This study uses a SLR methodology10, 11, which closely fits the topic of this paper: the clear steps involved in an SLR are specifically designed to reduce bias, increase transparency and facilitate replication of the review11. The precise nature of this paper’s research questions means that the SLR methodology is a better fit than other review methodologies because the focus lies on finding, extracting, and systematizing evidences12. The underlying philosophy and approach are pragmatism regarding the use of Blockchain technology for IoT in Smart Cities context13. In the SLR methodology, the work starts by identifying the need for the review, specifying research questions, and developing the review protocol11.
Review Method
Key concerns of the SLR method are the reduction of bias and the increase of transparency of a review11. The review protocol includes the search strategy, selection criteria, selection procedures, data extraction strategy and data synthesis strategy. The review protocol is reported in this section.
Review Protocol
This paper has conducted an initial systematic mapping review on the broader topic of Blockchain within IoT in Smart Cities11. This procedure forms an inductive, semantic approach whereby the three domains are chosen based on general knowledge of their current profile of interest. In addition, research into these three topics may offer new and improved methods for delivering more effective technological solutions in a variety of fields, including smart cities, Information Technology Infrastructures, and many others14-16. For this initial mapping review, a pilot search of the Science Direct database was conducted, the authors found minimal experimental or model-specific studies. Furthermore, when keywords such as Blockchain and Smart City were searched, the output from these search terms was not necessarily indicative of a direct relationship, and in many cases, it was found that Blockchain was a secondary or tertiary consideration. Finally, many studies' experimental nature was narrow, which resulted in purposive and selective interpretations that were irrelevant in the broader context of IoT-based smart city solutions. For all those reasons, a primary search strategy was developed to overlook the irrelevant or inconsistent studies and focus more explicitly upon the search terms and focus of the current study. Over the last 4 years (via manual search), this paper has identified 277 potentially relevant papers from science direct database only. This large number is primarily due to a significant increase in the number of IoT-research related papers published since then in dedicated venues. Examining those 277 papers, this SLR considered that experiments in merging Blockchain and IoT-driven smart cities (narrow focus) related to the broader use of “Blockchain for IoT” (wider domain). Blockchain for IoT includes smart cities contexts, but also includes other contexts for more effective solutions and due to the expansion of IoT industry into every aspect. Moreover, Blockchain for IoT attracts high current interest2-4. SLR further requires specifying the particular rationales behind research questions as asked. These rationales are presented in next section.
Research Questions and Rationales
This review aims to address the most essential and vital but confronting questions regarding the integration of the Blockchain throughout IoT-driven smart cities. The rationales and the research questions are summarized in Table 1 as an immediate reference.
Table 1: Research Questions and their Rationales
Research Question (R.Q.)
|
Rationale
|
RQ1. How much progress has been made in securing and protecting IoT-based integration into the smart city solution?
|
To assess the technical feasibility of blockchain-supported IoT integration in smart city applications.
|
RQ2. What role could Blockchain play in serving as a unified, central database and transaction ledger for IoT-based triggers?
|
To critically assess the central goals and technologies related to smart city applications and digital solutions.
|
RQ3. What are the advantages (and limitations) of blockchain integration within smart city applications?
|
To analyse the technological opportunities and limitations of IoT-related capabilities concerning the current standard of practice.
|
RQ4. Which components of the smart city solution can be standardized, and which elements will remain proprietary?
|
To assess the viability of blockchain-supported IoT solutions while considering an intermediary authentication ledger's applicability to support smart city scalability.
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RQ5. Which structural limitations or challenges will prevent blockchain integration, and how could these problems be resolved?
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To assess the factors restricting or limiting blockchain integration and to analyse the extant solutions and alternative models.
|
Search Strategy
The primary search strategy is relied on an automated online search using a specific search query in a set of academic databases. This SLR has targeted search of multiple academic and industrial databases consisting of peer-reviewed experiments and structural models related to blockchain-based solutions for IoT-Smart City applications. During the identification and selection process, the following strategic controls were used to streamline, focus, and legitimize the results17:
- Key Terms: Blockchain, IoT, and Smart City.
- Search Databases (Table 2): ScienceDirect, Taylor & Francis, IEEE Xplore, Wiley, Sage, Web of Science (WoS).
Table 2: Search Databases
Organization
|
Database
|
URL
|
Clarivate
|
Web of Science
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https://webofknowledge.com/
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Elsevier
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Science Direct
|
http://www.sciencedirect.com/
|
IEEE
|
Xplore
|
http://ieeexplore.ieee.org/Xplore/
|
Sage Group
|
Sage
|
https://journals.sagepub.com/
|
Taylor & Francis Group
|
Taylor & Francis
|
https://www.tandfonline.com/
|
Wiley
|
Wiley Online Library
|
https://onlinelibrary.wiley.com/
|
Quality assessment standards emphasized complete, experimental solutions that met all of the inclusion criteria and offered peer-reviewed or conference-based models with a high level of academic credibility. For this reason, some databases or journals were excluded due to their lack of mainstream credibility and academic value. An initial development of the inclusion/exclusion criteria created a baseline for defining the search terms and focus of the database search function. The SLR search strategy involved the following stages of investigation, interpretation, and analysis:
Stage 1: Conduct keyword search of specific academic databases in order to identify possible range of sources meeting conditions of inclusion criteria.
Stage 2: Eliminate or exclude studies failing to meet all criteria and identify studies which qualify for further screening.
