# Eco-Efficient Mobility in Smart City Scenarios
## 4 Proposed Solution  
### Architecture  
The conceptual design of the proposed OBU for sustainable personal vehicles is depicted in Figure 3. As observed, it is equipped with two different LPWAN-based technologies, namely, LoRaWAN and NB-IoT, which provide access to the Internet. Through these links, the device establishes bidirectional connections with servers in the cloud for reporting data and receiving information or commands. These servers can be accessed from personal mobile devices such as smart phones or tablets, to offer functions like those described previously, i.e., environmental monitoring, intelligent route guidance, security applications, etc.  
There are two fundamental elements composing the core of the unit: the main board and the communication board. The former is an embedded single board computer that includes a regular CPU or microcontroller, and memory, integrated in a System on Chip (SoC) fashion. In this case, memory accounts for both RAM and flash memories. Both Inter-Integrated Circuit (I2C) and USB buses are considered in order to communicate this board with external modules, i.e., sensors and a communication concentrator. Some sensors could be also connected to direct digital or analog input/output lines. Whereas sensors are simple devices including (i) an inertial unit, (ii) a radiation detector, (iii) a noise meter, (iv) an environmental sensor (temperature, humidity and pressure), and (v) a pollution monitor, the communication concentrator is the second principal element of the OBU. It integrates a USB-to-serial adapter to enable communications by including chips such as GPS and the respective transceivers for the two selected LPWAN technologies: LoRaWAN and NB-IoT. These modules incorporate their corresponding external antennas to obtain better signal levels.  
The OBU is completed with an external display to show status messages and a battery unit to power the whole set.  
The implemented communication stack is also shown in Figure 3. In addition to the physical and link levels provided by the LPWAN transceivers and their corresponding firmwares, IPv6 connectivity has been integrated into the stack to enable the OBU's interoperability with Future Internet services and devices. For this to be done, an adaptation layer based on the Static Context Header Compression (SCHC) scheme has been developed as described in [38]. In the upper layer, applications based on Representational State Transfer (REST) over TCP/IP are supported, but the Constrained Application Protocol (CoAP) for restricted IoT communications is also considered.  
With this architecture, the running software is in charge of aggregating data from sensors, deciding reporting interval, and choosing the most appropriate communication technology. For the case of LoRaWAN packets, they reach a LoRaWAN gateway that forwards the messages to a LoRaWAN server. The latter is in charge of extracting the data payload and sending it to the corresponding cloud service by using standard IoT protocols such as Message Queuing Telemetry Transport (MQTT). In turn, communications through the NB-IoT link are supported by the coverage provided by a cellular service provider and involves a series of 3 GPP protocols that are transparent for the OBU, as they are already implemented in the NB-IoT transceiver. While LoRaWAN deployments are usually composed  
Figure 3: Architecture of the connectivity solution for eco-efficient devices.  
of private gateways and servers, NB-IoT ones depend on rigid telcos infrastructures and the traffic flows should cross their premises.