# Survey of Blockchain-Based Applications for IoT
## 8 Blockchain Scalability and Efficiency
### Multi-Layer Blockchain  
A blockchain can be implemented in a multi-layer architecture by connecting several private networks to a public blockchain [61]. In this multi-layered architecture, users can control which data are shared on the public blockchain through access control policies that can be enforced by a smart contract. A multi-layer approach can enhance performance by implementing various blockchain technologies across different layers, each chosen to leverage its unique advantages. In this architecture, the blockchain is a medium for securely storing non-sensitive information, such as access control lists (ACLs) and metadata, and facilitating data exchange. The tamper-resistant nature of the blockchain ensures that this information remains protected against unauthorized modifications [62]. Figure 12 below illustrates this blockchain architecture. The layer 1 blockchain serves as the core network, whereas the layer 2 blockchains handle additional computation and scaling. Bridging nodes connect layer 2 to layer 1, keeping the blocks local for privacy, security, and performance. Only essential transactions are bridged to the main chain, reducing congestion and optimizing efficiency.  
Given that many IoT devices are limited by constrained computational power, storage, communication speed, and energy, a multi-layer approach is essential for IoT blockchain deployment. These devices often lack resources for intensive blockchain operations such as consensus mechanisms and encryption. IoT devices can benefit from blockchains' enhanced security and transparency without being overtaxed by offloading these tasks to  
Figure 12: Multi-layer blockchain.  
more capable layers. This approach supports a scalable, efficient, and secure IoT network that can address the limitations of resource-constrained devices.  
Figure 13 below shows a multi-layer blockchain system designed for IoT integration.  
At the access layer, IoT devices (e.g., home appliances and sensors) connect through IoT gateways, which manage communication via legacy protocols;  
The edge layer uses a sidechain (layer 2 blockchain) to process local transactions and reduce latency and computation load at the core layer, supported by bridging nodes that transfer data to the upper core-cloud layer;  
The core layer utilizes a Main Net (Layer 1 blockchain) for global consensus, high-security storage, and interoperability.  
The introduction of the edge layer offloads computational tasks from resource-constrained devices to more powerful edge servers or cloud data centers, significantly enhancing system performance and reducing latency [63]. This shift not only alleviates the processing burden on end devices but also improves the quality of service (QoS) and user experience by enabling real-time processing and analytics closer to the data source. Offloading computational tasks allows for the dynamic allocation of resources, optimizing energy consumption while maintaining high performance. Furthermore, it reduces network bandwidth usage by processing data locally whenever possible, rather than transmitting large volumes to centralized cloud data centers. This approach is particularly beneficial in applications like the IoT, smart cities, and autonomous systems, where rapid data processing and low latency are critical [63].