Stage 3: Screen and eliminate duplicate or incomplete studies failing to add instrumental value to the blockchain, IoT, and smart city discussion.
Stage 4: Identify the central findings and core theoretical insights developed by each of the included studies.
Stage 5: Compare and interpret the cohort of outcomes in order to draw conclusions to the core research questions and synthesize the models into a single, unified proposition.
Study Selection Criteria
This paper has defined a set of inclusion and exclusion criteria presented in Table 3 for papers to be included in the SLR based on its purpose11, 17. It worth noted that the extraction period for inclusion criteria was set in 2016 because of the beginning of the appropriate integration of Blockchain and smart city applications2, 18.
Table 3: Inclusion-Exclusion Criteria
Inclusion
|
Exclusion
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· Studies published from January 1, 2016, till Aug 2019
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· Studies published before 2016
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· Experimental research with targeted model
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· Non-experimental research or failure to include model or design
|
· Include blockchain and smart city solutions
|
· Only Blockchain or only smart city
|
· IoT-oriented or indicative
|
· No consideration for IoT
|
· Peer-reviewed journal or legitimate academic conference
|
· Non-peer reviewed or small-scale study (e.g., blog post, non-academic conference)
|
· Realistic or practical application
|
· Unrealistic, non-translational application or model
|
Search Process
The search query used for this SLR combines keywords that were directly derived from the aim and five research questions presented in Table 1. To develop the search query, the SLR has followed this process:
Stage 1: Utilize multiple search terms and the operators "AND" and "OR" to combine Blockchain, IoT, and Smart City and their synonyms into a single search function.
Stage 2: Eliminate any review or editorial studies and focus on smart city-specific research considering the broader, experimental implications of an innovative, blockchain-based solution.
Stage 3: Identify the primary and secondary studies relevant to this model and determine whether to include or exclude for some reasons (e.g., lack of experimental relevance).
Stage 4: Remove any redundant study that has been included in other databases. For example, WoS is an aggregation database and not a journal or article-specific database. Therefore, some articles like Sharma and Park19, Hammi et al.20, Rahman et al.21, and Fayad et al.22 were replicated from other databases. As a result, they were included in the SLR but counted only once.
Data Extraction
This research process acknowledges that the experimental procedures, technologies, and applications for each study were different from the other research; however, several overlapping elements were critical to reflect the meaning and significance of blockchain technologies in IoT-based smart city solutions. The data items included in this SLR were based on the following extracted elements: Title, Journal, Volume (No), Year of publication, Pages, Overview, and Findings (Appendix A).
Data Synthesis
The review utilized sources that met the inclusion and exclusion criteria. The analysis revolved around the emerging themes based on research questions. The research questions were independently carried out by the reviewers23.
Screening Process
A systematic approach was applied to the comparative and critical analysis of multiple studies that reflected the indicative characteristics of experimental solutions related to blockchain technologies for IoT in smart cities solutions. However, this study rejected many of these approaches to follow the central dimensions of the inclusion criteria17.
Any inconsistency was resolved through mutual consensus regarding the quality of reviewed articles. Several studies were excluded because of their lack of focus on the technical elements of blockchain technology and blockchain architecture.
Table 4 further refines these controls into the PICO(s) model proposed by Cochrane24 as a systematic solution for comparing evidence-based studies in the health care field. By refocusing this model on the problem essential to this investigative process, a solution which meets three different criteria, it was possible to narrow the search process down further and restrict the outputs of the SLR to a narrower, more manageable selection of appropriate studies. Most notably, studies without a practical or model-specific consideration that could be translated into a general, broad-spectrum application for a unified smart city solution were now excluded during this search process.
Table 4: PICO(S) Framework for Study Selection
PICO(S)
|
Inclusion Criteria
|
Exclusion Criteria
|
Problem
|
Blockchain AND IoT AND Smart City
|
Individual emphasis on keywords or focus outside of the immediate context
|
Intervention
|
Experimental, Model-Oriented, Inclusive
|
Non-Experimental, Outlaying, Exploratory, Conceptual
|
Comparator
|
Underlying Purpose Related to Smart City Applications
|
Underlying Purpose Related to Other Applications
|
Outcomes
|
Proposed Model or Contributory Solution
|
Revised Concept, Future Research Recommendations
|
Study Design
|
Experimental, Architectural, Non-Proprietary
|
Proprietary, Empirical, Application-Limited
|
Therefore, the summary output of this systematic exercise was indicative of multiple academic, peer-reviewed, and conference studies that were experimental and reflected meaningful contributions and innovations to a middleware layer of blockchain integration within the broader complexity of an IoT-navigated smart city environment. Table 5 presents the search queries that were used to determine the scope of the research.
According to the search strategy, the search on the selected six databases was conducted from May to July 2019. Variations of the keywords were used to complete the comprehensive search; however, those articles were ultimately selected, including the full set of keywords (IoT and Blockchain and Smart City). Other searches were just treated as a part of the process (e.g., a learning exercise) to assess the scope and depth of the available literature during the selection procedures and ensure that the distillation of relevance is consistent with the inclusion criteria. In the Taylor and Francis database, even though it contained more than 3,000 articles related to the IoT and Smart city applications, none of the articles were associated with blockchain technologies' experimental nature. Instead, these articles adopted a theoretical or summary approach reflected throughout many of these databases.
