# IoT and Blockchain Integration: Applications, Opportunities, and Challenges
## 7 IoT and Blockchain Integration  
The IoT has proven to automate domestic, industrial, and business monitoring and functioning. Nevertheless, it relies on centralized cloud computing for data storage, processing, and network command and control. With the centralized model, the integrity of the data and the process output of cloud computing are always in question because IoT owners have to rely on a third party (such as cloud vendors) for the integrity of the data and the process outputs [30].  
On the other hand, the basic features of a blockchain network include transparency, verifiability, data redundancy, and trustworthy [52]. These principal features can fill the gaps in providing the security-related guarantees required by an IoT network and related applications. Thus integration of an IoT and a blockchain network can serve a primary need for colossal storage, business/industry automation, fault-tolerance, and data/process integrity.  
Devices in an IoT network can be designed to play different roles in a blockchain network. Edge devices such as routers, routing switches, integrated access devices, multiplexers, and gateways can be designed to store the blockchain and perform transaction validations. Intermediate devices such as relay routers can be designed to execute services for issuing transactions to a BCN on behalf of the connected resource constraint devices such as sensors. Several integration models have been discussed in previous studies [30; 53]. Nevertheless, we summarize them into four major IoT and BCN integration models.  
1. Sensor devices as Transaction Issuer (SaTi): In this integration model(see the interaction (a) between sensor node \(s\) and BCN in Figure 6), IoT devices such as sensors take part in issuing transactions to the external BCN. Such IoT devices should be designed to accommodate computational power and bandwidth requirements. However, the typical IoT devices do not have the storage capacity to store a complete blockchain. In many use cases, such as industrial IOTs, this model may be too costly and ineffective for sensors to communicate with an external BCN. In that case, edge devices such as IoT gateways may be employed to issue transactions on behalf of all the low-power, resource constraint IoT devices.
2. Edge devices as Transaction Issuer (EaTi): In this integration model (see the interaction (b) between edge device \(g\) and BCN in Figure 6), specially designed IoT edge devices such as gateway routers may be actively issuing transactions to an external BCN, without actually storing a copy of the blockchain ledger. This integration model is efficient, given that it requires a limited number of edge devices for interacting with the BCN.
3. Edge devices as Transaction Verifier (EaTv): This integration model extends the EaTi model, where specially designed IoT edge devices issue transactions to the BCN and maintain an entire blockchain ledger for active block validations. In many cases, edge devices could handle both issues and validate transactions being an active node for a BCN. However, unless business interests require it, an industry may not use edge devices for transaction validations, which are computationally intensive and require higher storage and bandwidth. In the figure, interaction (c) is part of this integration model.
4. Hybrid: In a hybrid integration model, IoT and blockchain interact through edge devices and specially designed IoT sensor devices. It depends on the applications whose interactions go through edge and IoT devices. In the figure, interactions (a), (b), and (c) are part of the hybrid integration model. In other words, sensor devices issue transactions, and edge devices issue, and validate transactions for a BCN. Nonetheless, this model imposes redundancy in issuing transactions for a BCN, which is costly in terms of bandwidth.  
Having discussed the models of IoT and blockchain network integration, the following section discusses some compelling applications born out of IoT and blockchain integration.