## 4 The SensoTube Architecture's Implementation Reference Model
### Expandability and Inter-Layer Services Mechanisms
#### 4.2.1 Inter-Layer Signals Management Service Mechanism
Additionally, in the external row of through-hole pads, extra BECs can be soldered, in order to enable selected shield's signals connection to its top and bottom neighboring shields. This is a secondary provision for local signals connections among shields. This can be considered as a nested connection method, and allows a functional shield to have its own sub-functional shields without intervention to the rest of the system's functional shields. Figure 14 illustrates an example of the combination of a secondary BEC together with pin-headers and jumpers. In this example, just five of the S-BEC's signals have been connected to the shield's circuits, while six signals of the shield are ready for connection with its two neighboring shields (or with just one of them). All of these modifications, which are based on the particular usage of the two rows of pads, are not permanent in nature, they do not impose limitations to boards stacking, and they can be performed very easily by the end-users of the shields, e.g., researchers and any kind of developers. Also, there is a third alternative for signals connections, which is very convenient at the system-level signals physical connections. One or more screw terminal blocks can be placed at the pads  
Figure 14: Use of secondary BEC to local connections with neighboring shields. Figure 12: Detail of the S-BEC 1 showing the BEC’s pads row at the top and the two rows of pads added at the bottom. The electrical PCB connections (traces) are denoted with red color. Figure 13: Selective connection of shield’s signals to S-BEC’s signal by the use of pin-headers and short-circuit jumpers. of the external rows, which are directly connected with the S-BECs signals (Figure 15). This facilitates users' physical access to the various shields' signals by just wiring instead of risky soldering. Such, a function is particularly useful for easily adding and removing several types of sensors in WSAN agricultural applications. The proposed signal management mechanisms are low-cost and easily implemented in the PCB. The addition of the extra rows of pads (two rows per S-BEC) can be easily hosted in WSAN hardware systems for agricultural applications, due to the fact that, there are no space limitations in this application domain. Despite the fact of the space reservation from the added rows of pads, there is more than enough PCB space for the development of the shield circuitry. Following the aforementioned signals management mechanisms, the poor performance and low reliability of the WSAN systems, due to clumsy wiring of signals connections, are minimized. On the other hand, the various different shields can be designed as totally independent functional entities without signals connections and boards' expansion barriers. The SensoTube polymorphism in signals management is depicted in Figure 16.  
From the designers' perspective, the methodology of incorporating the proposed mechanisms is very convenient and straightforward. In Figure 17, on the left and right sides there are the sheet symbols of the S-BEC 1 and S-BEC 2. In each one of these sheet symbols there are the signal ports entities which represent both the common signals of the stacking BECs and the signals of the parallel pads row. The pads row signals are numbered as PIN_1 up to PIN_40. In the middle of the Figure 17 there is a third sheet symbol. This represents the new, under design, functional shield. Designers can choose to connect all, or just some, of the signals of their shield to the pads row's signals. Signals of similar function e.g., UART transmit and receive signals should be connected to the sheet port entities that are opposite to the TXD and RXD signals of the BECs. There is no physical connection amongst the signals of the shield and the signal of the predefined signals of the BECs. It is on the designers' discretion to use shorting jumpers (see Figure 13), or secondary BECs (see Figure 14), or screw terminal blocks (see Figure 15) to route the signals of the shield.