Prisma Flow Diagram
The summary output model presented in Figure 1 highlights the effects of the systematic distillation process which resulted in the parsing of many thousands of studies down to a much smaller, directly-relevant sample. Figure 1 provides a visual representation of the PRISMA (2019) flow diagram employed to increase the overall transparency of this SLR reporting process. It further highlights the critical assessment and rejection process based upon the increasingly specific search functions performed during the individual database reviews. At each stage in the PRISMA model, studies were either included or excluded on the basis of the screening criteria which included the inclusion/exclusion criteria, a determination of completeness or topical relevance (e.g. needed to include blockchain), and a quality assessment of the journal or source of publication. In many cases, studies were excluded because they failed to link blockchain models to smart city solutions. In other cases, smart city solutions focused on IoT nodes but failed to consider the blockchain. All three of these top-level keywords, blockchain, IoT, and smart city were critical to ensuring that this study met the central research objectives and answered all of the questions developed for this research process. As evident from this model:
- This review has identified 26 studies which were based on blockchain technologies, IoT capabilities, and smart city solution conducted in the period from 2016 to 2019. This list can be used by other studies to expand the work in this particular field (Appendix A).
- This review has selected 18 primary studies that fulfill the criteria mentioned in Table III for quality evaluation, were published in academic journals, contained some form of experimental or model-oriented solution, and most importantly, offer a distinct advantage over other proprietary research in this field: transferability and adaptability.
- The final primary studies can offer appropriate benchmarks for comparative analysis against the current review paper.
- This review paper has also conducted an in-depth review of the selected experimental studies to elaborate on the ideas, considerations, and research in blockchain technologies, IoT capabilities, and smart city solutions.
- This study has presented an SLR concerning applications in which Blockchain can be implemented with the collaboration of IoT capabilities and smart city solutions.
- Appendix A presents a structured summary and systematic evaluation of the central perspectives and findings relative to each of these included studies.
Threat to Validity
Although the studies identified over the course of this SLR are indicative of the leading research in this field of blockchain innovation and IoT applications, their reliability and practical functionality remains under-tested. Validity in the context of the IoT is about immutability and any potential repudiation, conditions that cannot be tested without an ecosystem in which to experiment, evaluate, and confirm the practical value of the originating solution. The review of these experimental models is derived from an assumption of reliability and validity that is conditioned both by the credibility of the source documents (e.g. journals, conference papers) and the reputation and identity of the authors in question (e.g. professors, students, researchers). These findings, therefore, assume construct validity on the basis of the comparable similarities between these approaches and the overlapping characteristics of the methods and models in question. Future testing and analysis will be needed to determine whether this proposition is reliable or sustainable over the full scope of implementation, testing, and usage. In this case, the comparable overlap between the individual studies has been used to establish the validity of the results from the SLR and to confirm that there is an interwoven theoretical and conceptual position that has evolved out of multi-directional study, model development, and experimentation in this field.
Moreover, there are numerous threats that may emerge when carrying-out a systematic review. For instance, not all studies or sources identified are relevant based on information. In this regard, different search criteria were identified and researched for different databases for eliminating this threat. Different criteria and logical operators were applied for increasing coverage. All relevant documents were found through different keywords combinations. Majority of the research attained after the exclusion criteria were published in 2016-2019 even though the subject was completely new. In this regard, it is assumed that the missing article review on the subject was too minimal for influencing the findings of this study.
A threat in this context is associated to unpublished works or related works that are not currently present in the selected scientific database. The excluded publications do not influence the internal validity since the selected databases were well-articulated and researched. It might be claimed that a margin of error might be present due to initial sampling even though the articles were covered based on the selection criteria. Thereby, the margin of error is used for calculating the internal validity in systematic literature review studies. External validity refers to the level the outcomes of the study can be generalized for other circumstances, times, and individuals. The data obtained, in systematic review, as an outcome of research questions are assumed for reflecting general outcomes with respect to existing blockchain patterns and research.
I. Research Questions and Evidences
Research Question 1: How much progress has been made so far in securing and protecting IoT-based integration into smart city solutions?
The original novelty of IoT-related smart city solutions has gradually evaporated from the academic compendium and has replaced uncertainty and optimistic innovation with a more pragmatic, utilitarian, and integrative focus. Ideology in this field of study was related to IoT-based innovations and indicative of a complex tension between uncertainty and innovative potential. This was attributed to the lack of structural vision and the competing spectrum of heterogeneous technologies and smart city applications25-27. In fact, Almirall et al.28 and Chen et al.29 have connected network security threats with the heterogeneity of IoT-related technologies and innovations in a scholarly manner. Nevertheless, the SLR has illuminated an alternative structural shift in the field of IoT research. Therefore, it has replaced the underlying questions of why, what, how, and where. Progress in this field has shifted away from the technical functionality of blockchain-based IoT integration in the smart city, and instead, towards understanding the means of unifying the intermediary standard of authentication and data management. Researchers are now pursuing understanding of how IoT nodes can be deployed, secured, and activated without requiring the proprietary limitations of existing applications and service provisions; instead, linking the capabilities of the IoT through a broader, unified proposition of blockchain-supported connections (e.g. smart contracts).
Innovation, by definition, in the modern fabric of the IoT architecture, is now defined by the integrative potential of the individual nodes within a broader digital footprint22. Khan and Salah30 once predicted an iterative security solution capable of leveraging ontological integration and threat identification to shape the security function. Studies established that the modern IoT agenda was predicated based on holistic, structure-integrated, non-repudiated, and ubiquitous cyber-security standards31-33. More importantly, the IoT was no longer object-specific; instead, it was a mesh-based solution that incorporates multiple decentralized nodes into a performance-broadened, function-diversified solution that emphasizes service execution over data encapsulation32.
