# IoT-Enabled Smart Sustainable Cities: Challenges and Approaches
## 2 Overview on a Smart Sustainable City
### Connectivity Infrastructure  
As stated in Section 1, the IoT represents one of the core technologies for a smart city implementation, involving the deployment, in different urban areas, of an always growing number of heterogeneous connected devices, as shown in Figure 2, and representing an overview, made by IoT Analytics [7], on the estimated number of connected IoT devices (also denoted as Smart Objects) in the period 2015 \(\div\) 2025. Following the increasing trend shown in Figure 2, it is clear that the communication infrastructure is the first key aspect to deal with in future smart cities, as connectivity in a smart city will be required anytime (day/night), anywhere (indoors, outdoors, or on the move), and between highly-heterogeneous entities (such as PCs, smartphones, tablets, battery-powered IoT devices, etc.) characterized by different communication needs and capabilities, and will involve Human-to-Human (H2H), Human-to-Thing, Thing-to-Thing, and Machine-to-Machine (M2M) interactions.  
Figure 2: Trend on the global number of connected IoT devices in the period 2015 \(\div\) 2025 [7].  
Taking into account the different application scenarios, a heterogeneous set of communication technologies and different network architectures can be adopted guaranteeing connectivity, depending on the characteristics of the services to be implemented. In most cases, connectivity among IoT devices is guaranteed through wireless networks, enabling the exchange of information with flexible and low-cost deployments [8]. As an example, mesh networks and short-range communication technologies are preferable when end-to-end communications are not feasible because of power limitations or obstacles. In this case, hop-by-hop communications from device to device can be employed to bring information from IoT nodes (data sources) to data consumers (e.g., servers, border routers, etc.), and backwards. At the same time, star networks and long-range communication technologies are preferable in all the cases where direct and reliable communication links between a central \"hub\" and all IoT nodes within its coverage are available. The most relevant communication technologies that can be used in the smart city context [9] can be summarized as follow, with a corresponding graphical classification (in terms of transmission distance and communication speed) is shown in Figure 3.
* Low-Power Wide-Area Networks (LPWANs): This kind of network is especially designed to interconnect battery-powered devices with low bit rates over long ranges. Due to their low cost, wide coverage, and straightforward set-up, LPWANs are being deployed in different applications where a small quantity of data needs to be transmitted [10]. LPWANs work on both unlicensed and licensed frequency bands and include different standards (that can be open or proprietary), the most relevant ones are LTE-M [11], Narrowband IoT (NB-IoT) [12], and LoRa [13].
* 5G: Thanks to its advanced features, such as very low latency (less than 1 ms) and very high bandwidth (10 Gb/s), IoT is emerging as one of the primary use cases for 5G. Therefore, 5G can be considered as an enabling communication technology for smart cities, allowing more and more IoT devices to be connected to the Internet, regardless of their location and time, supporting applications such as smart traffic systems, public safety, security, and surveillance in the context of smart cities [14].
* Wireless Local Area Networks (WLANs) and short-range networks: Many use cases in smart cities require the deployment of \"regional\" (namely, covering a limited spatial area) and, in some cases, of \"individual\" networks (such as Personal Area Networks, PANs). In this context, the available communication technologies are extremely diverse, spanning from protocols based on IEEE 802.15 to Bluetooth and Bluetooth Low Energy (BLE) [15].  
Figure 3: Network communication technologies.