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Smart world is envisioned as an era in which objects (e.g., watches, mobile phones, computers, cars, buses, and trains) can automatically and intelligently serve people in a collaborative manner. Paving the way for smart world, Internet of Things (IoT) connects everything in the smart world. Motivated by achieving a sustainable smart world, this paper discusses various technologies and issues regarding green IoT, which further reduces the energy consumption of IoT. Particularly, an overview regarding IoT and green IoT is performed first. Then, the hot green information and communications technologies (ICTs) (e.g., green radiofrequency identification, green wireless sensor network, green cloud computing, green machine to machine, and green data center) enabling green IoT are studied, and general green ICT principles are summarized. Furthermore, the latest developments and future vision about sensor cloud, which is a novel paradigm in green IoT, are reviewed and introduced, respectively. Finally, future research directions and open problems about green IoT are presented. Our work targets to be an enlightening and latest guidance for research with respect to green IoT and smart world.INDEX TERMS Smart world, Internet of Things, green, radio-frequency identification, wireless sensor network, cloud computing, machine to machine, data center, sensor-cloud.

16 0890-8044/13/$25.00 © 2013 IEEE hrough the utilization of elastic resources and widely deployed data centers, cloud computing has provided countless new opportunities for both new and existing applications. Existing applications, from file sharing and document synchronization to media streaming, have experienced a great leap forward in terms of system efficiency and usability through leveraging cloud computing platforms. Many of these advances have come from exploring the cloud's massive resources with computational offloading and reducing user access latencies with strategically placed cloud data centers. Recently, advances in cloud technology have expanded to allow offloading not only of traditional computation but also of such more complex tasks as high-definition 3D rendering, which turns the idea of cloud gaming into a reality. Cloud gaming, in its simplest form, renders an interactive gaming application remotely in the cloud and streams the scenes as a video sequence back to the player over the Internet. A cloud gaming player interacts with the application through a thin client, which is responsible for displaying the video from the cloud rendering server as well as collecting the player's commands and sending the interactions back to the cloud. Figure 1 shows a high-level architectural view of such a cloud gaming system with thin clients and cloud-based rendering.Onlive [1] and Gaikai [2] are two industrial pioneers of cloud gaming, both having seen success with multimillionuser bases. The recent $380 millon purchase of Gaikai by Sony [3], an industrial giant in digital entertainment and consumer electronics, shows that cloud gaming is beginning to move into the mainstream. From the perspective of industry, cloud gaming can bring immense benefits by expanding the user base to the vast number of less powerful devices that support thin clients only, particularly smartphones and tablets. As an example, the recommended system configuration for Battlefield 3, a highly popular first-person shooter game, is a quad-core CPU, 4 Gbytes RAM, 20 Gbytes storage space, and a graphics card with at least 1 Gbyte RAM (e.g., NVIDIA GEFORCE GTX 560 or ATI RADEON 6950), which already costs more than $500. The newest tablets (e.g., Apple's iPad with Retina display and Google's Nexus 10) cannot even meet the minimum system requirements that need a dual-core CPU over 2.4 GHz, 2 Gbytes RAM, and a graphics card with 512 Mbytes RAM, not to mention smartphones of which the hardware is limited by their smaller size and thermal control. Furthermore, mobile terminals have different hardware/software architecture from PCs (e.g., ARM rather than x86 for CPU), lower memory frequency and bandwidth, power limitations, and distinct operating systems. As such, the traditional console game model is not feasible for such devices, which in turn become targets for Gaikai and Onlive. Cloud gaming also reduces customer support costs since the computational hardware is now under the cloud gaming provider's full control, and offers better digital...