Blockchain, whether private, public, consortium or hybrid, is a peer-to-peer distributed ledger technology that records agreements, transactions, sales, and contracts. It also confirms transactions for creating a verified as well as unchangeable information ledger. The attributes of open, interconnected, transparent, and peer-to-peer sharing and storage are appropriate in the Blockchain to the interconnection, peer-to-peer, openness, and shared attributes of the energy internet.
Blockchain can develop significant smart grids and networks, changing how everything is developed from the vote and established credit to receive energy. In certain ways, it could be an essential constituent of what is required for circumventing outdated systems and providing lasting solutions to cities.
Six central themes have been identified within this SLR regarding IoT-related innovation and the contribution of research evolution to a revised frame of reference related to IoT adaptation for smart city solutions presented as a visual representation in Figure 2. The brief explanation of each iterative dimension of the IoT solution is as follows:
- Adaptive: For the IoT to meet the varying needs of urban residents, individual nodes and their underlying software technologies must be sufficiently adaptive to create a mesh network of valuable, specific, and interdependent information resources34,35.
- Integrative: Any IoT solution is sufficiently valuable as a standalone service (e.g., smart locks on the house); however, integrative IoT solutions provide users and companies incentives to link and connect multiple devices to a central database or information management solution24.
- Intelligent: By interpreting and purposefully triggering actions related to predetermined guidelines and authorizations, each node in the IoT becomes a critical service agent within the smart city solution36. Although adaptation and integration provide the basis for a connected overlapping network, it is the intellectual functioning of the IoT that ultimately meets the requirements of smarter, high-functioning services21,31,37.
- Secure: Given the critical nature of the information being collected regarding user behaviors, locations, and identities, the IoT must be sufficiently secured through confidence-enhancing technologies to motivate user participation38,39. Integration further exposes systemic vulnerabilities; therefore, some intermediary or standardized security protocol is required to protect users against internal and external threats32.
- User-Defined: User-definitions serve as conditions for shaping IoT services and shaping IoT services and protecting individuals against the loss of control over such critical, personal resources35.
- Service-Oriented: The IoT is service-oriented, and as a result, the data collection process is only secondary to the end outcome of the user-device relationship: the service36. By designing IoT-based solutions that meet specific needs or expectations, companies are reshaping the nature of human-technology interaction, and as a result, they are precipitating and accelerating the evolution of information-supported, autonomous service provision32.
There is an immediate need for a framework capable of fulfilling lightweight security requirements and performing information management functions40. Critical factors related to dependability, authentication, security, and decentralization are important antecedents to the future of smart city technologies40,41. Through a review of experimental studies, it has been made evident that the unified concept of a 'canopy framework' described by Paul et al.42 is an essential layer of innovation that will provide the next generation of system designs with a more efficient and unified solution. Central conditions of this standard include a lightweight, low-power, and low-computing standard of technology that transfers the responsibility for information management to either a central blockchain node or to outlying administrative nodes that limit the need for IoT-based information processing42.
This standard validates the model presented in Figure confirming the expectation of heterogeneity and adaptively and emphasizing a direct relationship between a predictable and standardized middle-layer technology (Blockchain) and outlying IoT-based proprietary nodes. In this way, the Blockchain as a service (BaaS) standard eliminates the companies' need to develop their database management solutions. It will ideally allow smart city technologies to be added and removed from the network solution according to the need rather than technological dependency35,37. Accordingly, API solutions will remain proprietary and can be developed to support the independent module and objectives of a company. Similarly, the revised homogeneity of the Blockchain's middle layer solution is an important innovation that will realize the broader objectives of a secure, scalable, and interdependent smart city standard18,43.
Research Question 2: What role could Blockchain play in serving as a unified, central database and transaction ledger for IoT-based triggers?
Throughout this SLR, the evidence captured has illuminated a form of transformative shift, whereby uncertainty regarding blockchain technologies exemplified by prior theoretical research has struggled to address the practical and the theoretical inconsistencies36,44,45. Whether it is conceptual or theoretical, the Blockchain serves as a placeholder or viable solution that needs additional testing and exploration in various exemplary contexts and system specifications36. When implemented practically, Blockchain becomes a modality or method of goal actualization; it transforms the underlying limitations of extant IoT technologies into a form of catalyst for innovation and model adoption17. The SLR has illuminated a gateway opportunity in blockchain technologies that can address many of the underlying limitations and deficiencies reflected in the innovative effects of IoT development and deployment in a technologically heterogeneous marketplace. The following five dimensions represent the blockchain solution's indicative characteristics and their roles in servicing and shaping integrative technologies of the IoT-based smart city.
- Security and Authentication: This characteristic of the blockchain-based ledger solution secures and connects with the hash-based, encryption-supported, proof-of-work standard of authentication and data protection18. IoT nodes are vulnerable to inner attacks or external tampering due to their low-security profiles and lightweight characteristics. By maintaining records of service-related transactions on the decentralized ledger, individual users, and support agents acquire the ability to provide or restrict access to the resources, databases, and actions necessary to make the IoT a high-functioning, service-centered solution46,47. IoT nodes self-authenticate autonomously without requiring a central authority and system changes can be detected relative to blockchain-level permissions and records. The base of security is to ensure the validity of identity of a device that access to the network in the IoT. Authentication is a mechanism by which a network refers whether the user has access to specific resources. The authentication can be classified into three classifications: (1) possession, (2) ownership, and (3) knowledge. Public key cryptography is used for preventing illegal devices in order to access the IoT for authenticating IoT entities. The major difference is that a peer-to-peer authentication methodology is introduced regardless of third-party based on blockchain. Security protection ensures the reliability of the IoT devices. It still has the risk of being attacked by malicious users because of the system or software even if a device has passed the authentication of other nodes. The network entity is modified by the intruder for leaving a backdoor in the device for preparing corresponding infiltration and modifying the fundamental configuration file in the device. Critical data have been regularly verified for discovering potential intrusions instantly.
