# Smart Cities of the Future as Cyber Physical Systems: Challenges and Enabling Technologies
## 1 Introduction  
Cyber physical systems (CPS) are complex, heterogeneous, distributed systems where the cooperation among cyber components (e.g., sensors, actuators, and control centers) and physical processes (e.g., temperature control, traffic management, and fire detection) is deeply intertwined. A CPS is defined as a system where the computation, networking, and physical processes are integrated to monitor and control physical environments [1]. The diffusion of CPSs is strictly related to the advent of the Internet of Things (IoT), a collection of devices with limited computational capabilities that expose their services to the internet, following a TCP/IP stack [2]. [PERSON] et al. [3] stated that IoT devices were born in the year 1999 in the MIT Auto-ID Lab as technologies that included bar codes, smart cards, sensors, voice recognition, and biometrics. In 2005, [PERSON] [4] identified the trend pushing the technology toward a pervasive dimension and, in particular, pushing devices in that direction. [PERSON], again in [3], deeply analyzed the IoT concepts and perspectives from several points of view, providing an interesting categorization. Available IoT devices may be equipped with a microcontroller unit (MCU) and/or a microprocessor unit (MPU) [5], exploiting their facilities to manage sensors (smoke, gas, fire, presence, cameras, and more), actuators (lights, valves, traffic lights, motors, and more) during their life-cycle; at the same time, they may run programs that pre-process the physical signals to produce data that is useful for several purposes. For example, a single smoke sensor is not sufficient to identify a fire (a cigarette could deceive it). A traditional fire system delivers the perceived signal to a central processing system that correlates the signals with other sources and decides whether to activate the alarms, while also informing firefighters and surveillance. A CPS typically operates side by side with a multitude of other CPSs (e.g., vehicles, factories, buildings, hospital, street, and more), making it difficult, or even unfeasible, to have a unique framework to manage the resulting whole system for several reasons:  
* Administrative: The environments belong to several owners, private or public, that are free to make their own choices according to various factors: financial, bureaucratic constraints, etc. * Technologies advancement: CPS realized at different times adopts different technologies. * Incompatibility with previously deployed technologies. CPS federation is the new direction followed by many researchers, which allows integrated autonomous CPSs through federation to manage, coordinate, and organize sensors, actuators, and host resources and to provide support for the development and maintenance of high-level services [6]. A first example of a CPS is the Smart Building (SB), historically defined in 1981 with the term Intelligent Building, coined by the United Technology Building Systems Corporation, then, implemented in the City Place Building in Hartford, Connecticut [7]. \"Smart\" buildings are mostly customized control systems able to provide basic automatic management facilities of the installed devices (e.g., smoke and fire sensors, ventilation peripherals, and heating systems). In 2009, the European Commission's Information Society provided a long and complex definition of a Smart Building [8]; a simplified version of this is the following: a Smart Building is an integrated system based on the IoT and Ubiquitous Computing facilities that is able to take advantage of a range of computational and communication infrastructures and techniques. The Smart Building concept is easily configurable in several scenarios, modifying its characteristics to obtain various results. For example, a SB specialization is related to the industrial context, where the physical processes commonly monitored by a SB (e.g., HVAC, fire, and intrusion control systems) are added to the controls related to the production processes.