# Distributed Ledger Technology Review and Decentralized Applications Development Guidelines
## 3 Scalability Tier
### Transaction Throughput
Additionally, some issues regarding the overall security of the systems need to be considered. Upon returning to the main chain, the transactions validated by the side chains are valid even if the validators' network of the side chain differs from the ones of the main chain. The Payment Channels mainly implemented in Lightning Network solutions [124] are one of the best choices for improving transaction throughput. The mechanisms of the Lightning Networks were firstly defined for Bitcoin in [124], and future implementations have followed for other networks: Raiden for Ethereum [125] or Bolt for Zcash [126]. The Payment Channels aim to combine all the small payments into one large payment at the \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Protocol and Network Tier** & \multicolumn{3}{c}{**Scalability Tier**} \\ \cline{2-5}  & **Fully Replicated** & **Sharding** & **Sidechains** & **Payment Channels** \\ \hline \hline Trusted Parties & None & None & Depending on implementation & None \\ Transaction Scalability & Low & Medium & Medium & High \\ Cost & High & Medium & Medium & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Transaction scalability solutions. end of a service period. The Lightning Network provides a point-to-point network that runs on top of the blockchain network as depicted in Figure 5.  
It relies on hash time locks and cryptographic secrets to ensure the reliability of off-chain payments. The nodes in the Lightning Network exchange cryptographic signed transactions among them. They represent the payments exchanged between nodes offline. At any point, the payments can be deployed on the main chain to securely redeem the associated coins. Whenever two parties aim at exchanging many micro-transactions, the first step is to open a channel between them, represented on the blockchain as an opening transaction that locks the maximum amount of money that the two parties could transact off-chain. The transaction opening is signed by both parties involved. To unlock it both parties need to agree on the spending of the amount. Afterward, the parties will continue to exchange messages off-chain, namely, commitment transactions, which are designed as fail-safe mechanisms against cheating off-chain. Whenever a transfer occurs, determining a change in balances, each party creates one commitment transaction specifying the updated balances, signs it, and sends it offline to the other party. Upon receiving the commitment any party can successively sign it and publish it on-chain to redeem the coins or can wait and make other off-chain transfers and return only with a future commitment transaction on-chain. To prevent any party to return to the chain with a deprecated and more favorable commitment transaction, secret-locking and time-locking mechanisms are incorporated. They provide a fail-safe mechanism such that any party acting fraudulently risks losing all the money in favor of the other channel party. The benefit of transferring over a route is the reduction of the cost associated with the opening transactions required on-chain each time a channel is required with a specific party. For sending the transactions through a path that requires intermediary parties (hop nodes) to route the messages, additional security mechanisms are required to ensure the correct delivery of the transacted assets. For successfully issuing a routed transfer, a commitment transaction is exchanged between every two parties involved in the path channels. Hash Time-Locked Contracts [127] have been defined over the previously presented mechanism, to prevent the intermediary to unlock the routed commitments before the receiver can confirm the payment. Upon confirmation, a proof is issued by the recipient and sent to the intermediary to unlock the hash-locked commitment. If the proof is not provided during an established period (time lock) then the intermediary will no longer be able to claim the payment. The Payment Channels offers the best scalability solution.