## 1 Introduction  
A smart grid is an extensive combination of power grids, communication networks and information management systems, which contributes to green and cost-effective energy generation. Generally, a smart grid comprises a data communications infrastructure integrated with an electrical grid that collects and analyzes data captured in near real time about power transmission, distribution and consumption [1]. Thus, the key to enabling a smart grid system is the seamless convergence of existing electronic grid systems and information-communication technologies (ICTs).  
The electrical power grid system can be divided into four elements: electricity generation plants, transmission substations, distribution substations and end users [2]. First, power plants generate power from a variety of sources, including solar, wind and nuclear sources, for distribution. As the power approaches the customers' homes, it is again stepped down to the voltage required for residential use. Finally, home appliances access power through their electric smart meters [2; 3; 4]. Even though the primary goal of these elements differs, time-critical information that is generated by each element needs to be robustly shared between all elements in a smart grid system. For example, power generation can be dynamically controlled by using the real-time energy consumption of the end users. Meanwhile, the end user can visualize the real-time power usage of the home and can obtain the real-time cost of the power supplied from the power provider. In a smart grid, electricity can also be returned to the grid by users. For example, home users may be able to generate electricity using photovoltaic (PV) systems and return it to the grid or electric vehicles may provide power to help balance loads by \"peak shaving\", _i.e._, sending power back to the grid when the demand is high. This backward flow is important. For example, it can be extremely useful in a microgrid that has been \"isolated\" because of power failures. With the help of the energy feedback from the customers, the microgrid can continue to function, albeit at a reduced level.  
By exchanging information between different elements, the smart grid provides predictive information and recommendations to utilities, their suppliers and their customers on how to manage power in an optimal manner [4]. In addition, this information can be collected at the data server and reproduced by the service provider in order to develop new services and business models. Potential smart grid services focus not only on power-related services, but they also aim to enable the development of utility services and business models, such as water, gas, facility monitoring and unmanned surveillance [5; 6]. Further, all electric grid elements must share the common understanding that the smart grid needs to be integrated with a data-networking infrastructure in order to be categorized them as \"smart things\" [7].  
In this paper, we revisit the current smart grid system architecture, and we investigate a new paradigm called SDN-enabled smart grid. In addition, we compare recent studies of the SDN-enabled smart grid based on its service functionalities, and we present further research issues regarding the SDN-enabled smart grid. To the best of our knowledge, this is the first work that deals with the identification and comparison of SDN-enabled smart grid architectures.  
The rest of this paper is organized as follows. In Section 2, we describe the essentials of the smart grid technologies. In Section 3, we describe the isolated subsystems of the current smart grid that are based on information management and communication layers, namely the information characteristics, communication architecture and its requirements. We present the survey and common design of theSDN-enabled smart grid in Sections 4 and 5, as well as its challenges and guidelines for the smart grid. In Section 6, we conclude the paper.