Random Early Detection (RED) can stabilize the instantaneous queue size at a router by controlling the average queue size within a given target. In this case, RED can achieve high throughput in the routers. However, the average queue size is quite sensitive to network scenarios and it is difficult to adapt RED parameters to changing network traffic. In this paper we use a previously developed dynamic model of TCP behavior together with a linear feedback model of TCP/RED to analyze and design a mechanism for RED parameter tuning in response to changing network conditions like traffic load, link capacity and round-trip time. Even though the values of four key RED parameters are determined by varying network conditions, they can be tuned independently without consideration of the interactions among these RED parameters. Simulation results show that this mechanism can keep the instantaneous queue size stable and maintain high link utilization in a wide variety of network conditions.

Introducing the Internet into process control systems has brought a lot of benefits and some difficulties to control engineers as well. The major challenge of Internet-based process control systems is how to deal with Internet time delay and data loss. This paper analyses Internet communication features and proposes a control structure with a variable sampling time to overcome the Internet transmission delay. In order to take advantage of the structure fully and attack the major difficulties, two compensation elements have been designed and implemented in the system. These two compensators are located in the feedback and feedforward channels in the architecture respectively. Our simulation and experimental results illustrate that the control structure and two compensation elements can efficiently deal with the Internet time delay and data loss. Our simulation results also show that the time delay and data loss in the feedforward channel seem to cause more serious influence on the control performance and is more difficult to be compensated for.

Industrial control systems (ICSs) in networked environments face severe cyber-security risks and challenges. A timely response to cyber-attacks is of paramount importantce for mitigating risks. However, the security policy developed for an ICS may be conflicting with the ICS's safety policy, on which much attention has been paid for a long time in industrial control. An inappropriate enforcement of the security policy may deteriorate the ICS performance or even result in severe unexpected consequences. To tackle this problem, a risk-based security task scheduling approach is presented for ICSs with consideration of the safety policy. It ensures a timely response to cyber-attacks without compromising safety. More specifically, the approach reconciles security tasks and safety tasks according to a designed resolution policy, so as to acquire contradiction-free security and safety (S and S) tasks. Then, a real-time risk assessment method is developed to characterize the subtle change of the system risk with the implementation of the reconciled S and S tasks. After that, a task scheduling method is designed with the risk as the optimization objective, i.e., it searches the optimal task scheduling scheme by minimizing the risk posture. The resulted scheduling scheme ensures the smooth implementation of the S and S policy, which reflects the optimal recovery process against the risk. Finally, case studies on a hardware-in-the-loop testbed are conducted to demonstrate the effectiveness of the proposed approach.

This article discusses the compensation of the transmission delay in a networked control system (NCS) with a state feedback, which possesses a randomly varying transmission delay and uncertain process parameters. The compensation is implemented by using a buffer in the actuator node and a state estimator in the controller node. A Linear Matrix Inequality-based sufficient condition for the stability of the NCS under the designed compensation is proposed. The simulation results illustrate the efficiency of the compensation method.