Knut Øvsthus

An Industrial Perspective on Wireless Sensor Networks — A Survey of Requirements, Protocols, and Challenges

Wireless Sensor Networks (WSNs) are applicable in numerous domains, including industrial automation where WSNs may be used for monitoring and control of industrial plants and equipment. However, the requirements in the industrial systems differ from the general WSN requirements. In recent years, standards have been defined by several industrial alliances. These standards are specified as frameworks with modifiable parts that can be defined based on the particular application of WSN. However, limited work has been done on defining industry-specific protocols that could be used as a part of these standards. In this survey, we discuss representative protocols that meet some of the requirements of the industrial applications. Since the industrial applications domain in itself is a vast area, we divide them into classes with similar requirements. We discuss these industrial classes, set of common requirements and various state-of-the-art WSN standards proposed to satisfy these requirements. We then present a broader view towards the WSN solution by discussing important functions like medium access control, routing, and transport in detail to give some insight into specific requirements and the classification of protocols based on certain factors. We list and discuss representative protocols for each of these functions that address requirements defined in the industrial classes. Security function is discussed in brief, mainly in relation to industrial standards. Finally, we identify unsolved challenges that are encountered during design of protocols and standards. In addition some new challenges are introduced and discussed.

Medium access for a military narrowband wireless ad-hoc network; requirements and initial approaches

This paper presents the initial requirements for a tactical military MANET and their implications on the design of a medium access control (MAC) protocol. The main focus is on TDMA based mobile VHF and UHF networks transferring both data and voice traffic within a bandwidth of 25 kHz. Two MAC approaches are discussed; distributed time division multiple access (DTDMA) and soft reservation schemes such as collision avoidance time allocation (CATA). Both approaches were identified as potential solutions fulfilling most of the requirements. It was found that DTDMA enables less signalling for longer traffic streams (for example voice) compared to the soft reservation approaches. Given a somewhat longer connection setup time, the number of supported voice conversations is expected to be higher than for the soft reservation approach. Shorter data bursts should be handled using a more contention like approach, avoiding lengthy connection setup times.

Field trials of two 802.11 residual bandwidth estimation methods

Ad hoc networks have attracted much attention due to their decentralized architecture and potential mobility. The latter promise has sparked research aimed towards routing and quality of service (QoS) admission schemes. The volatile links that are inherent to these networks have however spawned a need for a cross-layering service below IP for local route and QoS decision making, or information dissemination to participating hosts. We have conducted field trials of an active and a passive estimation method implemented in a real wireless driver to estimate the residual bandwidth. Through these tests we show how residual bandwidth estimates can be obtained in an 802.11 wireless network and identify the strengths and weaknesses of their underlying methods. The implemented wireless driver collects per link bandwidth data, local retransmission count and expected transmission time

Rerouting Time and Queueing in Proactive Ad Hoc Networks

In a MANET network where nodes move frequently, the probability of connectivity loss between nodes might be high, and communication sessions may easily loose connectivity during transmission. The routing protocol is designed to find alternative paths in these situations. This rerouting takes time, and the latency is referred to as the rerouting time. This paper investigates the rerouting time of proactive routing protocols and shows that the rerouting time is considerably affected by queueing. Simulations and analysis are conducted to explore the problem. Finally, we propose a MAC-layer solution that reduces the rerouting time problems due to queueing. Simulations and analysis show that the solution is so effective that it eliminates the entire problem in many situations.

Towards a Dual-Mode Adaptive MAC Protocol (DMA-MAC) for Feedback-based Networked Control Systems

Abstract Automated control systems play an important part in many industrial domains and the medium used for communication between devices in these systems is in transition from wired to wireless for cost reasons. Control systems have strict requirements on delay, throughput, and reliability, that vary with time during operation. Addressing these requirements requires predictable and robust protocols to be employed, and they must be adaptive to the varying states of the controlled process. In this article, we propose a dual-mode adaptive medium access control protocol that caters for two main operation modes in control systems: the steady mode operation, and the transient mode operation. We present initial performance analysis results focusing on energy consumption, including a comparison with a related single-mode industrial monitoring and control protocol.

Alternative Packet Forwarding for Otherwise Discarded Packets

Packets in multihop wireless networks are often discarded due to missing routes, packet collision, unreachable next-hop or bit error. As a remedy to these problems, this paper presents alternative packet- forwarding mechanisms that will reduce the likelihood of discarding packets. In addition to this, a new packet-buffering solution is proposed. By combining these two methods, reduced packet loss was achieved in addition to preserved network resources. This was confirmed through simulations, where a comparison between our proposed solutions and standard forwarding showed up to 50% reduction in packet loss.

