## 3 Key Technology of the A2S Integrated MIoT
### Routing Protocol
#### 3.3.2 Opportunistic Routing  
When it comes to A2S integrated MIoT routing protocols, opportunistic routing is one of the common routing methods. The schematic diagram in Figure 8 illustrates the opportunistic network. Opportunistic routing refers to a way to realize DT by transmission opportunities when there is no direct communication among nodes in the A2S integrated MIoT [135]. This opportunity can be either an overlap of two nodes in time or an overlap of two nodes in space. Specifically, this routing method can be divided into the following steps:  
* Data topology construction: In the A2S integrated MIoT, each node has a data cache to store the received data. When a node receives data, it first stores the data in the local cache, establishes a data topology relationship, and records the source and destination nodes of the data.
* Chance encounter: When two nodes come into proximity, they exchange data in each other's cache. If one of the nodes can forward data to the target node, the data are sent to the target node; otherwise, it continues to be stored in the local cache.
* Data transfer: When the data finally reach the destination node, the entire routing process is completed.  
The advantage of opportunistic routing is that in the MIoT, even if nodes cannot communicate directly with each other, data can be transmitted through various opportunities to improve the arrival rate of data [136]. For example, on a ship, direct communication may be unstable due to factors such as geographical location, weather, and sea conditions. However, through the opportunistic routing mode, nodes can make use of opportunities for DT to improve the efficiency and reliability of DT. It is worth noting that opportunistic routing also has some limitations, such as the need for a large cache space, a long time to discover opportunities, and also need to design effective routing protocols to ensure that data can be transmitted in a timely manner [137].  
In A2S integrated scenarios, the opportunistic routing technique is being used to optimize resource allocation and service scheduling to achieve more efficient operations [53]. Logistics distribution companies can use opportunistic routing technology to improve the efficiency of goods distribution. When new goods need to be delivered, the system can select the best delivery route and truck for distribution based on the destination of the goods, the location, and availability of the vehicles [138]. By dynamically distributing goods to idle vehicles, logistics and distribution companies can reduce idle miles and waiting times, thereby improving distribution efficiency. Opportunistic routing can also find application in intelligent transportation systems to optimize traffic flow and reduce congestion. For example, in urban road networks, traffic management systems can dynamically adjust the timing of traffic lights according to real-time traffic conditions and road condition information [54]. When traffic pressure is detected at a certain intersection, the system can extend the green time through the intersection and adjust the signal timing of the surrounding roads accordingly to optimize the traffic flow and reduce the traffic congestion. Table 4 analyzes and compares typical routing algorithms.