# Opportunistic Sensor Data Collection with Bluetooth Low Energy
## 1 Introduction  
Sensor nodes are being deployed worldwide to enable smart environments, whereby resources can be efficiently managed and/or human life quality can be enhanced. In some scenarios, such as homes or industrial automation, network infrastructure is typically available for the sensor nodes, therefore whenever the latter need to send their obtained data, some device must be ready to receive (and maybe forward) the data [1, 2]. However, there exist other scenarios where network infrastructure is expensive, or it is hard to deploy for practical reasons, such as smart cities, agricultural ecosystems, developing areas, etc. [3, 4, 5]. In those, as long as latency requirements for the collected data are loose, opportunistic sensor data collection (OSDC) carried out by a mobile entity may be more suitable. On the other hand, OSDC can also be viewed as a backup or disaster recovery option for data collection when infrastructure fails, similarly to the role of opportunistic networking to provide communications means after disaster recovery [6]. Useful mobile entities for OSDC include pedestrians, and vehicles such as public buses or even drones. Such mobile entities may approach an isolated sensor node on purpose (e.g., an agricultural intentionally walking, or flying a drone, close to the sensor node location) or due to the path they naturally follow (e.g., the daily route to work for a pedestrian or the scheduled route of a public bus). For the sake of flexibility and seamless, low-cost operation, communication between the sensor node and the mobile entity requires the use of a wireless technology commonly or easily available on the mobile entity equipment. At the same time, such technology should allow low-power operation for the sensor node, which typically is not mains-powered and has a limited energy source (e.g., a battery). A major candidate technology that fulfills such requirements is Bluetooth Low Energy (BLE) [7, 8]. In fact, since BLE is the wireless low power technology best established in the mobile market (massively implemented e.g., in smartphones, tablets, and wearables), it is a natural choice for developing an OSDC solution. Competitors such as Ant/Ant\({}^{+}\) are not supported by any standardization body, while other standardized technologies are today far from competing with BLE in this specific domain. IEEE 802.15.4 could be an option with the release of modular smartphones [9], but their adoption seems far in the near future. Secondly, the work conveyed in order to extend Wi-Fi to meet the requirements of IoT [10; 11] is focused on infrastructure-based scenarios. Finally, Visible Light Communication (VLC) technologies are progressing in their standardization [12], but their intrinsic characteristics make them only well suited basically for indoor and short range communications. Furthermore, BLE is emerging as a key enabler of the Internet of the Things (IoT). The methods to run IPv6 over BLE have been standardized [13], allowing the use of BLE devices such as smartphones as gateways [14]. On the other hand, there exist proposals to extend BLE for mesh network topologies which are progressing towards standardization [15; 16]. With regard to market products, the BLE ecosystem is growing [17]. The recent family of products called beacons, designed for opportunistic data collection, has achieved remarkable momentum. Beacons can advertise identifiers (e.g., URLs) and sensed data [18]. Considering all the above, BLE is a promising technology for OSDC. It is thus crucial to know the achievable performance of BLE in such scenario for the design, planning and practical use of OSDC solutions. However, use of BLE for OSDC has only been considered to a limited extent [19; 20; 21]. To our best knowledge, a detailed, comprehensive characterization of BLE for OSDC is not available in the literature. In this paper we provide an analytical and experimental study of the performance and trade-offs of BLE as a technology for OSDC.