“Network QoS Management in Cyber-Physical Systems”

1. “Network QoS Management in Cyber-Physical Systems” by Feng Xia, Longhua Ma, Jinxiang Dong, and Youxian Sun Nicole Ng

2. Goal of the paper • New generation of engineered systems => CPS • Major challenge that needs to be addressed: QoS (Network Quality of Service) • WSANs (Wireless Sensor/Actuator Networks) will play an essential role in CPS • Goal: Examine characteristics of WSANS and requirements of QoS in context of CPS

3. Network Design in CPS • CPS – “integrations of computation, networking, and physical dynamics, in which embedded devices are networked to sense, monitor, and control the physical world • Massive networking of embedded computing devices => CPS system • WSANs => serve as underlying network infrastructure for CPS

4. The Vision for CPS • Large number of embedded devices interconnected through WSANs that make up various autonomous subsystems • CPS may be composed of many subsystems • To share information globally, WSANs are connected to the internet • Cyber-physical city: cyber-physical subsystems for health care, smart home, intelligent transportation, facilities maintenance, public security

5. Physical Topology of a CPS • WSANS serve as the interface between the cyber system and the physical system • WSANs enable cyber systems to monitor and manipulate behavior of the physical world

6. WSAN and QoS requirements • WSANs – new generation of sensor networks, coexistence of sensors and actuators, active information gathering infrastructure • Characteristics • Resource constraints • Energy conservation is critically important for extending the lifetime of the network • Platform heterogeneity • For large-scale CPS, hardware and networking technologies used in underlying WSAN’s may differ from one subsystem to another • Dynamic network topology • During runtime, new nodes may be added or removed • Mixed traffic • Diverse applications may need to share same WSAN that induce periodic and aperiodic data • Sensors for different kinds of physical variables

7. WSAN and QoS requirements for CPS • CPS is application-oriented • Different applications have different QoS requirements • QoS – capability to provide assurance that service requirements of applications can be satisfied • Existing QoS mechanisms may not be applicable to WSANs in context of CPS => more research needs to be done in this area

8. SOA • SOA – (Service-oriented architecture) architectural style encompassing set of services for building complex systems of systems • Model where functionality is broken down into small distinct units (services) • Need to identify and specify services • Categories of services that should be classified • Functionality, interface, and properties of each service • Quality levels relevant to performance requirements • Dealing with different between sensors and actuators when specifying services

9. Communication Protocols • Paper looks at MAC (medium access control), routing, and transport protocols • To efficiently support QoS in WSANs, protocols need to be designed keeping in mind the heterogeneity between sensors and actuators involved in CPS => QoS-aware MAC, routing, and transport protocols developed for WSNs not suitable for WSANs • Necessary for new QoS mechanisms to be layered on top of existing networks

10. Resource Management in WSANs • Resource budgets need to be guaranteed in order to meet certain QoS levels • Higher level of QoS = need for more resources (ex. CPU time, memory size, bandwidth and/or energy) • Resource management in WSANs is challenging • Complexity of CPS, dynamic feature of the networks, unpredictable and changing environments • Need self-management technologies

11. Energy Conservation • Major concern in WSANs • Lifetime of nodes restricted by battery energy • Need to minimize energy consumption and maximize QoS, but these are two conflicting requirements • In-network computation can be exploited to reduce energy consumption of nodes (reduce traffic load at cost of slightly increased computation in each node)

12. Possible Solution: Feedback Scheduling • Previous work shows that feedback scheduling can handle uncertainties in resource availability by automatically adapting to dynamic changes • Role of feedback scheduler: dynamically adjust specific scheduling parameters of relevant traffic to maintain desired QoS level

13. Feedback Scheduling Framework • System output (QoS parameter) = controlled variable • Adjustable scheduling parameters = manipulated variable • Desired value of QoS parameter = setpoint

14. An Example: A Simple WSAN • s1, s2, s3 and s4 are source (sensor) nodes • s3 is an interfering source node • s6 is an immediate node • a1 and a2 are actuator nodes

15. A Simple WSAN • The nodes compete for the use of the same wireless channel for data transmission • Utilized communication protocol is ZigBee with date rate of 250 kbps • Size of data packets transmitted is 45 bytes • Sampling period for each source node is 10 ms • Running the system: • At the beginning all nodes except s3, s4, and s5 are active • s5 is switched on at time t = 20s and off at t = 40s • s3 and s4 are off until t = 60s • In real-time CPS, DMR (deadline miss ratio) from each source node to actuator is a major concern

16. Deadline miss ratios without QoS management

17. Deadline miss ratios with QoS management • Controlled variable is deadline miss ratio • Manipulated variable is sampling period of sensor

18. Conclusion • Many issues and challenges to implement CPS • This paper focused on one of those challenges: network QoS management • Feedback scheduling framework proposed as a sample solution