1. Cyber-Physical Systems 

Past decades witnessed great success and advances in the field of information and communication technology (ICT) which is expected to grow much more in the close future and attract so many attentions. Among the variety of applications in Information Technology field, ICT Embedded components in different devices have become an important component playing a significant role in different aspects of our life.

Embedded systems are defined as computer systems which are designed to perform specific functions, usually in real-time and are embedded as part of a complete system. Embedded systems include different types of devices such as portable devices (e.g. smart phones and MP3 players) and large systems (e.g. plant control systems) [1].

Nowadays, the relation between the cyber world and the physical world has distinguished CPS from traditional embedded systems. CPS main characteristic is the integration of computation and physical processes [1]. In these systems, different devices with computational components are working together in a network and are monitoring, sensing and actuating the elements in the physical world.

There are many different examples and applications of CPS. They include large-scale systems such as health care, automation, transportation and smart grid system. In addition, a new concept is coming up for mobile cyber-physical applications using smart phones and mobile Internet devices that employ multiple sensors [1]. In all different types of CPS, the most important issue is properly understanding and resolving the complicated interaction between physical and computational elements [1]. In these systems, the cyber section is set of control logic and sensor units and the physical section is a set of actuator units [10].

Term “Cyber-Physical Systems” has emerged as an important and critical research topic is the combination of computations, communications and control. Although it is difficult to have one specific definition for this term because of its vast span, it can be depicted as a physical engineering system that its operation is being controlled, monitored and integrated by a computational core.

In CPS, understanding physical and computational processes separately is not enough and sufficient. CPS is about the intersection and it is important to understand the interaction of physical and computation components. [4]

As the connection diagram demonstrates in Figure 2-1, sensors capture and transmit the physical world’s status to CPS and the computational components process the received information. The system then decides which action should be taken based on the result given by computational components and the actions are performed by the actuators. The following steps have been defined by Eric Ke Wang showing the main steps in CPS workflow: [5]

1. Monitoring: This is the fundamental function which monitors the physical environment and its processes. Using this function CPS can send feedbacks on previous actions and make sure that operations are correctly done in the future.
2. Networking: This function is responsible for aggregating data received from sensors. There are many sensors networked in CPS which are generating data in real-time. Meanwhile, different services need to interact and communicate with the network.
3. Computing: The data monitored in step one and aggregated in step two should be analyzed in step three which is computing step. This function is responsible for checking the criteria which are defined previously and decide if the result coming from analysis satisfies that. If not, the computing section executes corrective actions. As an example, the temperature rise can be detected using a data-center CPS.
4. Actuation: The results coming from computing elements are sent and executed by actuators. The actions that can be taken by actuators are different types of activities such as correcting the cyber behavior and modifying the physical process. For example, shutting down a system before a probable explosion.

CPS have much more functions, capabilities and services which could not be included in embedded systems. Users can not influence embedded systems. Embedded systems can just help with automated tasks and they are not usually visible to the user. Any possible action needed in embedded systems would be performed under complete control of the user [7]. In opposite to embedded systems, CPS acts based on the data gathered from the physical world in real-time and reacts to this data through predefined orders while cooperating with services, local systems, Internet of Services (IoS) and Internet of Things (IoT) [7].

In summary, CPS focuses on the connection between the physical world and the cyber world while traditional embedded systems concentrate on computational elements. Figure 2-2 shows the structure of a CPS more clearly.

CPS exceeds embedded systems in different aspects. CPS are more reliable, safe, efficient, robust and adaptable [1]. As an instance, with help of a fast response captured from the sensors, the damages of an explosion in gas stations or in a car accident can be avoided. These systems can also help in having more precise robotic surgeries and result in less pain, blood loss etc. As a result, research on CPS is growing faster and is significantly important nowadays. Considering the unlimited applications CPS can enhance humans’ life quality.

