An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> D. NETWORK FUNCTION VIRTUALIZATION (NFV) <s> The ocean covers nearly two-thirds of the surface on the Earth, and there has been great interest in developing underwater wireless sensor networks (UWSNs) to help us explore the ocean realm. A great deal of efforts have been devoted to it, and significant progress has been made since the beginning of 2000s. However, most of the networks are isolatedly developed currently, inherently hardware-based and application-oriented with inflexible closed-form architectures, which are difficult to reconfigure, reprogram and evolve. They also lack the capability in sharing resources, and are far from service-oriented networks. These limitations impair their capacity for wide range of applications. To further propel the development of UWSNs, next-generation UWSNs have been proposed recently, which are robust, flexible, adaptive, programmable, support resource-sharing feature and are easy to manage and evolve. Moreover, a number of novel software-defined techniques and paradigms, such as software-defined radio, cognitive acoustic radio, network function virtualization, software-defined networking, Internet of Underwater Things, and sensor-cloud, have been emerging. These software-defined technologies have the capability of softwarizing network resources, and then redefining them to satisfy diverse application requirements, improve resource utilization efficiency and simplify network management. Consequently, these evolving technologies are envisioned as critical building blocks and major driving forces, which will transform conventional UWSNs toward software-based, programmable, user-customizable, and service-oriented next-generation UWSNs. In this paper, we provide a comprehensive review of existing works on implementing these techniques, and also present discussions for future research. We hope to inspire more active research on these areas and take a step further toward realizing next-generation UWSNs. <s> BIB001 </s> An Overview of Next-Generation Underwater Target Detection and Tracking: An Integrated Underwater Architecture <s> D. NETWORK FUNCTION VIRTUALIZATION (NFV) <s> Network Functions Virtualization (NFV) and Software-Defined Networking (SDN) are new paradigms in the move towards open software and network hardware. While NFV aims to virtualize network functions and deploy them into general purpose hardware, SDN makes networks programmable by separating the control and data planes. NFV and SDN are complementary technologies capable of providing one network solution. SDN can provide connectivity between Virtual Network Functions (VNFs) in a flexible and automated way, whereas NFV can use SDN as part of a service function chain. There are many studies designing NFV/SDN architectures in different environments. Researchers have been trying to address reliability, performance, and scalability problems using different architectural designs. This Systematic Literature Review (SLR) focuses on integrated NFV/SDN architectures, with the following goals: (i) to investigate and provide an in-depth review of the state of the art of NFV/SDN architectures, (ii) to synthesize their architectural designs, and (iii) to identify areas for further improvements. Broadly, this SLR will encourage researchers to advance the current stage of development (i.e., the state of the practice) of integrated NFV/SDN architectures and shed some light on future research efforts and the challenges faced. <s> BIB002
NFV is a concept of virtualization of network resources into software-based network functions BIB001 which is at its early stage for terrestrial sensor networks. NFV provides the services in virtual machines that perform different operations. NFV can use SDN as a part of service function chaining (SFC), and SDN can provide connectivity between virtual network functions (VNFs) BIB002 ,  . A VNF is the virtualization of specific network function that should function autonomously. Both SDN and NFV are complementary to each other, by this means they can be applied to network of different types and can be used in simplifying network management. Therefore, for the real-time implementation of next-generation underwater sensor networks, integration of NFV and SDN facilitates multiple underwater applications under the same infrastructure. 