# Challenges, Threats, Security Issues and New Trends of Underwater Wireless Sensor Networks
## 3 Threats and Challenges in Underwater Wireless Sensor Networks (UWSNs)
### DoS Attacks
#### 3.3.1 Physical Layer  
The physical layer of UWSNs is in charge of the acoustic communication between nodes in the network. As discussed in Section 3.1 Passive Attacks, the malicious attacker can eavesdrop transmissions, and then predict the nature of communication by analyzing the packets traffic, observe the exchange of the packets, identify communicating hosts, and determine the location of nodes. With this information, the attacker can perform several DoS attacks in the physical layer, including _eavesdropping attacks_ and _jamming attacks_. An eavesdropping attack is a kind of passive DoS attack that listens for acoustic transmissions between nodes and captures packets transmitted in the channel. If the packets are not encrypted, the attackers could easily read the data and privacy information. As a consequence, it is difficult to detect if eavesdropping is actually occurring since the attacker does not disrupt the normal communication. In [18], the authors presented an analytical model which investigates the probability of eavesdropping attacks with consideration of underwater acoustic channel conditions, including signal attenuation and ambient noise. The relationship between the eavesdropping success condition and the underwater acoustic signal channel is established. The authors derive the eavesdropping probability from consideration of both isotropic eavesdropper and array eavesdropper, respectively. Moreover, the probability heavily depends on the acoustic signal frequency, spreading factor, wind speed and node density. As shown in Figure 4, in a jamming attack, one or several malicious nodes constantly emit useless or undesired signals, and thus normal communication with other legitimate nodes in the affected region will be interfered with. Worse, the malicious nodes can paralyze the entire network by jamming only a few special nodes such as cluster-head node, root node or base station. These special nodes are responsible for aggregating data, transmitting packets, and managing the network. For example, as shown in Figure 1, the underwater sink node aggregates data from sensor nodes and then sends this data to the on shore base station or surface sink. Jamming attackers can continuously send packets or communication requests to the underwater sink node. In this manner, the underwater sink node is not able to respond to requests from legitimate nodes or receive packets. Moreover, the attacker may simply keep requesting an opportunity to send data to realize the goal. A jamming attack can effectively disrupt localization protocols in an underwater environment. In [19], the author defined two types of attackers with four different attack methods and introduced the effects of jamming attacks on underwater acoustic networks (UANs). These attacks are evaluated on three commonly used underwater acoustic modems in a real-world experimental testbed in Mansfield Hollow Lake, Mansfield, Connecticut and in a lab testbed. But no feasible solution to defend jamming attack is given in this article. To defend against jamming attack, the majority of anti-jamming mechanisms used in WSNs cannot be directly used in UWSNs. Spread-spectrum techniques are widely used in underwater acoustic communication. Among these techniques, the frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) are drawing interest for their good presentation over noise and multipath interference [20]. To an extent, FHSS and DSSS techniques are resistant to interference from jammers. Even if a FHSS technique is used, the jamming attacker can jam a wide band of the spectrum. Worse, the DSSS technique is also vulnerable to a high-power jamming signal. Another way to defend against a jamming attack is that the nodes switch to sleep mode and periodically wake up to check the attack is over. Unfortunately, for some kind of UWSNs, these spread-spectrum techniques are not suitable. In [21], a multi-armed bandit-based acoustic channel access algorithm is proposed to achieve the jamming-resilient cognitive acoustic communication.