## 1 Introduction
### From WSNs to IoT  
WSNs were originally proposed for military surveillance purposes [5]. Due to its early success, this technology was then envisioned for use in applications such as habitat monitoring [6], weather monitoring [7], agriculture monitoring [8] and wildlife monitoring [9]. The revolution that is necessary to ensure WSN technologies flourish in a similar manner requires more effort in simplifying design, implementation, deployment and usability. The societal impact of WSNs is highly influenced by the number of the users, which is itself determined by the quality and the quantity of the available applications. Currently, there is an urgent need to inspire developers to build more useful Over The  
Figure 1: An Example of Cluster-Based WSN. Top (OTT) applications to improve people's life quality or experience. Therefore, the concepts of Internet of Things (IoT) [10] and Internet of Everything (IoE) [11] have been proposed and popularised through concepts such as smart buildings/home, smart cities, smart roads and connected cars [12; 13]. WSNs came first, providing the enabling technological foundations for IoT. The fundamental difference between WSNs and IoT is that the dynamics and diversities in IoT are much higher than that in WSNs, which include the following aspects:  
1. **Applications:** Traditional WSN applications are mainly focusing on monitoring the environment and then collecting the data. The sensors are deployed at fixed locations in the field. The volume of the data transmitted in the network is small. However, IoT applications can range from smart kitchen to smart city monitoring. All the applications in these systems have specific requirements on the network performance. For instance, the smart road lighting application in a smart road system requires short delays. The communication for the cameras in smart monitoring systems requires high bandwidth. 2. **IoT devices:** WSN systems normally have homogeneous sensors, in terms of sensing ability, processing capability and power supply. This feature has simplified the design of the relevant protocols, such as routing protocols. However, in IoT systems, this feature is no longer applied. For example, in a smart home system, the fridge, the vacuuming robot and the light sensing devices all have their own functionalities and characteristics in this system. Furthermore, the devices in IoT systems are no longer limited to those basic sensors. 3. **Communication ability:** Since the features of the connected devices in IoT systems vary a lot, their communication capabilities including communication range, communication band and communication power consumption are all different. 4. **Number of connected devices:** By the end of 2020, according to Cisco, there will be over 50 billion connected devices [11]. Managing such a large number of devices along with the generated data is challenging in IoT systems. 5. **Objectives:** As mentioned, WSN systems are mainly used for monitoring the environment and collecting data. User profile and system context are rarely discussed. One of the most important objectives for IoT systems is to improve people's life and their personal experience. User oriented and context aware design is demanded in IoT systems. From the communication perspective, each IoT device may be embedded with multiple radio access interfaces enabling Multiple-In and Multiple-Out (MIMO) communication. Network selection among the available interfaces has drawn many attentions from both the 3 GPP organisations and researchers [14]. However, traditional clustering strategies for WSNs typically assume the sensors in the networks are homogeneous with only a single communication interface available. If a heterogeneous network is considered, the degree of the diversity is still rather low (For example, some heterogeneous networks simply assume that some sensors have doubled power supplies than others). This assumption has become unrealistic recently, especially in IoT systems.