## 2 The \"Bottini\" of Siena
### Data Transmission Performances in the \"Bottini\" Environment  
Basing on the above discussion, a data transmission technology suitable for the realization of the \"Bottini\" monitoring system must satisfy three main requirements: it has to be low power, it must enable multi-hop communications, and it must guarantee a minimum transmission range of 200 m. Accordingly, the most suitable solution can be identified among the Sub-GHz technologies that offer superior propagation properties compared to higher frequencies, e.g., the 2.4 GHz ISM bands [31]. In this context, LoRa technology has drawn a large interest and represents one of the most widely employed solutions, thus, enabling the use of a wide range of off-the-shelf devices. LoRa technology is a Semtech's proprietary technology [32] that is specifically aimed at the realization of low power monitoring systems. A similar proprietary solution currently available on the market is the Sigfox [33] technology. However, while SigFox technology exploits a global proprietary  
Figure 1: (**a**) A view of one of the “Bottini” galleries and (**b**) one of the historical inscriptions. infrastructure and requires a subscription to transmit data to a cloud infrastructure, LoRa allows to set up private networks without any kind of subscription. For this reason, LoRa is provided with the adequate degree of flexibility to fit the needs of the specific application at hand. More specifically, in order to minimize the power consumption of each nodes, LoRa networks are typically arranged according to a star topology. In this setting, a sensor node can wake-up, sample the data from the sensors, transmit them and go back to the sleep state, regardless of the behavior of the other sensor nodes. Then, upon receiving data from each node, the gateway (that is typically power supplied from the grid) collects and forwards them to the remote data management center. In order to identify the best system configuration for the \"Bottini\", the performances of LoRa were tested on the field, considering the transmission range as the main performance indicator since power consumption is only marginally affected by the environmental conditions. The LoRa test infrastructure was composed of the following devices:  
* An Arduino UNO board provided with a Libelium SX1272 LoRa module, connected through an ad-hoc Multiprotocol Radio Shield, powered by 4 1.5 V AA lithium batteries, in charge of sampling data from a test sensor (in this case an LM335 temperature sensor by Texas Instruments);
* A Waspmote USB Gateway provided with an SX1272 module and connected to a laptop to check in real time the actual data transmission. The settings for the LoRa modules, chosen in order to achieve the largest transmission range, are shown in Table 1.  
An important characteristic of the considered LoRa radio interface is the possibility of achieving very low sensitivity levels of the receiver (\(-137\) dBm), when compared with possible alternatives. As an example, the different radio modules at 868 MHz present sensitivities in the \(-100\) dBm/\(-120\) dBm range [34]. On the other hand, lower sensitivity entails higher transmission range for a given transmission power, that is a fundamental requirement in the considered scenario. The data transmission range was first tested outdoor in an urban environment. In this case, a transmission range of around 3 km was experienced: this value is in line with the ideal performances expected for the system. In a second step, we carried out experiments in the \"Bottini\" scenario. For this purpose, a section of around 500 m length of the so-called \"Bottino Maestro di Fonte Gaia\", i.e., the gallery bringing the water to the fountain in the main square of the city (\"Piazza del Campo\"), was chosen. Figure 2a shows the network of galleries below the city centre while Figure 2b highlights the chosen section. It is worth noting that, while the gallery runs along a single direction (from south to north), the path is not perfectly straight.