- Encryption and Access Keys: Through the hash-based encryption capabilities of the Blockchain that employ a standard like Ethereum, companies can anonymize and protect user information and identities35,47. Yet in spite of this protocol, the pseudo-anonymous character of these alt-coin solutions may potentially expose the blockchain to linking attacks along downstream nodes35. Encryption also can be used to secure cloud-based storage solutions and to reduce the likelihood that any usable data could be scraped or accessed by individuals with malicious intent37. Access keys (public and private) afford the owner the right to initiate a transaction or access a resource, and are secured by complex cryptographic signatures that make reverse-engineering and replication a nearly impossible exercise48. Shared secret key technique is used for delivering secret key in the header request for accessing the endpoints. REST protocol is used for providing a standard approach to access the endpoints. Basic HTTP Authentication is used to secure all REST endpoints or by using a shared secret key.
- Structure and Network Integration: The middleware status of the Blockchain was a critical antecedent to the universal integration of this data management solution across all IoT-based networks38. Even in scenarios where companies choose to maintain a private blockchain, Blockchain's unified centralization ultimately indicates its adaptability across several network configurations and service-based solutions47. The blockchain is characterized by a range of public, private, and hybrid solutions that can be customized according to the unique protocol and computing needs of the intermediary network (e.g. public entity, private enterprise, consortium)32.
- Decentralization and Transactional Management: One of the smart city vision's primary goals is to automate and decentralize service solutions and letting smart city vision's primary goals are to automate and decentralize service solutions and let artificial intelligence and algorithmic controls respond to user behavior in productive ways43. By decentralizing these responsibilities, government agencies have reduced the weight of resources and information technology agents invested in system monitoring and task execution processes32. Similarly, companies participate in a peer-to-peer solution by integrating additional modules and technological mechanisms to expand the scope of smart information and services49,50.
- Smart Contracts and Activity Triggers: The antecedent to a secure and efficient blockchain solution for smart city applications is the smart contract36. This contract is defined by its predetermined authorizations and triggered by both the user and service providers30. The reciprocity of this input-output-based system ensures the protection of users during the information transfer process51.
Components such as being distributed, secure, shared, smart, and encrypted offer the mechanism for being democratized, automatic, private, and transparent in the computing of blockchain-based sharing services. The computing block-based sharing services support the automation of business services and transactions. IoT devices can take part in trust-free transactions, and contracts can be acquired in computing codes for automatically performing the obligations that users have committed to in accord when enabled by blockchain technology.
It should be noted that a smart contract has now been integrated into the Blockchain. It is also noted that code functions are integrated within a smart contract and can connect with other contracts, store data, send ether to others, and make decisions. A blockchain is integrated into Watson IoT that facilitates information from devices such as device-reported data, barcode scanned events, and radiofrequency identification (RFID) to validate smart contracts or to be shared with device-reported data52. Software agents can be adjusted for dynamically managing each distributed independent organization, which connects physical nodes in a network to devices throughout a series of contextualized smart contracts. Sharing business will be automated by computing blockchain-based sharing services and the highly efficient IoT supported by the internet and a series of contracts, smart transactions, and agents53.
Research Question 3: What are the advantages (and limitations) of Blockchain's integration within smart city applications?
One of the major revelations of this SLR was that any limitation to blockchain integration was due to a delayed response to the gradual and niche-based research development in this field of study. Researchers have envisioned a new pragmatic future for blockchain integration; the transition from an enigmatic and misunderstood status to this progressive frame of reference has been checked thoroughly for any uncertainty16,18,47. Specifically, this SLR has revealed multiple layers of uncertainty related to the following critical advantages of blockchain integration:
- Immutability: Once the data has arrived at the Blockchain, it remains immutable, as the hash serves as a mechanism to validate the unaltered data32,35. As an advantage, immutability ensures accuracy in the smart contract exchange and reduces user vulnerability to unauthorized or inconsistent manipulations of the agreement35.
- Limitation: If the data enters the corrupt Blockchain, it will remain corrupt when recorded within the same blockchain ledger35. This vulnerability creates challenges for IoT developers, and device-level security is required to restrict the possibility of misleading or inaccurate transactions.
- Decentralization: More than 5 billion IoT devices are connected to the internet, and it is predicted that more than 29 billion IoT devices will be connected by 2022; therefore, a decentralized and autonomous database is needed to process large amounts of data without threatening the privacy of the individual users31.
- Limitation: The lack of centrality in the administration of smart agreements and contracts could lead to conflicts between proprietary designs and organizational solutions when organizations fail to embrace a blockchain-based proof of work standard17.
- Non-Repudiation: Contractual conflicts and legitimacy are restricted in blockchain-supported smart networks because of the hash-based, smart contract-controlled processing, and storage of data within each block30. By optimizing the number of blocks within a given system and restricting outlying variants through mining-based hash-maps, Blockchain offers a non-reputed solution that is confirmative and transaction-validated31.