1.1.                       CPS characteristics

Key characteristics of CPS can be summarized as follows:

1. System of systems: In contrast to embedded systems, CPS is a complex system, consisting of many subsystems that interact with each other and can also stand-alone. Therefore, the CPS complexity is much more than a traditional individual embedded system [1].
2. Interactions between control, communication and computation: CPS should be automated and non-technical factors (human factors) should be all omitted in the control loop. Therefore, the control, communication and computing element should be considered in parallel while designing the system. [1]

3. Coupled cyber and physical world: The physical world in CPS should be tightly coupled with the cyber world. Consequently, large-scale wireless and wired networks become significantly important. [1]

1.2.                         CPS background

In 2008, in the US, President’s Council of Advisors on Science and Technology (PCAST) organization has prioritized CPS as a top federal research investment. Because of the recommendations coming from PCAST about CPS importance, the CPS program was started at the National Science Foundation (NSF) in 2009 [1]. The NSF CPS program concentrates on fundamental issues concerning different sectors such as health care, automotive, energy, transportation and aerospace.

In addition, this program supports developing different components including methods, hardware, software and tools to accelerate the realization and the usage of CPS in different domains. The NFS CPS program also further implemented the CPS Virtual Organization[1] to prepare and increase research and education community for emerging innovations and applications of CPS.

In 2010, several workshops and conferences started regarding CPS and their applications. These conferences were mainly initiated by the cooperation of the ACM and the IEEE. The first conference on CPS (ICCPS), was successful with many interesting paper and sessions. A timeline of CPS history is shown in Figure 2-3.

1.3.                       CPS applications

Nowadays CPS are being used in various domains including health care systems, assisted living, advanced automotive systems, traffic control and safety, energy conservation, environmental control, critical infrastructure (e.g. power, water), robotics and manufacturing [6]. In this section, three examples of CPS applications, health care and medicine, aerospace, electric power grid and automotive systems are explained.

1.3.1.     Health care and medicine

The health care domain consists of home care, operating room, robotic surgery, national health data, electronic patient record, etc. These systems which are mostly controlled by computer systems and some work in real-time require too much safety and accuracy in timing [3].

Health care domain in CPS that is usually called Medical CPS (MCPS) helps doctors and patients to interact with each other easier using the cyber section in CPS to receive better treatment [3]. With the help of MCPS, doctors can monitor patients from far away on remote systems rather than the local stand-alone systems.

The wireless technology significantly improves the safety of health care systems; current complicated massive connections between systems using wires in health care environments often results in a crisscross of cables named ‘‘malignant spaghetti’’, which is a serious vulnerability that puts patients’ lives in danger [1]. Systems’ reliability is of high importance in MCPS and it is one of the researchers’ priorities to enhance the overall reliability by bringing new technologies and theories.

1.3.2.     Aerospace

CPS research has a significant impact on the design of aircraft as well as on air traffic management with the aim to significantly improving aviation safety. Some key research issues in aerospace CPS are as follows:

(i) New functionalities to achieve higher capacity, greater safety and more efficiency as well as tradeoffs among their possibly conflicting goals; (ii) integrated flight deck systems, moving from displays and concepts for pilots to future autonomous systems; (iii) vehicle health monitoring and management; (iv) safety research relative to aircraft control systems.

One of the main challenges is design verification and validation of extremely complex flight systems. Since the complexity of flight systems is constantly increasing, the cost of verification and validation also increases. The research on verification and validation of aviation flight-critical systems includes how to provide methodologies for rigorous and systematic high-level validation of various system safety properties and requirements. This is evaluated in all phases ranging from initial design through implementation, maintenance and modification; it is also highly required to understand tradeoffs between.

1.3.3.     Power grid

A power grid CPS is formed from the power electronics, power grid and embedded control software. Designing this type of CPS requires a high-level of security, fault tolerance and decentralized control [6]. Recently, research on smart power grid has gained tremendous interests. Development of smart power grids has been of great public interest, which results in a high priority for policymakers. It is a top priority to protect the energy infrastructure from failure as well as outside attacks. For example, under certain unexpected situations, a failure in one location of the electric power grid can propagate across the grid, which leads to plenty of failures and blackouts.

The key objective is to design a robust power grid network by introducing real-time control in the composition of cyber and physical elements in the grid. In particular, security policy, intrusion detection and mitigation must deal with possible outside attacks and should be carefully considered.

