The Extreme Edge at the Bottom of the Internet of Things: A Review

The Internet of Things is being established nowadays, and the deployment of devices to be interconnected with each other and to the Internet is becoming a more common issue. The Internet of Things is structured in several levels and, in this regard, depending on the layer in which these devices are located, the complexity range and specific constraints arise. In the lowest level of this structure, the so-called edge is found, where the data are gathered from the environment by tiny electronic devices which are very limited in energy, computing, and memory resources. These devices, in turn, present different levels of complexity, which lead to a distinction between the edge layer and the sensor or extreme edge layer. In the extreme edge is where the most essential and limited tasks are carried out, these are sensing and sending data. Meanwhile on the edge, in addition to sensing, some processing can be performed in order to offload upper layers and to save precious energy by not communicating useless data. In this review paper, the structure of the Internet of Things is presented and the extreme edge is detailed, presenting the implementation options and requirements, with the purpose of having a better understanding of the needs when a specific application has to be developed and a set of devices has to be deployed. A special emphasis receives the bottom layer, focusing on devices oriented to last unattended for tens of years.