- Limitation: Corruption of the Blockchain via unauthorized or false contracts could create mistrust and network vulnerability17. Structural solutions that restrict tampering potential, such as edge-aware and smart semantic contracts, could potentially eliminate the majority of these threats in a blockchain-integrated network49.
- Network Framework and Integration: For smart cities to realize their optimal, integrated structure; there is a critical need for an intermediary layer capable of objectively and autonomously negotiating smart contracts and providing agents and users with appropriate access17. The Blockchain’s decentralized character is valuable for a multi-layered system fulfilling a critical gap in IoT-based smart city design that would potentially have been filled by some form of limited proprietary technology49.
- Limitation: Organizational buy-in and user support is a critical antecedent to the investment in this process, and as a result, it acts as a limitation that could potentially restrict such engagement in short to medium term34.
The smart city experiences several challenges. Some of the most substantial challenges are associated with an elevated amount of data transfer that ensures security. This study encourages developing the capabilities to use novel blockchain technologies that can overcome challenges by taking benefit of the opportunities and advantages of Blockchain and other associated technologies. Blockchain technology offers massive possibilities for shaping the improved smart communities in the forthcoming events that are more efficient and offer a better quality of life. The technology should be enabled for changing the existing situation, which should be replaced via innovation for delivering on its promise. This study further demonstrates how blockchain-based sharing services can assist in creating smart cities and discusses the foundations and concepts of Blockchain and the application of blockchain technology. Clear examples of the practicality of this technique are indicated in different economies in different sectors. It indicates the comprehensive, continuous, and successful experiments, offering a clear perception of the benefits and advantages. The technique also highlights the challenges that could be encountered in the integration of such technologies in other contexts.
Research Question 4: Which components of the smart city solution can be standardized, and which components will remain proprietary?
For a smart city design that meets the diversified requirements of complex urban environments, any unified solution must consider the scalability and adaptability of the distributed yet interconnected nodes33,36. As a result, blockchain technologies' middleware solution presents a potential for standardization, whereby data processing, cloud-based storage technologies, and security authentication are connected to smart contracts and transaction signatures39. In each of the studies reviewed within the SLR, it became evident that the blockchain solution's conceptual underpinnings remained constant, while the outer level nodes were adaptive according to their user and administrative guidelines. There are several central dimensions of the standardized blockchain solution that need to be incorporated into any smart city solution. Figure 3 visualizes four central traits of a unified and standardized blockchain solution to facilitate lifecycle usability and maximize the blockchain solution's efficiency and intra-transactional value. These core components are further defined as follows:
- Scalability and Centrality: The blockchain solution must be sufficiently scalable and maintained centrally in order to allow for network connections across broadly defined public channels. This means that additional IoT nodes can be added or subtracted freely and without damaging the continuity and reliability of the network.
- Trust and Security: There is a unified expectation that any connections to the network will be sufficiently trustworthy and that they will afford the security to restrict access to user data or identity (e.g. anonymity, encryption, protections). This means protecting against both insider and outside attacks, creating stop-gap solutions that restrict unauthorised access, and affording users the ability to set, modify, and remove permissions.
- Authentication and Authorisation: Established as the formal basis of the API contract between the user and the service provider, the authentication protocol should create a consistent basis for authorising actions on both ends of the smart contract.
- Transaction-Oriented Ledger: Based upon immutable record keeping, consistent transactional processing, and predefined agreements, the blockchain operates as a central ledger or clearinghouse for the IoT providing the basis for fulfilment and execution of the contractual agreements.
Where the blockchain, the proof of work concept, and the data storage protocol will need to be standardised in order to address the scalar complexity of the smart city architecture, the need for proprietary and heterogeneous software and hardware solutions must also be considered. The varying contractual terms required for smart energy meters and self-driving automobiles are different, and for this reason, contractual dimensions cannot be established by the service provider39,51. Besides, the type of information collected by the IoT node should be considered, and while the terms of the smart contract will define the access to this data, consumers will also need to be able to maintain control over how and when their data is accessed and used31,52.
Finally, proprietary APIs and data interpretation software will be needed to make sense of the broadening scope of big data that will ultimately be captured via these diversified smart nodes. Decentralized data collection will have opportunities for both public and private enterprises; therefore, as consumers recognize the potential advantages of data sharing and access granting, this smart mesh network's net benefit will become contingent upon the proprietary leverage of fog-based information resources32. The SLR has demonstrated how purpose-built experimental models are designed to connect individual technologies (e.g., one IoT node) with specific transactions or behaviors (e.g., smart electricity monitoring). Simultaneously, the broader complexity of a more dynamic solution that considers the integrative potential of a centralized data management solution (e.g., a public database) must be considered within the context of both public and private enterprises.
Research Question 5: Which structural limitations or challenges will prevent blockchain integration, and how could these problems be resolved?
One of the major limitations observed within this SLR was the uncertainty and misunderstanding that has made blockchain adoption so inconsistent over the past decade. While academic vision and innovation provide the primary antecedents, more productive structural solutions such as experimentation are entirely inadequate when considering smart city applications' breadth. Independent studies conducted by Ouida et al.47 and Rahman et al.18 have established a framework of progress and proof of work design capable of facilitating change and transforming the structural expectations regarding blockchain integration; however, additional real-world demonstrations are needed to highlight the scalability and connectivity of such solutions. Further, the commercialization of any Blockchain integrated solution will require both consumers and corporations to trust in an enigmatic technology that has generally been associated with crypto-currencies and controversy. With security threats publicized and the risks of unchecked and potentially vulnerable technologies are debated at public forums, the blockchain solution’s legitimacy and consistent fidelity have yet to be proven.
