# Cognitive Routing in Software-Defined Underwater Acoustic Networks
###### Abstract  
There are two different types of primary users (natural acoustic and artificial acoustic), and there is a long propagation delay for acoustic links in underwater cognitive acoustic networks (UCANs). Thus, the selection of a stable route is one of the key design factors for improving overall network stability, thereby reducing end-to-end delay. Software-defined networking (SDN) is a novel approach that improves network intelligence. To this end, we propose a novel SDN-based routing protocol for UCANs in order to find a stable route between source and destination. A main controller is placed in a surface buoy that is responsible for the global view of the network, whereas local controllers are placed in different autonomous underwater vehicles (AUVs) that are responsible for a localized view of the network. The AUVs have fixed trajectories, and sensor nodes within transmission range of the AUVs serve as gateways to relay the gathered information to the controllers. This is an SDN-based underwater communications scheme whereby two nodes can only communicate when they have a consensus about a common idle channel. To evaluate our proposed scheme, we perform extensive simulations and improve network performance in terms of end-to-end delay, delivery ratio, and overhead. c 2017  
issues in underwater networks [3] because it can utilize the spectrum in an environmentally friendly manner (i.e., avoiding harmful interference with natural acoustic systems) and in an efficient manner (i.e., providing high spectrum utilization). For this purpose, a sensing technique is required to detect the presence/absence of the primary user (PU), thereby ensuring that PU activity is safeguarded. An acoustic link is only formed when two acoustic users reach a consensus about a common idle channel. Several routing protocols have been proposed for underwater networks, but for cognitive underwater networks, the number of protocols that consider a cognitive capability with a routing technique is limited. Nevertheless, safe and stable communications issues for underwater cognitive acoustic networks (UCANs) are still under investigation in order to reach a robust and distinguished solution. Moreover, existing underwater networks are composed of thousands of nodes that are deployed to collect data in an area of interest in the ocean, thereby satisfying the requirements of a single application. Another issue in these networks is that due to application constraints and vendor dependency, it is difficult to use these nodes for other services in the same area [4]. To overcome the shortcomings with existing architectures in underwater networks, we intend to apply a new concept of software-defined networking (SDN) in this domain. SDN is an emerging technology that increases network intelligence by separating the control plane from the data plane [5]. SDN technology has been progressively diffused into different types of network systems, including UANs, in order to overcome the limitations in these networks [1; 4; 6]. It offers the flexibility to adapt and satisfy different applications. Our main objective is to combine a cognitive capability with a routing technique in underwater networks by using the SDN approach. We intend to overcome the problems of spectrum scarcity and high latency in UCANs. Our goal is to select the best route between source and destination that maximizes the capacity and minimizes the duration among all the paths. Spectrum sensing is done with an orthogonal frequency division multiplexing (OFDM)-based energy detection scheme. The SDN controllers programmatically configure the traffic and have a global view of the network. Our proposed scheme has a single main controller (MC) and several local controllers (LCs). LCs are mobile autonomous underwater vehicles (AUVs) that have a localized view of the network. Sensor nodes send requests to LCs querying them for a route to the target node. The LC quickly responds to the request if it has a route to the destination; otherwise, it forwards the request to the MC.