**3. Scalability Tier**

The DLT scalability limitations are mainly driven by the restrictions imposed by the Protocol and Network Tier solutions (e.g., the consensus algorithms). To achieve higher scalability, one option would be to curtail some of the features of Protocol and Network Tier. This can be done either by compromising the security, the immutability, or the consensus of the DLT. Because most of the time this is not acceptable, the scalability challenges are open for research for all DLT variations. In this sense existing concepts such as distributed databases or file systems, have been reconsidered and integrated with Protocol and Network Tier, to allow the implementation of solutions for the Scalability Tier [7,100].

Anyway, due to the advent of blockchain platforms and applications, most of the nowadays literature is focused on the scalability limitations of this type of ledger. They are imposed either by maximum block size (e.g., 1 MB Bitcoin) or by a cost constraint (e.g., gas consumption and gas price in Ethereum). These constraints are combined with the strict periodicity of the block generation (e.g., 10 min for Bitcoin, 15 s for Ethereum) imposing limitations in the number of transactions processed. Moreover, they are impacting both the storage and the processing capabilities (e.g., due to the gas consumption costs in the case of Ethereum smart contract execution). Bitcoin reportedly can allow 7 transactions/second on average [101], while Ethereum registers 13 standard transactions/second or 7 transactions/second in case smart contract execution is involved [102]. Even private deployments reach certain limitations. Hyperledger is advertising 100,000 transactions/second, although reports show a lower limitation of 700 transactions/second [60].
