# An On-Demand Scheduling-Based MAC Protocol for UW-WiFi Networks
## 3 Application Scenario  
Underwater Internet-of-Things (UIoT), as a significant extension of Internet of Things in underwater environments, is regarded as the network of interconnected underwater smart objects. UANs, considered as a promising network infrastructure to support UIoT, have wide application prospects in marine scientific investigation, ocean environment observation, marine ranch monitoring, disaster prevention, underwater rescue, coastal guard, military reconnaissance, blockades, etc. Usually, UANs, meeting the above application scenarios, are composed of heterogeneous nodes with heterogeneous data. For example, the nodes may include surface buoys, submerged buoys, unmanned underwater vehicles (UUV), gliders, unmanned surface vessels (USV), and so on; and the data may include sonar detection data, image, hydrological data, status data, command data, etc. Additionally, these UANs need to have the following functions: data acquisition in the uplink, remote command control in the downlink, and interaction between some nodes.  
In view of the aforementioned actual requirements, and in order to enable flexible networking of heterogeneous nodes with diverse missions, the UW-WiFi network architecture is introduced, as shown in Figure 1, in which the given UW-WiFi network is two-level, the first level consists of a surface gateway (called SG) and base stations (called BS) within SG's communication range, and the second level is composed of multiple subnets consisting of base stations and terminal nodes within BS's communication range. All terminal nodes in the whole network can be static or mobile. The radius of each subnet is the maximum communication range of the BS. Subnets are spatially far away from each other without interference between them. The BS of each subnet is responsible for providing wireless access services for terminal nodes, which can aggregate a wide variety of data from terminals and forward them to the SG through the upstream underwater acoustic link, and can relay commands from the SG or data from other nodes to its terminal nodesthrough the downstream underwater acoustic link. In this work, the designed MAC protocol is intended for each subnet, and the MAC protocol for the entire hierarchical UW-WiFi network, including a solution to switching among subnets for AUVs, is not in the scope of this paper, which will be our future work.  
Designing a practical MAC protocol is of great significance to the UW-WiFi network. The MAC protocol is the basis for guaranteeing good quality of service in wireless networks. Taking into account the inherent characteristics of underwater acoustic signal with long propagation delay, low available bandwidth, and high bit error rate, as well as the diverse data transmission demand of nodes in the UW-WiFi network, the functional MAC scheme constructing is a challenging work. Firstly, in the case of different data transmitting needs of each terminal node in the network, how to effectively manage the access of multiple nodes to the shared channel to optimize the channel utilization. Secondly, how to resolve the issue that different types of data have different demands for reliability and real-time performance. In the following sections, we will discuss the MAC protocol design strategy.