The hardware platform for the Edge Gateway, the Edge Device, must be able to to be able to properly execute the aggregation, processing and transmission tasks as described above, and allow the possible integration of further future components.
In addition to these technical requirements, the choice of the hardware platform must also take into account other requirements related to the addressed user. The target users for these sensors, in addition to the institutional one, is, in fact, increasingly made up of high school students and private citizens who voluntarily intend to buy or assemble the devices themselves. Specific experiments in this sense will be made in the TDM project, but this mode of use is increasingly widespread worldwide.
It is necessary, therefore, that the hardware platform has a limited cost and is easily available, possibly with a large community of developers and users possible. It must also be free from commercial constraints on royalties and licenses, and possibly have a 'educational' connotation.
In particular, two of these general requirements, the low cost and the wide user communities narrow down the scope of choices to:
- hobbies Single Board Computer (SBC) for the platform, and
- Linux distribution as the operating system.
The main role of the Edge Gateway is to collect and aggregate measurements and data acquired locally by the various sensors and measuring stations in the field and prepare them for sending to the cloud for subsequent storage and processing.
From the point of view of I/O capabilities, the hardware platform must therefore be able to communicate with the sensors using both simple electric buses and more sophisticated network protocols. In particular, for sensors directly connected to the Edge, it is possible to use mainly I2C, USART and custom protocols implemented on GPIO, while for remote detection units such as energy consumption meters and weather stations / air quality, the easiest and most effective way is the WiFi connectivity through home network, if present in the place of installation, or possibly creating it through the Edge Gateway itself. The possibility of future expansions should also be taken into consideration: the desired and non-binding requirement for the hardware platform is therefore the presence of additional electrical buses and communication protocols between embedded devices.
As regards communications from the Edge Gateway to a Cloud, such as that of the TDM project, it was decided to use Internet connectivity in the installations it has reached, while for installations in rural areas or not yet served, it is possible to provide the use of LTE connectivity through specific chips or modems/USB dongles.
To sum up, the hardware platform for the Edge Gateway must have the following peripherals:
- I2C bus/port;
- UART/USART bus/port;
- free GPIO pins.
The presence of the following protocols, although not binding, is a desired one:
- CAN bus/port;
- ADC ports/peripherals;
- ports with PWM output;
- Video outputs and/or LCD controller;
Must have network devices:
- WiFi;
- Ethernet;
- USB Host.
As for the computational requirements, i.e. those requirements that allow the software architecture of the Edge Gateway to be executed effectively, these are of three types: the processor and the computing power, the amount of RAM memory, and the storage capacity.
The requirements for the Edge Gateway processor are as follows:
- must be supported by the Linux kernel and be able to run a full Linux operating system;
- have low power consumption to allow use via low voltage battery;
- have good software support and possibly a large community of users and developers;
- have free and royalty free SDKs and compilers available;
- have low cost and full development boards accessible;
- given the architectural choice to use multiple microservices running concurrently, the processor must have more than one core.
Many processors for embedded applications are capable of running a complete Linux operating system. Among these, the ARM-CORTEX family offers several choices, both as processors and development boards, that meet these requirements.
As far as RAM memory is concerned, the above points are worth noting. The provision of RAM for the Edge Gateway must:
- be sufficient to run a complete Linux operating system;
- have a low power consumption;
- be large enough to serve multiple competing microservices running on Docker containers;
- support access from multiple cores.
Storage on the Edge Gateway means the storage medium both for the operating system and the Edge applications, and for the storage of data collected over time by the various sensors and measuring stations connected. More in detail, the requirements for storage are:
- economic, accessible and flexible, i.e. solutions easily available on the market, in different capacities and easily replaceable and upgradable;
- sufficient capacity to hold the operating system, the Edge applications and a reasonably large history of data collected;
- wide speed of data transfer.
The most common solution that addresses to these requirements is the SD technology, and, therefore, it is considered that the hardware solution for the Edge Gateway will need to have support for SD/microSD cards. Empirically it has been established that a size of 8GB, class 10, is sufficient for the purposes of the Edge Gateway, however, given the availability of higher capacity SD with reduced costs, we recommend the minimum size of 16GB, class 10.
The above requirements can be met by many hardware solutions, with various cost-performance ratios. In the following section, we describe the software architecture implemented, which relies on the above requirements and has no dependencies on the specific solution chosen. In section 5 we will discuss, instead, the hardware implementation of the reference prototype.