## 3 The SensoTube Architecture
### Data Acquisition and Control Layer (DCL)  
In real-world WSAN implementations, it is very common for the specifications of the data acquisition (DAQ) and control to change [6]. For example, a new type of sensor may require a higher conversion resolution and a higher sampling rate, or the need for some extra sensors may require extension of the existing analog inputs, or a new actuator may need more energy and special driving circuits in order to be driven, etc. Regarding the field of agriculture, the measuring and monitoring of various physical parameters require, very often, the use of complex sensory devices [5]. For such  
Figure 8: Representation of the SensoTube architecture for WSAN hardware systems design. The seven functional layers are shown as discrete horizontal layers. The four inter-layer services are shown to vertically penetrate the seven functional layers. reasons, in [9] it is pointed out that, regarding the agricultural domain, a data acquisition daughter card is required. Today, the support of the data acquisition and control aspect of the WSAN systems seems to be rather underrated in the existing architectures. For instance, in COTS (e.g., motes), the emphasis has been entirely put on the RF communications. In particular, some of them have a couple of sensors soldered on-board, just as to be able to demonstrate the networking capabilities in measured data from the wireless nodes [26; 43]. In most of the cases, these on-board sensors are not of the proper type and form, in order to be useful in agricultural applications. In addition, as reported in [54], there are serious limitations in data sampling periods, when a single MCU is responsible for both the networking protocol and the DAQ functions under an operating system. In general, the COTS-based systems leave the development of the DAQ and actuators circuitry in the users' hands. On the other hand, the existing expandable OSH architectures provide limited support for a sound DAQ and control function due to their inherent structural constraints (see Section 2). Furthermore, these solutions are not energy optimized so as to support battery-operated WSAN applications. In particular, the various analog or digital sensors which are connected to an OSH main-board, are always activated, regardless the fact that the sampling rate may be very low. This is particularly evident, for example, in the management of the soil sensors which are based on the SDI-12 bus [112]. Also, the existing expandable OSH-based systems suffer from scalability, in terms of processing power and communication peripherals. Thus these architectures appear to be convenient only for the limited scope of short-term experimentation. On the contrary, the SensoTube architecture with its inter-layer services facilitates the design and development of flexible and scalable DAQ and control shields. According to the SensoTube, a WSAN system is capable to use more than one DCL shield. Each DCL shield can employ its own MCU. This ensures the ability for reconfiguration at shield's level, as well as the capability to undertake the execution of measurements scenarios locally without disturbing other functional layer shields. Regarding the MCU, designers can make their choice either by selecting a commercial ultra-low power one, or by using an FPGA [113], or an analog mixed-signal processor [114]. Furthermore, with the introduction of the energy management service, the DCL shield(s) can be entirely powered-ON or OFF, according to the application scenario. In this way, the maximum level of energy consumption control is achieved. Also, a dedicated MCU-enabled shield can help the designers to take all the necessary PCB design precautions for the highest performance in signals integrity (SI) and EMC. Moreover, SensoTube aims to provide the necessary polymorphism, in terms of signal connections, to allow for increased flexibility and versatility.