Another problem with the aspirational development of a unified blockchain solution is collaborative innovation or a centralized, publicly-supported framework. Independent innovations such as the multiple adaptations of the Ethereum protocol create conduits to structural progress, but they also raise questions about the viability of a singular, centralized standard19,35,48. For Blockchain to become an effective middleware solution; there must be a regulatory body or overarching standard established for smart city service management. Proprietary IoT nodes are invaluable and must be supported to ensure that the smart city reality is achievable; however, these decentralized nodes represent just one layer of a mesh solution that places outlying edge-level nodes into a purpose-built technological fog34,35. Therefore, to overcome the myriad structural, security, and data management limitations that affect the practical advantages of the IoT-based smart city solution, a unified blockchain framework is needed to establish, control and legitimize the underlying smart contracts.
Big data management might be eased through the secure and verifiable blockchain structure54. On the contrary, data analytics via blockchain structure, implying too much overhead. All transactions will not be essential, and therefore, efficient or intermediate auxiliary structures might be integrated to increase the overall efficiency. Thereby, solutions must be implemented on an ad-hoc basis. Nonetheless, there already remain blockchain-based structural solutions for big data storage55.
II. Overall Discussion
Many issues have yet to be addressed while blockchain applications are being broadly deployed, which would make them durable, scalable, and efficient56. The components they provide are not unique if evaluated individually, and most of the platforms are well-represented for years. However, combining these components make them competent for several applications to justify the intense interest of several sectors57.
Their applications are anticipated to penetrate additional domains or industries compared to the ones covered in this survey as blockchains’ become more mature 58. However, this is far from the truth; while many applications propose blockchains as a solution and an alternative to databases, there are many other solutions where conventional databases must be utilized instead59. The individual attributes are identified that are mainly needed per each application domain. This enables selecting the appropriate Blockchain and the subsequent platforms for tailoring the Blockchain to the actual requirements of the application60.
It has been noted that the smart city is becoming much smarter because of the existing expansion of digital technologies. Smart cities comprise different kinds of electronic equipment integrated by some applications, including sensors in a transportation system and cameras in a monitoring system. Also, the use of individual mobile equipment can be explored. Therefore, different terms such as participants, motivations, security policies, and objects' characteristics should be considered by taking a heterogeneous environment. Smart cities' essential elements include smart energy, smart buildings, smart technology, smart healthcare, smart infrastructure, smart mobility, smart citizens, smart governance, and education.
Interoperable Imperative
A critical antecedent to identify an adaptive, information-rich, and fluid smart city ecosystem is represented through networked interdependencies. The intersection between dynamic and static resources will enable a process of data-rich, behavior-aware and smart transactions that will essentially change the scope of technology-improved decision-making in unified urban spaces using iterative enhancements61-63.
The competitive precedent for siloed technological development has caused structural fissures as algorithms, networks and proprietary modules, and network-spanning information exchange of smart nodes in the smart city ecosystem despite all benefits64,65. A unified proposition for interconnected and integrated solutions is indicated via recent decentralized wireless networks such as Helium and IOTA proposed that allow positioning data ledgers as intermediary clearing houses for transaction-aware smart networks65,66. The existing solution suggests a non-proprietary, standardized, and unified protocol for inter-connecting IoT-based smart city solutions with a blockchain middle layer for protecting both user and commercial preferences in a distributed and circulated smart city ecosystem.
Blockchain and the IOT
The IoT is accomplishing a widespread technological perspective of multi-nodal and integrated communication across distributed networks via multi-function, low-power, and lightweight devices68. The scale of IoT-based solutions is rapidly expanding from smart home solutions to critical city features. The potential to mine and interpret these data resources is restricted because of the paucity of interoperability across proprietary systems with several devices attaining usage, environmental and behavioural information32.
Each hardware unit of the IoT is restricted in both communication and computing power when classified as restricted nodes that constrain its competency to effectively secure and monitor unauthorized activities and security breaches57. It becomes essential for systematically mitigating the code size by modifying the connection network's infrastructure via a distributed solution with both light and full nodes, whereas blockchain solutions can address the authentication stress of the IoT57.
This network topology depends upon what has been realized by69 as a distributed consortium network and outlying side-chain networks connecting IoT devices to intermediary notary nodes on the blockchain-based on an edge-computing protocol. A parallel consensus and transaction-verified authentication have been facilitated from the central conceptual foundation for the Helium network or Tangle proposed by67, which ensures paucity of conflict between the existing and any previous transactions69. Blockchain technologies were proposed as a decentralized solution that is dynamically susceptible to double-spending, security breaches, and exploitation for online transactional systems31,37,39. The authenticity and non-repudiation of the blockchain exchange are immutable as it forms a foundation of distributed accountability and trust that resolves the essential conflicts surrounding agreement-based legitimacy through hash-based registration of outgoing and incoming transactions39. Therefore, this trustworthiness addresses most of the security and susceptibility-based issues, which undermine lightweight IoT nodes' resiliency.
Existing IoT solutions work on proprietary networks with limited interoperability, which result in single-stream transaction outcomes such as user-device-service provider. Cross-service communication between devices is needed for integrated smart IoT applications for facilitating authorized trigger-command-response outputs and enhances the comparative smart value system70. For instance, an in-vehicle module can use geographic information system (GIS) data for triggering home automation throughout the final phases of the drive, which could open the garage, illuminate the home and maintain a preferred temperature based on user-prescribed settings. Integrated commands should be communicated and responded based on several APIs developed for each IoT-based node, which an unrealistic process is considering a lack of multi-modal communication across individual devices and proprietary challenges71.
