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This scenario consists of a wearable ECG sensor attached to the human body through a smartwatch and a smartphone that acts as an edge device, as presented in Figure 2.5. As for the communication, Bluetooth is used to connect the ECG sensors with the edge device, while WiFi is used to connect the smartphone to fog devices and cloud.

Figure 2.5 A wearable ECG sensor.

Generally, users prefer smartwatch devices that provide monitoring heart functions while they continue normal physical activities. Due to the limited battery life and storage capacity of the smartwatch, we assume that the data produced by this device is around 1 KB per second and it is stored in the smartphone. Based on this assumption, daily produced data by a wearable device is around 86 MB per day and 2.6 GB monthly. One must note that smartphones have limited battery life and storage capacity. Hence, the smartphone at some point has to transfer the gathered data to another device that provides more storage capacity.

Referring to Figure 2.5, one can witness that data streaming is realized between the wearable device and the smartphone. Both devices remain connected to each other during the operation time. In case of any critical event, the wearable device interacts with the edge device and notifies the user for any situations. The process (1) start with getting real-time values from a wearable device to the smartphone. The smartphone application checks (2) periodically the wearable device to see if the connection between them is active. In addition, the smartphone may run out of free disk space and one can configure the application for daily synchronization (3) with another storage capability device, or with a central cloud storage or even with a fog node.

Since the wearable device and the edge device has limited resource capabilities, one must consider the energy consumption of both devices. In such system architecture, the first recommended approach is to decide what data to transmit to the cloud, what to store locally, and the last is to develop better monitoring algorithms. In the other words, when designing such systems, the critical point is to consider the energy consumption, which is affected by three main functions that are realized between devices, such as (1) communication, (2) storage, and (3) processing requirements. Hence, developers have to code software with highly efficient streaming algorithms, storing essential monitoring information, and avoiding continuously data transfers with the central cloud.