## 1 Introduction  
The Internet of Things (IoT) has become an emerging key technology for future, in which a myriad of sensors, actuators, and smart objects in our daily life are connected to the Internet. These sensors and actuators (e.g., surveillance cameras, home appliances, and environment monitoring sensors) are typically equipped with different kinds of microcontrollers, transceivers, and protocols for communication of sensing and control data [1; 2; 3]. These real life objects, either sensors or actuators, are connected with each other to transfer their sensed data to centralized servers, where information is collectively stored and made available for particular users with proper access rights. The transfer of data from one sensor/actuator node to another sensor/actuator node or an IoT server is performed through a new communication paradigm called Machine Type Communications (MTC) or Machine-to-Machine (M2M). The communication technology for the first-hop of a path between an IoT device and an IoT server is mostly expected to be wireless radio access for the ease of installation and deployment. IoT devices use wireless radio access technologies such as Wireless Wide Area Networks (WWAN) and Wireless Local Area Networks (WLAN) to communicate with servers [4]. In some cases, constrained IoT devices may first communicate with intermediate entities called IoT gateways or M2M gateways through Wireless Personal Area Networks (WPAN) or WLAN. The gateways in turn forward data from these devices toward IoT severs, and act as a translator between IoT devices and servers [5; 6; 7; 8]. Connectivity between IoT devices and other IoT gateways or servers can be provided by using different kinds of wireless technologies such as 3 GPP Long Term Evolution (LTE) and LTE-Advanced, WiFi, ZigBee and Bluetooth or other standard wireless technologies [9; 10]. The network characteristics of IoT using these wireless technologies are quite different from those for traditional wired or wireless networks because the number of devices participating in communication is very large. In addition, traffic per IoT device is typically not so much because each IoT device senses and transfers a small amount of data to a corresponding IoT server, although data generated from a huge number of objects may collectively have some impacts on the network performance. Furthermore, IoT networks should operate stably and sustainably for a longer period without any need for human intervention [11]. Another aspect is that gateways may incorporate multiple radio interfaces for versatile purposes such as throughput, latency, and energy efficiency [12]. Devices in such IoT networks will typically operate based on battery power sources, and hence, energy efficiency is naturally of utmost importance in device management. Looking into a particular Wireless Sensor Network (WSN) domain, energy efficiency for battery operated sensor nodes and lifetime prolongment have been research issues for so long [13; 14], where Medium Access Control (MAC) layer protocols focus on adjusting the duty cycle for sensor nodes, and routing layer protocols are designed for data aggregation and many-to-one transmission. Similarly, since IoT devices operating in the IoT network paradigm are also battery operated, battery consumption should be kept in mind during IoT network deployment [15]. However, IoT network characteristics and deployment scenarios are more complex than traditional WSNs in various aspects, e.g., the volume of IoT devices, bidirectional traffic between IoT devices and servers, heterogeneous data for sensing and actuation, usage of heterogeneous wireless radio access technologies, existence of IoT gateways, _etc._ For this reason, some conventional WSN power saving strategies such as homogeneous data aggregation and clustering are not directly applicable to IoT cases. Extensive research is being conducted for energy conservation forbattery-constrained IoT devices from various aspects such as standardization, academic research, and industry development.