## 4 State of Art about Data Availability in L2 Rollups
### Off-Chain (Validium Mode)
#### 4.2.2 Data Availability Layer (DAL)  
The traditional monolithic blockchains are designed with a single layer that handles the transaction execution, consensus, data availability, and settlement. However, this tight integration has led to issues such as low throughput, scalability, and transaction cost. For instance, Bitcoin has low transaction throughput, and Ethereum has high Gas prices. These challenges led to a novel modular blockchain design with separate layers for execution, consensus, data availability, and settlement [42]. Figure 3 illustrates the difference between modular and monolithic blockchains. This modularity allows for improved throughput and lower transaction costs, resulting in more scalable systems. Therefore, a dedicated Data Availability Layer (DAL) can be a storage and consensus layer for rollup, including transaction data, settlements, and off-chain data. A DAL concept is similar to DAC; instead of relying on permissioned nodes, DALs are permissionless networks and employ the Proof-of-Stake (PoS) validator systems to store data off-chain without relying on trusted third parties. DAL is a dedicated layer that ensures the integrity and availability of transaction data without burdening the underlying L1 blockchain.  
DAL distributes transaction data across multiple nodes in a decentralized network while ensuring data redundancy and fault tolerance through techniques such as erasure coding. Erasure coding splits data into multiple segments and allows the original data to be recovered from a subset of these fragments. This ensures that the data are still recoverable even if certain fragments are lost or distorted. A DAL ensures security by making all data available to nodes and preventing malicious activities. Moreover, a dedicated DAL helps achieve a more flexible and modular blockchain design, significantly reducing transaction costs and increasing scalability.  
Celestia [43] is a modular pluggable and pre-configured DAL solution that offers rollups to store large datasets off-chain. This allows the rollups to integrate DAL into existing solutions easily. Celestia incorporates data availability sampling (DAS), allowing light nodes to verify data availability without downloading the entire block. Randomly sampling small portions of data ensures that the expected data are consistent. A consistent dataset indicates that the entire dataset is available. Moreover, Namespaced Merkle Trees  
Figure 3: Modular vs Monolithic Blockchains.  
(NMTs) allow the node to download only relevant transaction data, which leads to an efficient settlement and execution. These two novel approaches enable rollups to achieve a significant cost reduction of approximately 100x and improved scalability.  
Another example of DAL is Avail [44], a spin-off of Polygon. Avail splits the network into light peer-to-peer nodes and employs a sampling technique to verify data availability. Moreover, Avail uses the innovative polynomial KZG commitment schema [45] and the erasure coding for proof validity. The erasure coding distributes data across a decentralized network, ensuring it remains available and verifiable. This separation improves the overall efficiency and scalability of the rollup.  
The NEAR protocol [46] is another DAL introduced by the NEAR foundation in 2023, which employs a sharding mechanism called Nightshade. Each shard splits the data into chunks and then integrates them into a single block. These data are available for around 60 h and are then transferred to archival nodes to retain them. In this way, the network distributes the responsibility of data availability, which enhances scalability and reliability.