# Energy Efficiency for Green Internet of Things (IoT) Networks: A Survey +
## 3 Literature Review  
IoT brings an enormous number of different devices and infrastructures under the same umbrella and the consequent massive growth in connectivity of 80 billion smart devices on the Internet in 2025. Figure 1 reveals the growth of the IoT devices compared to the growth of world population [6, 7, 8, 9, 18, 19]. These networks generate a large amount of data collected via these smart devices. Every two years, data doubles in size and is expected to arrive 163 Zettabytes in 2025 [20]. The volume of the IoT data will increase from 2% in 2013 up to 10% in 2021 [21]. Figure 2 reveals the growth of data from 2010 to 2025. It observes that by 2025, the volume of data will increase ten times compared to the data generated in 2016 [20, 22, 23]. Figure 1: Estimated number of connected devices vs world population. Figure 2: The growth of data over the years. The concept of IoT has attracted considerable attention by governments, businesses, military, healthcare, industries and researchers [24]. It can be expanded to almost everything from refrigerators to televisions (TV), smartphones to wristwatches, home security and alarm systems, etc. [25]. For instance, smart refrigerators can tell us the end of the validity of food using RFID or which items to buy during our next shopping trip to the market. Another example, users can use their smartphones or tablets with just a single touch to control items in a house such as turning lights ON/OFF, or setting the desired temperature before arriving home, this latter is now so well developed as to be advertised by APPLE, SAMSUNG, etc. as apps for their latest phones [26]. IoT with Cloud computing both serve to increase efficiency for business and industry. The IoT generates a massive amount of data, with cloud computing providing storage and pathways for those data to travel to their destination [27]. For instance, Amazon web services are one of several IoT cloud platforms that helps people to interact with their items to be purchased through its website [28]. Additionally, the integration of IoT with medical technologies enables real-time monitoring system and data collects to improve patient health [17; 29]. For example, IoT devices can be applied to track the real-time location of medical equipment such as blood pressure, heart rate, smart inhaler, oxygen pumps, wheelchairs, and other monitoring equipment. Additionally, IoT based on WSNs technology is used to monitor a wide range of sensors in the field that can detect and measure various physical phenomena such as volcanic activity, flooding, and wildfires [30]. These examples are just a few examples out of millions being implemented in IoT. Figure 3 shows some of the IoT applications in different fields. The literature review reveals that communications and IoT applications normally take the form of one of the following connections, see Figure 4:  
* People to People (P2P) connection: is the data transfer/share from a user to other. For example, telephone calls, video calls and social communications. It is usually named a collaboration connection [4]. * Machine to People (M2P) connection: is the data transfer from devices such as sensor nodes, smart devices, computing devices or others to the users for analysis. For instance, weather forecasting uses smart sensors to collect the information from the sensing field and dispatches it to the remote control center for further analysis [31]. * Machine to Machine (M2M) connection: is the data transfer between devices without human interplay. For example, a car connecting and talking to another car about its lane change, congestion, accident, distance, speed, or braking intentions, etc. [32]. Figure 3: Some applications that benefit from IoT technology. There is several studies that cover different aspects of IoT communication. For example, the survey by [PERSON], et al. [33] reported the major technologies that enable the IoT evolution, both wired and wireless and the components of the wireless sensor networks (WSNs).