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Abstract

The message scheduling is an essential mechanism in the context of Networked Control Systems (NCS). It strongly influences the stability of the closed loop distributed control applications. The goal of this paper is to present a study of different priority schemes which can be used for the message scheduling of a process control application implemented through a network (the network CAN will be considered). Two great schemes will be presented: static priority, hybrid priority including a dynamic part and a static part. The dynamic part is based on the dynamic performance of the controlled application. Considering the hybrid priority scheme, several types will be considered for the dynamic part: dynamic part alone, dynamic part and a static time strategy, dynamically controlled time strategy. The comparison of these schemes is done with respect to the stability of a process control application.

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Abstract

In this paper, we address the problem of route fragility in backhaul wireless mesh networks. Mesh routing protocols rely on intermediate nodes in the multihop path to forward data and detect route breakage. An intermediate node reports the link to next hop as broken after a fixed number of unsuccessful packet transmission attempts at the MAC layer. Transmission failure of a single packet on any link in the multihop path thus renders the route unusable. In practice, many transmission failures are isolated and temporary, caused by transient congestion on the wireless link. False route failures triggered by temporary transmission failures render otherwise good routes as unusable and degrade performance. We propose a novel route maintenance mechanism Congestion Aware Routing (CAR) which addresses this problem and makes a more "informed" decision about the link status. CAR uses estimation of medium congestion and the frequency and distribution of link failures to distinguish between broken (or poor quality) links from those experiencing temporary losses. Simulation results for TCP traffic show that CAR can provide up to 15% higher throughput, 50% delay reduction with 8 times smaller routing overhead.

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Abstract

One of the main Quality of Service mechanisms, allowing mainly the protection of already active flows, is admission control. This is even truer on the MAC level of 802.11 based networks due to scarce resources. We present in this paper a hybrid admission control algorithm for IEEE 802.11e EDCA. The algorithm uses the synthetic Markov chain model of EDCA that we developed in previous work injecting into it network state measurements. Simulations and results presented in this paper show a good protection of admitted flows and a good utilization of the network resources.

Abstract

This paper proposes a very simple reduced abstract model of the behavior of the EDCA (Enhanced Distributed Channel Access) access mode with respect to an access category under saturation. This model is derived from a Markov chain model of EDCA using reduction techniques. Closed-form expressions of common performance metrics (i.e. mean throughput, mean access delay and packet drop probability of an access category) are given and insight into the potential applications of this model are briefly presented.

Abstract

Wireless Mesh Networks offer promising solutions for extending the coverage of wireless local area networks along with increased communication reliability. The wireless mesh backbone serves as an access network for a large number of heterogeneous networks, providing them internet connectivity. Comprising of dedicated routers, the backbone differs significantly from conventional ad hoc networks due to lack of mobility and energy constraints. Classical ad hoc routing protocols developed for typical ad hoc networks, concentrate on mobility and energy conservation and their suitability and possible adoption for the backbone warrants careful evaluation. In this paper, we analyze the performance of four popular ad hoc routing protocols including two reactive (DSR, AODV) and two proactive (OLSR, DSDV) protocols for the mesh backbone using simulation. We provide results for routing overhead, packet delivery fraction and end-to-end delay metrics. Optimizations for these protocols are proposed and design directions for future backbone routing protocols are outlined.

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The Networked Control Systems (NCS) are systems very important in the technological context of today. This paper wants to show that the design of such systems requires to master both a basic knowledge on the distributed systems (message scheduling, message transfer protocol, task scheduling in a computer) and a basic knowledge on the control theory (transfer function, stability of the closed loop system) and to make bidirectional interactions between these knowledges. The example of a distributed system based on a local area network and of a regulation application is considered. We place emphasis on the network CAN and the priority scheme used to schedule the frames.

Abstract

The message scheduling is an essential mechanism in the context of Networked Control Systems (NCS) which are systems based on communication networks and which allow, in particular, the implementation of closed loop distributed control applications. The scheduling of the messages of the flows of these closed loop distributed applications strongly influences the stability of these applications. The message scheduling is, in several networks, mainly based on fixed (static) priorities associated to the messages (for example the network CAN). In previous works, we have shown that the static priority scheme has intrinsic limitations and so, we propose in this paper, in order to overcome these limitations, the concept of hybrid (hierarchical) priority. Different hybrid priority schemes are presented and evaluated with respect to the stability performances of the closed loop distributed process control applications. We show, in particular, the advantage of a scheduling based on a scheme "hybrid priority + time strategy".

Abstract

IEEE 802.11e specifies the Enhanced Distributed Channel Access (EDCA) for distributed access with traffic differentiation to a Wireless Local Area Network (WLAN). Traffic differentiation led to introducing several traffic queues within each station, thus introducing virtual collision. In this paper, we present a pattern based Markov chain model of EDCA including the virtual collision phenomenon not found in previous work. We also correct the misconceptions found in existing Markov chain models of EDCA (Kong et al., 2004). We thus obtain a more realistic model of IEEE 802.11e EDCA which will allow a more precise performance analysis that may be used for admission control.

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