# A Multi-Technology Communication Platform for Urban Mobile Sensing
## 2 Related Works
### MAC in LoRa  
Within smart city scenarios and IoT applications the network nodes operate under shared-medium conditions, i.e., the nodes will have to compete over a shared common communication channel. Thus, an efficient MAC scheme is an essential requirement to grant a successful operation of the network under such conditions. Wireless sensor network (WSN) and IoT applications require the fulfillment of some specifications in terms of the MAC protocol, such as multi-hop communication, resilience, and sometimes low-latency, among others [21]. Moreover, it is essential to provide fair access to the communication channel and avoid possible packet collisions. For Low-Power Wide Area Network (LPWAN) technologies, besides the underlying radio frequency characteristics, much of the technology value consists in satisfying the end user requirements such as the ability to create a network, control it, and offer bi-directional data flow. Long-range (LoRa) technology can be supported by the LoRaWAN (LoRa Wide-Area Network), a MAC protocol based on the Aloha method. This MAC layer is very lightweight and essentially implements pure-Aloha with Listen-Before-Talk (LBT), resulting in low channel utility under high traffic load due to packet collisions [22]. Promoted by the LoRa Alliance, LoRaWAN [23] defines the communication protocol and system architecture for the LoRa physical (PHY) layer. This protocol and network architecture has the most influence in determining the battery lifetime of a node, the network capacity, the quality of service, the security, and the variety of applications served by the network. The LoRaWAN architecture is typically laid out in a star-of-stars topology in which gateways act as connection bridges between end-devices and a central network server (_NetServer_) in the backend. Gateways are connected to the _NetServer_ via a standard Internet Protocol (IP) backhaul interface while end-devices use single-hop wireless communication to one or more gateways. The _NetServer_ is responsible for the management of the overall network. For instance, it filters the duplicated packets from different gateways, does a security check, and sends Acknowledgements (ACKs) to the gateways. The nodes in a LoRaWAN network are asynchronous and communicate when they have information \begin{table}
\begin{tabular}{c c c c c c c} \hline
**IoT Platforms** & **Multi- Technology** & **Long Range** & **Short Range** & **Multi- hop** & **Pos** & **Cons** \\ \hline
[11] & **-** & **-** & **-** & **-** & Highly-populated network. & Lacks a low-cost and long-range network. \\  & **-** & **-** & **-** & **-** & Multiple communication technologies. & Lacks an opportunistic network. \\  & **-** & **-** & **-** & **-** & Multi-hop capabilities with buses & Lacks low-cost and long-range network. \\  & **-** & **-** & **-** & **-** & Extensive LoRa experimental network setup. & Lacks multi-technology, multi-hop and opportunistic communication capabilities. \\  & **-** & **-** & **-** & **-** & Mobile sensing (through trans). & & **operputuristic communication capabilities. \\  & **-** & **-** & **-** & **-** & Energy-efficient management. & Lacks multi-technology, multi-hop and opportunistic communication capabilities. \\  & & & & & Multiple communication technologies. & \\ Our proposal & **-** & **-** & **-** & **-** & Opportunistic and delay-tolerant communications, providing a multi-hop data forwarding approach. & Still a small-sized network, currently under expansion. \\ \hline \end{tabular}
\end{table} 
Table 1: An overview of IoT platforms for smart cities. to transmit whether event-driven or scheduled. This asynchronous communication is a major driver of the battery lifetime increase. LoRaWan end-devices can serve different applications, each one with its own requirements. In order to optimize a variety of end application profiles, LoRaWAN defines three different device classes (Class A, Class B and Class C). The device classes trade-off network downlink communication latency over the battery lifetime.