## 2 Related Work
### IoT End and Super Nodes  
The open source initiative is capable of supporting the free use of open source-licenses. It is through this open source movement that the popularity of open-hardware has been triggered and consequently made available with different options for open hardware microcontrollers-based platforms. The question of which platform would be ideal to use, depends on the requirements of the system. These requirements comprise power consumption, cost and type of connectivity among others. An exposition of some of the most notable open-hardware platforms follows: Arduino Genuino 101: The module contains two tiny cores, an x86 (Quark) and a 32-bit ARC architecture core, both clocked at 32 MHz [22]. Orange Pi: Orange Pi 3 is a single-board computer with an Allwinner H6 system-on-a-chip (SoC) combined to a quad-core, Arm Cortex A53-based 64-bit processing cores running at 1.8 GHz and 1 GB LPDDR3 memory and a Mali T720 graphics processor [23]. Raspberry Pi 3 B+: Raspberry Pi uses the Broadcom BCM2837B0 system-on-chip (SoC) and includes four high-performance Cortex-A53 (ARMv8) 64-bit processing cores running at 1.4 GHz with 32 kB Level 1 and 512 kB Level 2 cache memory, a VideoCore IV graphics processor, and is linked to a 1 GB LPDDR2 (900 MHz) memory module on the rear of the board [24]. From the above-mentioned open hardware platforms, we are utilizing Raspberry Pi 3 B+ jointly with our proposed End Node in order to formulate the Super Node. An IoT system can function and transfer information only when the devices are safely connected with wires or wirelessly to a communication network. This connection is established by choosing the appropriate type of IoT protocol based on our system's specific requirements, like bandwidth and power consumption. The two major M2M (Machine to Machine)/IoT protocols are the Constrained Application Protocol (CoAP) [25], and the Message Queuing Telemetry Transport (MQTT) [26]. Both the MQTT and the CoAP are designed as a long-term vision that will enable their use in a lightweight environment. They both work well with low power and network constrained devices, so the choice depends on the application use case. In our particular use case, for reasons of simplicity and fast deployment Post Office Protocol (POP3) [27] was adopted. Hardware-wise our proposed IoT node is built around a state-of-the-art microcontroller, a high-performance RF and mixed-signal system, Flash/EE memory and SRAM as thoroughly described in [28]. Its power supply system is based on an integrated boost regulator that converts DC power from a PV cell, charging a Lithium-ion storage element for unceasing operation. Moreover, the node provides connectors for various sensors and system debugging. Software-wise, there is no operating system but the firmware code in C language with an approximate size of 10 Kbytes including the radio and sensors control libraries. Its main modules are the Microcontroller and Radio Module, the Power Supply Module and the Input/Output Module. The Microcontroller and Radio Module uses Analog Devices ADuCRF101 microcontroller which is a fully integrated single chip data acquisition solution designed for low power wireless applications. It features a 12-bit ADC, a low power CortexTM M3 processor from ARM(r), a 431 MHz to 464 MHz and 862 MHz to 928 MHz RF transceiver, and 128 KBFlash/EE memory packaged in a 9 mm \(\times\) 9 mm LFCSP. The Power Supply Module is built around Analog Devices ADP5090 chip which is an integrated boost regulator that converts DC power from PV cells or TEGs. The ADP5090 provides efficient conversion of the harvested power from 16 uW to 200 mW. A 4 \(\times\) 4 cm 2V monocrystalline PV panel has been used with 45 mA peak output. The PV panel charges a 3.7 V-650 mAh Li-ion battery. The output from the PV panel and the battery drives the ADP190 (linear voltage regulator) which finally provides a 3.3 V stable output.