## 3 The SensoTube Architecture
The first step towards the foundation of the SensoTube architecture, was to identify every single possible function that should be exist in an ideal hardware WSAN node and to classify the functions into certain groups according to their similarities and their scope. Next, these groups are considered as seven discrete functional layers (see Table 2) by which any WSAN hardware system can be studied, designed and built. As it is reported in [96], any efforts towards the substantial hardware abstraction can increase the fidelity of the characterization and classification of WSAN systems. According to SensoTube, each one of the suggested functional layers has to be able to be implemented as a separate expansion shield. In particular, such functional shields have to be: \begin{table}
\begin{tabular}{c c c} \hline \hline
**Layer Level** & **Layer Name** & **Abbreviation** \\ \hline
1 & Data acquisition and Control Layer & DCL \\
2 & Wireless Networking Layer & WNL \\
3 & Data Gateway Layer & DGL \\
4 & Application-Specific Layer & ASL \\
5 & Programming and Debugging Layer & PDL \\
6 & Power Management Layer & PML \\
7 & Evaluation and Testing Layer & ETL \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The seven conceptual functional layers of a WSAN hardware node. * _Autonomous_: Each functional layer shield should be fabricated on its own PCB. * _Dedicated_: Each shield should be designed in order to implement the tasks of the functional layer at which it belongs. * _Intelligent_: Local intelligence in every shield is necessary, in order to take care of its functions and to allow for reasonable reconfiguration. * _Uniquely identified_: When a system needs to incorporate several shields of different functional layer, as well as more than one shields of the same functional layer, then each shield should be able to be uniquely identified by the system. * _Addressable_: The system, according to the execution of its application scenario, should consider the functional layer shields as addressable units. * _Self-expandable_: In cases where the PCB surface area is not enough to host the necessary circuitry of a particular function, then one or more complementary extra PCBs should be able to be added without, at the same time, to disturb the rest of other functional layer shields. * _Context aware_: Each functional layer shield should be aware of its environment, that is, to be able to interact with other shields. * _Testable_: Each functional layer shield should provide plain testing facilities, e.g., connection points for signals testing. * _Compatible_: Each functional shield should be designed with respect to the homogeneity in form factor and expansion mechanisms. From the above characteristics, which form the profile of the ideal functional layer implementation, it is evident that the WSAN system should be a multi-processor system. In part, this is a mandatory in several COTS approaches which use MCUs together with MCU-controlled radio modules [97]. On the other hand, the provision of a multi-processor ability will help designers to escape from the egocentrism of the existing multi-board expandable architectures (e.g., Arduino and the like), which whilst theoretically support the concept but in practice only the MCU of the main-board has the total control of the common BECs. The commercial MCU solutions present in the market today [40], together with the ongoing research on ultra-low power MCUs can guarantee multi-processor operation, even for battery-operated WSAN nodes [98; 99]. In particular, among the most significant commercial achievements are the new low-power high performance ARM-based MCUs, which already have found their way into WSAN node implementations [100; 101] and the ultra-low power 16-bit MSP430 FR MCUs based on non-volatile ferro-electric RAM [102]. At the same time, researchers are striving towards the elimination of MCU leakages [103], the lowering of the MCUs' operating voltage level [104; 105] and the improvement of the internal power management circuits of the MCUs [106; 107].