is used on the Bitcoin network. Consensus is what secures the network by way of validating a block and adding it to the blockchain where it becomes immutable so it is no longer subject to change. This prevents tampering and data manipulation. The consensus requires nodes called miners who must compete with one another by solving a cryptographic puzzle using a probabilistic zero-sum game approach. The miner who solves the puzzle first becomes the block validator and is rewarded with Bitcoin (BTC) for their contribution.
A DAG is a finite graph which is directed forward in one direction with a topological ordering. It consists of vertices that lead to other vertices, which are paths called edges. The vertices are like points in a network. The system uses an “Efficient Teacher Grading” method instead of miners doing PoW. A DAG uses peers to help validate transactions in the network. When a new transaction is made, a new vertice representing the transaction is created and must be validated by other peers on the network. It doesn’t require solving a puzzle, but relies on confirmations as the consensus using a gossip protocol mechanism. When other peers on the network can confirm the transaction as correct, it will be validated.
A DAG network is much faster than PoW since it doesn’t rely on compute intensive puzzles. This allows it to run on lighter devices in contrast to PoW systems that rely on power hungry ASIC devices that perform large numbers of calculations to solve the puzzle. This is rather inefficient, so it requires more energy to produce coins or tokens. A DAG is much faster and scalable since it doesn’t require the same overhead as a blockchain network when it comes to consensus. Costs are lower too because there is no need to purchase expensive equipment that use plenty of electricity. A DAG can utilize mobile devices like smartphones to help confirm transactions on their network. This also makes DAG more suitable for micro-transactions which require instant validation of transactions. DAG offer less barriers to entry because practically anyone can become a peer using their low-energy consuming smartphones while PoW requires more investment in hardware that require a consistent supply of electricity to operate.
Scalability has been the main reason for DAG over blockchains. A blockchain like Bitcoin has scaling issues because of the consensus mechanism it implements and the protocols used on the network. It was not developed for high throughput transactions like the VISA or Mastercard network. VISA claims it can process 1,700 TPS (Transactions Per Second) or 150 million transactions in single business day. Bitcoin’s blockchain can only process between 3 to 7 TPS only. Security has been a consistent strength of the Bitcoin blockchain, as it has never been successfully attacked (e.g. 51% attack) as of this posting since it started in January 3, 2009. DAG have not been in production for that long and have mostly been used on experimental and concept networks. A DAG is mainly used for DLT (Distributed Ledger Technology) implementations while blockchains are used on trustless permissionless public networks.