Kristina Kunert

Towards Reliable Wireless Industrial Communication With Real-Time Guarantees

Increased mobility coupled with a possible reduction of cabling costs and deployment time makes wireless communication an attractive alternative for the automation industry and related application areas. Methods compensating for the high probability of bit errors accompanying wireless transmissions are, however, needed. This is predominantly important in industrial applications with strict reliability and timing requirements, which cannot be met by standard communication protocols as, e.g., TCP. In this paper, a way of combining retransmissions with real-time worst-case scheduling analysis is presented that can offer both a high grade of reliability and hard real-time support. The presented solution handles one or several retransmission attempts of erroneous data without jeopardizing already guaranteed delay bounds of other packets. A real-time analysis for a full-duplex, asymmetric link, utilizing the novel retransmission scheme and supporting both piggybacked and nonpiggybacked acknowledgments, is provided. A simulation study is presented that evaluates the performance of the retransmission scheme for bit-error rates typically experienced in wireless communication. The results clearly indicate a possible reduction of the message error rate by several orders of magnitude.

Reliable hard real-time communication in industrial and embedded systems

This paper presents a framework for how to use ARQ (automatic repeat request) in combination with real-time worst-case scheduling analysis to be able to support reliable hard real-time communication. We show how to handle retransmissions of erroneous data packets, while still not jeopardizing stated delay guarantees of other packets. We demonstrate this by taking a point-to-point link as an example. Through our simulation studies we have shown that a reduction of the message error rate by several orders of magnitude is possible with a reasonable utilization penalty.

Predictable real-time communications with improved reliability for IEEE 802.15.4 based industrial networks

Emerging industrial applications requiring reliable wireless real-time communications are numerous. Using existing standards such as IEEE 802.15.4 is essential for reasons of interoperability and cost efficiency. However, since 802.15.4 is unable to provide predictable channel access, real-time guarantees cannot be given. Further, the noisy wireless channel makes reliable communications particularly challenging. By adding a deterministic medium access method and a transport protocol with a truncated retransmission scheme to 802.15.4, we jointly enforce reliability and predictability. We evaluate our solution analytically by real-time schedulability analysis including retransmissions, and by computer simulations. We show that the message error rate can be improved by several orders of magnitude while keeping the utilization penalty at reasonable levels.

Exploiting time and frequency diversity in IEEE 802.15.4 industrial networks for enhanced reliability and throughput

Industrial networks based on IEEE 802.15.4 are spreading, even though the joint requirement on predictability and reliability from industrial applications is hard to fulfil in wireless networks, and the data rate of IEEE 802.15.4 is rather low. With the goal of providing real-time guarantees, with increased reliability and throughput, we propose two multichannel network architectures based on IEEE 802.15.4 with predictable medium access, real-time analysis admission control and transport layer retransmissions. We evaluate the architectures in terms of reliability, utilization, delay, complexity, scalability and energy efficiency. The evaluations show that throughput and reliability can be enhanced through redundancy and concurrency in the frequency domain.

Meeting reliability and real-time demands in wireless industrial communication

Employing wireless communication in industrial applications requires methods that deal with the high fraction of packet errors common to wireless transmissions. At the same time, industrial applications have real-time demands that protocols like TCP are unable to support. This paper combines ARQ (automatic repeat request) with real-time worst-case scheduling analysis to achieve both high reliability and real-time support. One or several retransmission attempts of erroneous data packets are handled, while still not jeopardizing stated delay guarantees of other packets. We present the real-time analysis for a full-duplex link using our retransmission scheme. In simulation studies, we demonstrate a possible reduction of the message error rate by several orders of magnitude for bit error rates typically experienced in wireless communication.

Deterministic real-time medium access for cognitive industrial radio networks

Industrial communication often has to work in an environment where other networks or radiation create different levels of interference for the data traffic. Additionally, industrial applications often demand predictable real-time performance of the network. One way of trying to utilise the available frequencies in an effective manner is to include cognitive functionality in the network. We present a medium access control protocol for a cognitive radio network, providing deterministic medium access for heterogeneous traffic and dynamic spectrum allocation. Spectrum sensing abilities in the nodes open up for the possibility of increasing successful data transmissions, and a real-time analysis framework provides upper-bounded medium access delay in order to guarantee timely treatment of hard real-time traffic.

