# Real-Time Communication Support for Underwater Acoustic Sensor Networks +
## 8 Conclusions and Future Work  
The use of UW-ASN is still in a developing state, far from reaching standard consensus on basic aspects like carrier frequency or modulation techniques. However, their relevance in making the development of underwater ubiquitous systems possible, guarantees the research on this topic for the years to come. The main limitation for the development of these networks has been the lack of models to represent the network performance in several application scenarios, particularly in those requiring real-time communication. In this paper, we advance the state-of-the-art in this topic, by proposing a network analytic model that represents the behavior of UW-ASN that cover wide areas involving a few nodes (i.e., low-density networks) and using an any-to-any communication scheme. The model considers real-time message-delivery, which is an aspect little explored by the research community, and mandatory for implementing some underwater ubiquitous-solutions, such as tsunami-warning systems. The real-time analysis for an underwater acoustic sensor network was done using two approaches. In the first one, the network is analyzed with integer linear programming techniques. The shortest path is used as routing policy combined with a message or node slot-allocation procedure in a TDMA frame. Based on this, we presented the schedulability condition for the case in which the messages are transmitted following a FIFO policy. This scheduling strategy is quite simple and requires little processing within the underwater nodes, thus reducing the computing complexity and the demand on the processors. However, better results may be obtained if some real-time priority policies are implemented, such as fixed priorities or earliest-deadline first. The performance of the network by using priority policies is left for future work. The second approach is based on a heuristic, where messages are scheduled following a per-link processing, finding the route with shortest delay. The solution obtained improves the two-step approach of finding the SPF in the first place, and then allocating the slots within the frame. As this heuristic only considers the messages actually being transmitted, unnecessary restrictions are avoided. In order to illustrate the applicability of the proposal, we modeled an UW-ASN for a simulated scenario where real-time transmission is necessary. Particularly, we implemented a network similar to the one required to support tsunami-warning systems, and we provided a routing proposal that helps meet the real-time requirements. This network capability increases the opportunities of developing underwater ubiquitous systems for other application scenarios such as environmental monitoring, study of marine species and climate change, biodiversity protection and defense. Given that the network model is analytic, the designers of these systems can use software tools to quickly create, evaluate and adjust their models until reaching suitable results. Therefore, they can proceed to implement these solutions by knowing the expected performance of these systems in the real world. This clearly contributes to reducing development efforts and risks, and making the solutions more effective. An alternative to the use of the proposed model is to perform iterative design based on simulations; however, such a process even needs an UW-ASN model, and it is much more  
Figure 11: End-to-end message delays for various frame-lengths. time-consuming. The next step in this initiative is to use the proposed model to design a real-world UW-ASN that helps us empirically verify the capabilities of the proposal. This work has been partially supported by the Spanish government under contracts TIN2016-77836-C2-1-R, TIN2016-77836-C2-2-R, and DPI2016-77415-R, by the Generalitat de Catalunya as a Consolidated Research Group 2014-SGR-881 and by Universitat Rovira i Virgili under grant OPEN2017. [PERSON] and [PERSON] designed the main aspects of the proposal, and its analytical evaluation.