Implementation and Deployment Evaluation of the DMAMAC Protocol for Wireless Sensor Actuator Networks

Abstract The increased application of wireless technologies including Wireless Sensor Actuator Networks (WSAN) in industry has given rise to a plethora of protocol designs. These designs target metrics ranging from energy efficiency to real-time constraints. Protocol design typically starts with a requirements specification, and continues with analytic and model-based simulation analysis. State-of- the-art network simulators provide extensive physical environment emulation, but still have limitations due to model abstractions. Deployment testing on actual hardware is therefore vital in order to validate implementability and usability in the real environment. The contribution of this article is a deployment testing of the Dual-Mode Adaptive MAC (DMAMAC) protocol. DMAMAC is an energy efficient protocol recently proposed for real-time process control applications and is based on Time Division Multiple Access (TDMA) in conjunction with dual-mode operation. A main challenge in implementing DMAMAC is the use of a dynamic superframe structure. We have successfully implemented the protocol on the Zolertia Z1 platform using TinyOS (2x). Our scenario- based evaluation shows minimal packet loss and smooth mode-switch operation, thus indicating a reliable implementation of the DMAMAC protocol.

Simulation-based evaluation of DMAMAC: a dual-mode adaptive mac protocol for process control

Control systems automation is widely used in many industrial domains and have strong requirements on delay, throughput, robustness, and reliability. In the domain of networked control systems, the medium of communication is increasingly involving wireless communication along-side conventional wired communication. Issues ranging from energy efficiency and reliability to low-bandwidth have to be addressed to enable the transition to increased use of wireless communication. In earlier work, we have proposed the Dual-Mode Adaptive MAC (DMAMAC) protocol relying on a combination of Time Division Multiple Access (TDMA) and Carrier Sense Multiple Access (CSMA). The DMAMAC protocol is able to dynamically adapt to the two main states found in process control: the steady state and the transient state. Key requirements to the DMAMAC protocol are energy efficiency, low probability of state-switch failures, and a low state-switch delay. The contribution of this paper is a comprehensive simulation-based evaluation of the original DMAMAC protocol along with the evaluation of a new pure TDMA-based variant of the DMAMAC protocol. Our results show that for processes where the steady state dominates, both variants of the DMAMAC protocol can reduce energy consumption by up to 45% in comparison to the closely related single-mode GinMAC protocol. Among the two variants of DMAMAC, the pure TDMA-based variant has the better energy efficiency and higher reliability. The simulation results also show that the hybrid TDMA-CSMA variant of the DMAMAC protocol has a probability of less than 0.3% for a state-switch failure in a given MAC superframe. The simulation study has impacted the design of the DMAMAC protocol by providing insights that have led to design changes in the originally proposed DMAMAC protocol in order to further reduce the state-switch delay between the steady and the transient state.

TCP challenges in hybrid military satellite networks; measurements and comparison

Military tactical networks require reach back connectivity, and in most cases the only viable solution is a combination of satellite and radio links. This hybrid network is characterized by long delay, transmission errors and relative low capacity. Geostationary satellites add a substantial time delay. Terrestrial wireless packet networks might introduce comparable delays and jitter. Mobility and radio interference inherently introduce packet losses. In such environments, the widely deployed Internet Transport Layer protocols (TCP) are known to suffer severe performance degradation. This paper assesses the performance of widely deployed TCP variants. The default TCP algorithm in computers running Windows 7 is TCP NewReno, while Compound TCP is available. For Linux, a wide variety of variants are available, including the satellite-tailored variant TCP Hybla - the default flavor is CUBIC. This study shows that these TCP implementations exhibit significant different performances over wireless links. The evaluation and analysis is based on real TCP/IP traffic over an emulated link with characteristics of a combined terrestrial radio and satellite link. It was found that Windows 7 TCP variants performed poorly in lossy networks compared to CUBIC and Hybla, where the latter two outperformed the other two. Differences in performance of up to 40 per cent in favor of Hybla and CUBIC were measured. Furthermore, an unfair division of capacity, at the cost of Windows TCP implementations, was observed when CUBIC and Hybla competed for bandwidth against a Windows 7 transmitter. Depending on several factors, including the buffer size implemented in the network, this unfairness was measured to starve Windows-originated traffic with as low as 5 per cent of the total available bandwidth. Possible solutions improving fairness include tuning of the TCP parameters and avoid utilization of different TCP implementations simultaneously.

Development of software application dedicated to impulse-radar-based system for monitoring of human movements

The importance of research on new technologies that could be employed in care services for elderly and disabled persons is highlighted. Advantages of radar sensors, when applied for non-invasive monitoring of such persons in their home environment, are indicated. A need for comprehensible visualisation of the intermediate results of measurement data processing is justified. Capability of an impulse-radar-based system to provide information, being of crucial importance for medical or healthcare personnel, are investigated. An exemplary software interface, tailored for non-technical users, is proposed, and preliminary results of impulse-radar-based monitoring of human movements are demonstrated.