Context-Aware Retransmission Scheme for Increased Reliability in Platooning Applications

Recent advances in cooperative driving hold the potential to significantly improve safety, comfort and efficiency on our roads. An application of particular interest is platooning of vehicles, where reduced inter-vehicle gaps lead to considerable reductions in fuel consumption. This, however, puts high requirements on timeliness and reliability of the underlying exchange of control data. Considering the difficult radio environment and potentially long distances between communicating platoon members, as well as the random channel access method used by the IEEE 802.11p standard for short-range inter-vehicle communication, those requirements are very difficult to meet. The relatively static topology of a platoon, however, enables us to preschedule communication within the platoon over a dedicated service channel. Furthermore, we are able to set aside parts of the available bandwidth for retransmission of packets in order to fulfil the reliability requirements stated by the platoon control application. In this paper, we describe the platooning framework along with the scheduling algorithm used to assign retransmission slots to control packets that are most likely to need them. This retransmission scheduling scheme offers a valuable tool for system designers when answering questions about the number of safely supported vehicles in a platoon, achievable reductions in inter-vehicle gaps and periodicity of control packets.

Data age based retransmission scheme for reliable control data exchange in platooning applications

Recent advances in cooperative driving hold the potential to significantly improve safety, comfort and efficiency on our roads. Platooning of heavy vehicles, where automated or semi-automated driving allows minimal inter-vehicle gaps, has shown considerable reductions in fuel consumption. Although using the same wireless communication technology, a platoon differs from a VANET (Vehicular Ad-hoc NETwork) in several points. It is centralized in its nature, with a well-defined group leader, its topology is fairly stable and it has very challenging requirements on timeliness and reliability of its control data exchange. Therefore, the IEEE 802.11p protocol suite and its recently defined message types do neither support the needs of a platooning application nor take advantage of its properties. A platoons control loop must continuously be fed with fresh data, so the information age is an important parameter to be closely monitored. In this paper, we therefore propose a framework for centralized channel access and retransmission capabilities for safety critical inter-platoon control data based on the data age of earlier received messages. A simulation evaluation compares our solution to a) the decentralized, standard-compliant IEEE 802.11p MAC (Medium Access Control) method, and a time-slotted scheme b) with and c) without retransmissions and shows that the centralized, data age based retransmission scheme clearly outperforms its competitors in terms of maintained data age.

Increased Communication Reliability for Delay-Sensitive Platooning Applications on Top of IEEE 802.11p

Cooperative driving in platooning applications has received much attention lately due to its potential to lower fuel consumption and improve safety and efficiency on our roads. However, the recently adopted standard for vehicular communication, IEEE 802.11p, fails to support the level of reliability and real-time properties required by highly safety-critical applications. In this paper, we propose a communication and real-time analysis framework over a dedicated frequency channel for platoon applications and show that our retransmission scheme is able to decrease the message error rate of control data exchange within a platoon of moderate size by several orders of magnitude while still guaranteeing that all delay bounds are met. Even for long platoons with up to seventeen members the message error rate is significantly reduced by retransmitting erroneous packets without jeopardizing the timely delivery of regular data traffic.

Increasing the probability of timely and correct message delivery in road side unit based vehicular communication

Intelligent transport systems provide a multitude of possibilities when it comes to increasing traffic safety on our roads. Many of the proactive traffic safety applications under development today demand timely and reliable treatment of deadline dependent data traffic. Unfortunately it is not possible to provide any timing guarantees when using the current IEEE 802.11p standard for wireless access in vehicular environments. Additionally, a difficult wireless channel environment makes successful data transmissions very challenging. We suggest adding a real-time layer, comprising a deterministic medium access control protocol and transport layer retransmissions, on top of IEEE 802.11p in order to enable guaranteed real-time behaviour and to improve reliability. In a simulation study we show that the packet error rate can be decreased by several orders of magnitude while being able to guarantee timely treatment of both ordinary transmissions and retransmissions by the help of a real-time schedulability analysis.