# Blockchain Interoperability for Future Telecoms
## 3 Literature Review
### Interoperability Techniques  
Exploring various popular descriptions, use cases, and interoperability techniques in this research field is essential for a better understanding of interoperability. According to [PERSON] et al. [46], interoperability is \"the ability of a source blockchain to change the state of a target blockchain, enabled by cross-chain or cross-blockchain transactions, spanning across a composition of homogeneous or heterogeneous blockchain systems\". This definition highlights that even though telecom operators tend to opt for standard and widely used private blockchain solutions, achieving homogeneity in blockchain networks remains challenging. Therefore, an ultimate solution should support diverse types of private blockchain networks. [PERSON] et al. [6] defined blockchain interoperability as \"the ability to flexibly transfer assets, share data, and invoke smart contracts across a mix of public, private, and consortium blockchains without any changes to underlying blockchain systems\". They further expanded the definition of the \"ability to change the state of target blockchain\" from [PERSON] et al. according to three essential features:  
* Asset transfer;
* Smart contract execution;
* Data sharing. These are the most common types of interoperability. Asset transfers and swapping are the most popular public cryptocurrency-based interoperability solutions. These use cases can also be applied to private blockchains, providing more flexibility in the telecom domain. [PERSON], the founder of Ethereum [47], described the general concept of interoperability techniques in 2016. [PERSON] discussed three scenarios:  
* Centralized or multi-signature notary schemes;
* Sidechains/relays;
* Hash-locking. Figure 6: Relay scheme from [PERSON] et al. These initial definitions are necessary to explore and expand our understanding of future interoperability schemas. According to [PERSON] et al. [6], in 2023, interoperability techniques were categorized into five schemes:  
* Notary Schemes: Consider trusted parties for centralized and decentralized exchanges. Notary schemes are trendy in cryptocurrency, as there is a high demand for the exchange of cryptocurrency coins worldwide. A famous example is the largest cryptocurrency exchange platform, Binance [48]. Binance is considered a trusted party when users require currency exchange. Notary schemes, such as trusted intermediaries, are considered centralized solutions and are not candidates for a final solution. * Hashed Time Lock Contracts (HTLCs): Automatically swap currencies for permissionless networks. Unlike notary schemes, which require a trusted third party, the HTLC protocol automatically swaps currencies between cryptocurrency-based blockchains. There is no intermediary for an HTLC-based solution, which makes an HTLC a decentralized solution. * Relay Schemes: Carry out transactions by transmitting them across blockchains, which can be either trusted or trustless. Trustless schemes agree on the communication protocols between the two blockchains and do not require trustees. If a relay scheme is built as a trusted relay, the solution is an intermediary-based centralized solution. The trustless relay approach is an ideal candidate if intermediaries need to be prevented. * Blockchain-Agnostic Protocols: Build an abstraction layer to communicate and interoperating among diverse blockchains. Building an abstraction layer is essential for defining the solution as centralized or decentralized. In 2019, [PERSON] et al. [49] proposed a relay component that abstracts the existing blockchain modules from the interoperability layer. The architecture presented in [49] is constructed as a decentralized solution without a trustee and is one of the earliest solutions for permissioned blockchain networks built with Hyperledger [50; 51]. The design offered by [PERSON] et al. also provides a side-by-side architecture and serves as a foundation for future projects using Hyperledger. * Sidechains: Transfer assets from the mainchain to the sidechain.