The wireless mesh network is one of the pragmatic developments developed through current long-range and high-frequency wireless technologies67,72. Autonomous IoT solutions are developed to negotiate with thousands of clients and obtain data appropriate to service support and provision by implementing a large-scale advanced metering and monitoring system73. An open-source and centralized blockchain-enabled solution is developed by involving the proprietary-network architecture for data transaction procedures while restricting data access and unauthorized transactions for extending this model64,74. This middleware layer will reduce the requirement for high-bandwidth and complex network connections using cloud-limited access and blockchain authentication. Instead, IoT nodes to use any authorized network to communicate when fundamental tolerances or circumstances are fulfilled.
Trust, Security, and Smart City Applications
Data exchange’s scope and specificity are considerable across smart city nodes, and security and authorized access are fundamental issues for service providers75. The extent of corruptible intermediaries related to networked interfaces is limited, and trust-based exchanges are ensured by applying cohesive standard IoT devices for a centralized and Blockchain negotiated database76. The proprietary APIs and user interfaces are retained by navigating transactions to edge computing and centralized blockchain requirements, while centralized transaction navigation is transmitted to cloud-based intermediary services throughout the blockchain solution77,78.
The benefits of transmitting data and computing management restrict corporate entities from small-scale experimental models of this interface and offer users the competence to navigate and limit access to their device-shared data79. Smart city interfaces offer an opportunity to navigate and interpret the noise and impairments featured through human behaviour for city officials. These solutions entail establishing real-time models, and predictive metrics merged with multiple layers of inputs for presenting a pragmatic aspect of supply and demand postulations18. Therefore, the effectiveness of future responses relies upon the connection between real-time updates and informational awareness, which increases the significance of a standardized and unified middle layer competent to connect several proprietary devices across the urban ecosystem. The adaptability of infinite loop architecture regulates data management and security abilities while encompassing the edge-to-edge elasticity of technology innovators, service providers, and developers11,80. In smart city solutions, there are multiple paradigms of the application of this model:
- Smart Energy Meters: Supply and demand data are transmitted by user-defined interfaces customized for both energy consumers and city managers via demand profiles and IoT tracking nodes to monitor route energy and enhance the overall efficiency of non-activity periods39,51.
- Smart Trash Management: Real-time data can be reported by smart waste bins prepared with IoT sensors for waste management administrators to trigger consistent and on-demand pickups81,82.
- Smart Route and Environment Monitoring: The ability to track carbon emissions, urban flow and population density over extended periods are monitored through real-time analytics for proposing solutions for flow-routing, pedestrian awareness, and capacity-building83,84.
The Solution: Standard-Setting and Blockchain Integration
It becomes vital that any coordinating solution is competent enough to address the particular restrictions about security and scalability due to the confined computing power in each node, the susceptibility of always-on or on-demand network connections, and the scale of the IoT85. The fundamental aspect of blockchain-based IoT is to use the decentralized agreement mechanism for guaranteeing information security before transactions are executed with possibly corruptible external nodes86. Transactions are predictive of IoT processing solutions where user events trigger downstream responses once identities are confirmed based on the contractual agreement87.
The blockchain middleware solution offers a high-efficiency and high-integrity solution to realize the overall intersected objectives of smart city solutions via mining-verified decentralized ledger, encryption, and security keys. Therefore, this study proposes the integrated solution based on five fundamental conditions that will achieve the overall objectives of interoperability, smart city deployment, and decentralization once the IoT is integrated with the blockchain middle-layer. These conditions are listed as under:
- Scalability and IoT Information Exchange: The maximum potential for expanding information processing abilities comparative to demand or supply network scale while integrating nodes on-demand80,88.
- Unified IoT Networking and Interdependency: Cooperative consensus, exchange-based architecture, user-defined controls, standardized protocol, and trans-organizational network solution89.
- Digital Authentication: Restriction and authorization are ensured by digitally signed transactions based on pre-existing permissions and contracts76,77.
- Autonomous Exchange and Transaction Processing: Permissions-based intuitive decision-making, independent transaction processing and restricted human navigation and network interaction79,90.
- Security and Threat Mitigation: Continuity-ensuring and access-restricted controls based on adaptive, independent, and encryption-protected interventions91,92.
The Model
A Blockchain solution is proposed in Figure 4, based on a current standard of exchange, even though it is dependent on the transactional modality, which has yet to be examined and investigated in practice. This model features an infinite-loop foundation for IoT consortium-based integration. Two external notary nodes are introduced by the model present beyond the mechanism of transactional control in the Blockchain, which includes user-defined preferences, software support, terms and consensus of the service solution, and privacy controls offered by the legit companies. The IoT nodes are brought online, and the privacy preferences are developed, and the terms of quality of service and sharing are ensured once they have physically transferred ownership from corporation to consumer.
At the IoT node, transactions are triggered in the side-chain network, which activates the Blockchain's smart contract. This will allow the network to approve and record responses as per the company's software solution at the notary node. There is a transmission of blockchain-encrypted information between companies according to the terms of the agreement. Return response to another IoT node will be triggered through responses following the information sent from Company A to Company B, which will result in the pre-approved action. In this regard, each IoT remains in its decentralized autonomy, but it is also competent for responding immediately and efficiently to the trigger or signal sent from the service provider.