# Challenges, Threats, Security Issues and New Trends of Underwater Wireless Sensor Networks
## 1 Introduction  
Underwater wireless sensor networks (UWSNs) have proven strength in various underwater applications of ocean monitoring, resource exploration, surveillance and military use in harsh underwater environments [1; 2].  
As shown in Figure 1, UWSNs are composed of several components: onshore sink, surface buoy, underwater sink node, and underwater sensor nodes. Moreover, satellite, vessel, and autonomous underwater vehicles (AUVs) can be used to expand the sense and communication range. Underwater sensor nodes monitor physical or environmental conditions, such as pressure, sound, temperature, etc. and cooperatively transmit data to the underwater sink node. The data are transmitted to a surface buoy via wired link, and finally received at an onshore sink or surface sink via radio communication. There are three different architectures for UWSNs. Static two-dimensional architecture: all the nodes are anchored to the ocean floor. The underwater sink node collects data from sensor nodes by the horizontal transceiver. Then, it relays data to surface buoy by the vertical transceiver or wired link. Static three-dimensional architecture: underwater nodes are anchored to the seabed and fitted out with floating buoys. The buoy pays the sensor towards the water surface. The lengths of the cables are different for the required depth of sensor nodes. Three-dimensional architecture with AUVs: as discussed above, AUVs can be used to expand the sense and communication range. The AUVs could be considered as super nodes, which have more energy, can move independently, and could be routers between fixed sensors, managers for network reconfiguration, or even a normal sensor.  
In UWSNs, to prolong the lifetime of whole network, cluster-based network architecture is widely used. A cluster-head (CH) node is elected to be the sink-node of the cluster, which aggregates and relays packets intra-cluster and inter-cluster. Hence, the energy consumption of CH is greater thanmember nodes. Sensor nodes are elected to be the CH in turn to balance the energy consumption and further prolong the lifetime of the whole network. The election protocol may be based on the residual energy [3], the best energy consumption [4], or the optimal number of CHs [5].  
Existing research on UWSNs is mainly focused on communication, self-organization, processing capabilities, cover ability, connectivity, adaptability and low energy consumption. Unfortunately, this existing research is constrained in terms of countering security threats in UWSNs because the resources are much more constrained while the security situation is more server-based due to the particularities and networking environments [6].  
The contributions of this article are as follows:  
* In this article, the special particularities and constraints of UWSNs and underwater acoustic channels are presented and discussed in detail. Based on the analyses, we conclude that UWSNs are vulnerable to various threats and attacks and security issues should be discussed.
* Threats and attacks in UWSNs are classified and discussed in this article. In addition, denial of service (DoS) attacks and feasible countermeasures in each layer are analyzed in detail.
* Compared with WSNs, some especial security requirements of UWSNs are discussed and existing security mechanisms and specific protocols are presented.  
The remainder of this paper is organized as follows. In Section 2, the peculiarities of UWSNs and the underwater acoustic network environment are introduced. The threats and challenges in UWSNs are discussed in Section 3. The security requirements and current security researches for UWSNs are presented in Section 4. In Section 5, open research problems and future research topics are outlined. Finally, Section 6 concludes the paper.