# Internet of Things in Marine Environment Monitoring: A Review
## 2 Overview of IoT in Marine Environment Monitoring
### Common IoT-Based System Architectures for Marine Environment Monitoring and Protection
Sensor nodes are used to sense and monitor environmental parameters such as water temperature and pH, salinity, turbidity, oxygen density, and chlorophyll levels, and transmit the collected data to sink nodes via ZigBee or some other wireless communication protocols. Actuator nodes execute commands from the upper layers. Communication between a sink node and sensor or actuator nodes is usually point-to-point. A sink node collects data from a group of sensor nodes and sends collected data to the base station or passes execution commands from upper layers to actuator notes, via mobile communication networks (2G/3G/4G). The base station is connected to a system server through the Internet. The system server stores and processes the received data from the base station, completes data analyses according to corresponding applications, sends commands to actuator nodes according to the pre-defined rules as appropriate. Various kinds of user terminals (desktops, laptops, pads, and smart phones, etc.) connect users to the system server over the Internet (including mobile Internet). The design and development of a durable and scalable IoT system for marine environment monitoring and protection should carefully consider a number of factors: The harsh marine environment, the communication protocols and network topology, the number and distribution of nodes, buoys and mooring systems, oceanographic sensors, energy supply and harvesting options, and so on. As described above, an IoT system consists of many sensor and actuator nodes and a gateway for the connection to the Internet. Usually, sensor and actuator nodes are organized into a connected network according to a certain topology. Physical topology and density are entirely dependent on applications [24], therefore the design and development of an IoT system need to consider its application and deployment environment. Even though more sensors can be densely deployed to enhance data availability and accuracy, a dense deployment also brings negative issues: High energy consumption, data collisions, and interferences, etc. [25]. Sensor nodes normally have four typical kinds of network topologies: Linear, star, cluster/tree, and mesh topologies, as shown in Figure 3.  
Figure 2: Common physical architecture of IoT-based marine environment monitoring and protection systems. While we have discussed in detail the star, cluster/tree, and mesh topologies in reference [18], the linear topology is a series connection of all the nodes. Its main advantage is that it has a simple structure thus that the cost is relatively low. As a consequence, the transmission volume is large, and the speed is slow. The selection of a right network topology for a particular application depends on the amount and frequency of data to be transmitted, the distance of data transmission, the requirement of battery life as required for maintenance, and the mobility of the sensor nodes [26]. On the other hand, the physical network topology of an IoT system may change during its operation due to its energy availability, node position variations, equipment or sensor malfunction, node reachability (due to noise, severe weathers, moving obstacles, etc. ), and task details of sensor nodes [27].