# IoT-Enabled Smart Cities: Evolution and Outlook
## 3 Key Technologies Developed
### Networking Technologies  
The IoT environment consists of an enormous number of smart devices, but with many constraints. Reduced processing capability, storage volume or available energy are among these constraints. Therefore, the IoT implementation requires communication protocols that can efficiently manage these conditions. As far as wireless IoT is concerned, many different wireless communication technologies and protocols have been used to connect the IoT devices. They can be coarsely catalogued into two main groups, namely Wireless Personal Area Networks (WPAN) and Low Power Wide Area Network (LPWAN), both of which can be found in the smart city deployments described in Section 2. The first ones are characterized by supporting short range communications with minimum complexity and power consumption while keeping medium/low data rates. IoT devices equipped with these technologies can establish centralized networks but, typically, they are arranged in multi-hop ad-hoc networks so that they can be deployed in relatively large areas. These kinds of technologies were the ones that were firstly available and monopolized the pioneering IoT-enabled smart city deployments. The most used technologies falling in this category are specified by the IEEE 802.15.4 Working Group and Bluetooth Special Interest Group (SIG) respectively. The IEEE 802.15.4 standard defines the lower-layers' communications protocols (i.e., PHY and MAC) for low-data-rate wireless connectivity with fixed, portable, and moving devices with no battery or very limited battery consumption requirements. Many existing IoT deployments employ Internet Protocol Version 6 (IPv6), over Low power Wireless Personal Area Networks (6 LoWPAN) [55] to set up the network among IoT devices equipped with IEEE 802.15.4 interfaces and connect them to the Internet. However, the majority of cases uses a clustered approach where IEEE 802.15.4 IoT devices create a network among themselves and include in it a gateway that is used as sink of all the information generated by the IoT devices in the cluster. The gateway then proxies that information to the Internet. While there are some proprietary solutions following this architecture like DigiMesh [56] or Z-Wave [57], the most well-known one is Zigbee [58], which uses IEEE 802.15.4 as a baseline and defines networking and service provisioning protocols on top of it. Other technologies like Bluetooth [59; 60] or and NFC [61] have been also considered and largely used as solutions for enabling IoT-enabled spaces in general, and smart cities, in particular. Concerning Bluetooth, the profile that has been mainly used for IoT environments is the Bluetooth Low Energy (BLE). The advantages of BLE include lower power consumption, lower setup time, and supporting star network topology with an unlimited number of nodes thus providing networking capacity that is well-suited for IoT scenarios. Both RFID and NFC use similar technology principles enabling very short-range communication technologies that enable data transmission among devices touching each other or being within a distance of no more than a few centimetres. Besides its use for instrumenting Points of Interest with unique information that can be retrieved by the reader if it comes close enough, NFC enables the commissioning and control of IoT devices. Pertaining to the second category, the LPWAN technologies, they appeared much more recently, but they have quickly gained momentum and the current trend in IoT deployments for smart city scenarios is to use this kind of communication protocol. They provide the main features demanded by technologies for IoT, namely low-power consumption and low-complexity, but provide much larger coverage (in the range of hundreds of meters to kilometres) and the capacity for supporting hundreds, or even thousands, of nodes in one network. In its basic mode, which is the one typically used in the existing deployments, they all share a star-like topology where a base station serves all the IoT devices under its area of coverage.