1.3.4.     Automotive systems

Nowadays vehicle systems are way more advanced than pure mechanical systems. Automotive systems are being used everywhere. Around 30-90 processors are embedded and networked in each car in different sections such as brake system, engine control, airbag system and door locks. [1]. Furthermore, cars may connect to each other and communicate using the Internet, vehicle-to-vehicle networks, or cellular networks [1]. In this situation, safety and security of systems become highly critical. These systems should guarantee reliability for complex networked software.

Automotive CPS has one of the most usages in our everyday life and is one of the most critical ones that should be highly secured as a small accident can damage a lot and take many lives. In the US, currently, almost 42,000 fatal accidents happen each year which could dramatically be reduced using more intelligent systems to help the drivers. Current technologies for collision avoidance are passive and heavily depend on driver’s interaction. Consequently, the automation of collision avoidance is of great interest. With advanced technologies for onboard sensing and in-vehicle computation, as well as with global positioning systems (GPS) and inter-vehicle information exchange, it is expected that near-zero automotive traffic fatalities and significantly reduced traffic congestion are achieved. With the growing development of CPS, some new solutions can be applied to unmanned vehicles. Researchers are working on a program to integrate unmanned vehicles and intelligent roads as a CPS. [6]

1.4.                       SCADA

SCADA is one type of industrial control systems (ICS) that help to monitor and control operation remotely via communication channels [2]. Existing industrial processes in the physical world can be monitored by ICSs. SCADA is cooperating with many various types of CPS. It can be employed to acquire raw data about the remote equipment’s status and it usually uses different communication channel for each remote station.

SCADA is the core of different industries such as transportation, manufacturing, power, gas, oil, water and many other areas. It cooperates well with many different types of CPS since they can range from simple to complex large configurations and projects. SCADA can be found in our daily life nowadays almost everywhere. It is not easily seen because it is used behind the scenes. You can find SCADA at your local supermarket, wastewater treatment plant, or more importantly the gas stations [2]. SCADA systems employ many software/hardware elements allowing industrial organizations to: [2]

• Aggregate, control, monitor and process data.
• Control devices and interact with them. Devices are connected through human-machine interfaces (HMI).
• Store all events in log files.

In SCADA, PLCs (Programmable Logic Controllers) and/or RTUs (Remote Terminal Units) receive data from sensors and/or manual inputs and send these data to computers which have SCADA software on them. These computers then analyze the data and display the result. SCADA reduces the wasting time and improves efficiency the manufacturing process and may result in significant savings of money and time. Figure 2-4 shows SCADA architecture and the connection between sensors, RTUs, PLC and SCADA system.

1.4.1.     Security challenge in SCADA

In contrast to ICSs, SCADA systems are used in large distances for large-scale processes including multiple sites [2]. Many security attacks have been reported against utility assets. Security vulnerabilities in critical systems can lead to fatal disruptions and they may disclose sensitive information. An attacker can execute an attack in less than one hour as soon as system vulnerability is known and its security is compromised. The growing usage of the Internet helps attackers to form an attack from multiple locations. The most dangerous type of attack is when attackers gain access to the supervisory control access and execute disruptive commands [2].

1.5.                       Architectural topology

There are three main topologies for CPS:

• Centralized topology: In this topology, data is collected from distributed sensors and all sensors and actuators are monitored using one deployed middleware [11]. In this topology, it is easier to manage and control CPS and a more secure environment is provided. However, there are more devices being added every day and it makes every system more complex, therefore, it would be problematic to use this tightly coupled centralized topology.
• Distributed topology: In this topology, a very small “middleware” is implemented on each physical device. This middleware is responsible for controlling the physical part and connecting with other peer-to-peer sections. For instance, a middleware may consist of agents and actors. These entities provide adaptive load balancing and monitoring while moving across networked sensors. As the name of this topology shows it provides scalable systems as it is possible to add as many as physical devices and computing elements are needed without any interference with other elements. On one hand, this topology can minimize the network congestion as there is no bottleneck point in it. But on the other hand, it is not possible to execute complex computation since the physical devices have limited resources and managing the devices gets harder. [11]
• Nested topology: If combine both pr