A comparison of control structures for error correction in packet-switched networks

New applications in areas such as factory automation, power distribution, and transportation rely on a networked infrastructure. These networked embedded control systems pose many research challenges on predictability, reliability, and scalability [1]. One important subproblem is on how to execute feedback control applications over unreliable communication channels. There are two natural approaches to address this problem. One is to modify each control algorithm to cope with the variations and uncertainties posed by the network, e.g., [2]. Another approach is to develop generic interfaces between control applications and transport protocols that are suitable for a large class of control applications and network conditions. The latter approach is less studied in the literature, but considered in this paper. We focus on adaptation mechanisms for packet losses, but the goal of our research is to develop interfaces that are aware of general features of both control applications (e.g., control quality, stability) and networks (e.g., delay, jitter, packet loss, disconnection). The main contribution of the paper is a comparison of a set of end-to-end error correction algorithms that support control applications on top of an unreliable packet-switched network. By adapting the amount of transmitted redundancy, it is possible to cope with quite drastic changes in the packet loss probability. It is shown that the amount and type of data used in the control algorithm influence the performance. In particular, interesting features of feedforward and feedback controls are discussed. The presented schemes are modifications on an adaptive forward error correction (AFEC) that was recently proposed by Park and Wang [3] for end-to-end transport of real-time traffic. Similar AFEC schemes have been applied to Internet telephony [4] and MPEG streaming [5]. Our paper presents some preliminary but quite encouraging simulation results. Ongoing work includes control theoretic analysis to support tuning rules for the proposed scheme, extensive simulations with real data, and control application implementations. Consider the following communication model for an end-toend connection. Let each data block consist of a fixed number of N > 0 data packets. For block m, there is a variable number of 0 ≤ km ≤ N packets that contain data (application information) and um = N − km packets of redundancy.