# Smart Cities of the Future as Cyber Physical Systems: Challenges and Enabling Technologies
## 1 Introduction
This way, the system (in this case a smart factory) will be able to monitor and quickly react to emergencies coming from the security system. Another category of CPS that is relevant and interesting is that of public infrastructures. Streets, public buildings, and undergrounds are commonly considered part of a smart city [6]. Efforts in this direction are devoted, both at the institutional level (as in the EU community) and at the academic and industrial levels as well, focusing on how to orchestrate traffic vehicles, enhance the security of the citizens, monitor air conditions, support public transportation systems, make faster and safer rescue operations, and so on. A smart city experiences a combination of several CPSs that have to be interconnected with the others by exchanging data, raw or pre-processed, enabling workflows involving the city to support everyday citizens' lives. With regard to the realization of such interconnections, the following question arises: Do the CPSs belonging to the same administrative domain? The answer guides us toward one of the following two solutions:* If the answer is yes, we can model such interconnection in a tightly coupled way by adopting a Software-Defined Building approach. * If the answer is no, we are obliged to consider a loosely coupled interaction where not all the capabilities of the systems are shareable, according to a federated cooperative approach. A Software-Defined Building (see Figure 1) can be defined as a building where, in line with the Software-Defined principles, the infrastructure and its composing devices are managed in a common way, offering the upper layer the functionalities to be managed. The upper layer is the management layer, where several facilities are available:  
* to control and manage devices of the lower layer,
* to orchestrate, aggregate, filter, and preprocess data coming from the infrastructure layer, and
* to offer functionalities exploitable by the applications that provide abstraction of the IoT devices available in the infrastructure layer. A summary of this vision is depicted in Figure 2, representing the well known four-layer model [9]. Following a bottom-up approach, the Information and Communication Technologies (ICT) infrastructure composed of sensors and actuators represents the smart city foundation. These objects are spread in the urban area and include public and private devices, such as traffic lights, smart billboards, bus GPS, lamp posts, air pollution and weather stations, smart cameras, and citizens' mobile devices. The infrastructure layer manages Cloud and Edge computational resources, the networking resources, and the storage and commuting facilities to collect, manage, and elaborate data. In this way, the infrastructure becomes very complex and has to be properly managed, therefore this layer has to provide core mechanisms for the smart city, enabling as many as possible users/citizens to access all the available resources. Evolving 5G and future technologies for network and computer infrastructure management have to be considered as well. On top of the infrastructure layer is the **management** layer, which provides platform-advanced features and, at the same time, enhances the infrastructure core mechanisms. Figure 1: An example of coexistent cyber physical systems (CPSs) in a software-defined building. This may include AAA, monitoring, profiling, SLA and QoS mechanisms, security, privacy, policies, orchestration strategies and placement, credit-reward systems, incentive mechanisms, and so on. The **Application layer** enables the smart city scoped services and applications to operate on several urban contexts, such as mobility, waste, public safety, energy, e-health, water, and BIM, to name a few. Finally, the **stakeholder** layer includes all the players involved in the smart city everyday life (e.g., the municipality, medical doctors, officers, enterprises, telecommunication operators, citizens, and vehicles). The #SmartME project [10] is a crowd-funded that plans to transform Messina into a smart city.