CZECH INSTITUTE OF INFORMATICS ROBOTICS AND CYBERNETICS INDUSTRIAL INFORMATICS DEPARTMENT Makespan minimization of Time-Triggered traffic on a TTEthernet network Jan Dvoˇr´ak,Martin Heller and ZdenˇekHanz´alek arXiv:2006.16863v1 [cs.NI] 20 Jun 2020 DOI: https://doi.org/10.1109/WFCS.2017.7991955 Cite as: J. Dvoˇr´ak,M. Heller, and Z. Hanz´alek.Makespan minimization of time-triggered traffic on a ttethernet network. In 2017 IEEE 13th International Workshop on Factory Communication Systems (WFCS), pages 1–10, 2017 Source code: https://github.com/CTU-IIG/TTEthernetScheduler c 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. Makespan minimization of Time-Triggered traffic on a TTEthernet network Jan Dvoˇr´ak Department of Control Engineering, FEE Czech Technical University in Prague Prague, Czech Republic Email: [email protected] Martin Heller ZdenˇekHanz´alek Department of Control Engineering, FEE DCE FEE and CIIRC Czech Technical University Czech Technical University in Prague Prague, Czech Republic Prague, Czech Republic Email: [email protected] Email: [email protected] Abstract sized instances. The universality of the provided method allows easily modify or extend the prob- The reliability of the increasing number of modern lem statement according to particular industrial de- applications and systems strongly depends on in- mands. Finally, the studied concept of makespan terconnecting technology. Complex systems which minimization is justified through the concept of usually need to exchange, among other things, mul- scheduling with porosity according to the worst- timedia data together with safety-related informa- case delay analysis of Event-Triggered traffic. tion, as in the automotive or avionic industry, for example, make demands on both the high band- width and the deterministic behavior of the com- 1 Introduction munication. TTEthernet is a protocol that has been developed to face these requirements while This paper focuses on the problem of creating providing the generous bandwidth of Ethernet up schedules for Time-Triggered traffic on the TTEth- to 1 Gbit/s and enhancing its determinism by the ernet network. The objective is to minimize Time-Triggered message transmission which follows the makespan of the Time-Triggered traffic and, the predetermined schedule. Therefore, synthesiz- thus, maximize the continuous gap for the Event- ing a good schedule which meets all the real-time Triggered traffic. The aim of the paper is to de- requirements is essential for the performance of the velop the scheduling algorithm and, consequently, whole system. to compare the concept of the makespan minimiza- In this paper, we study the concept of creat- tion with the concept of the porosity optimiza- ing the communication schedules for the Time- tion proposed by Steiner in [1]. The TTEthernet Triggered traffic while minimizing its makespan. network is a network protocol which offers Time- The aim is to maximize the uninterrupted gap for Triggered communication and preserves the back- remaining traffic classes in each integration cycle. ward compatibility with Ethernet. The provided scheduling algorithm, based on the Ethernet (IEEE 802.3) has been the prevalent Resource-Constrained Project Scheduling Problem technology for home and office networks over the formulation and the load balancing heuristic, ob- last few decades. Thanks to its widespread adop- tains near-optimal (within 15% of non-tight lower tion, it has developed into a mature technology of- bound) solutions in 5 minutes even for industrial fering high bandwidth with cheap and readily avail- 2 ECU been a trend of using Ethernet-based networks for s industrial applications as well. This trend has be- kn,s come even more pronounced with the ever increas- ing amounts of data transfers necessary to facilitate k n n,r ECU features like real-time image processing and recog- ECU r nition or communication among individual units in km,n k t o,n a smart system. Therefore, extensions of Ethernet kt,m 1 are being developed to meet the demands of indus- 2 3 4 m km,o o trial applications. An overview of the development ko,p 1 2 k 3 of industrial networks is given by [4]. ECU l,m 4 1 1 2 TTEthernet is a promising extension of Eth- l 2 3 p 3 4 4 kp,u ernet, which provides determinism and fault- kp,q tolerance while being compatible with standard ECU ECU u q Ethernet. Besides the TTEthernet standard, there has been ongoing effort on the standardization Figure 1: Example of the TTEthernet network in- of extensions to Ethernet for scheduled traffic by frastructure with routing and scheduling of message the IEEE 802.1 Time-Sensitive Networking Task m1 from node l to node q Group. Determinism with the strictest guarantees is achieved through a fixed schedule for the traffic. able hardware. Therefore, synthesizing a good (exactly what this Conventional computer networks are mainly used means will be discussed later) schedule which meets for on-demand data transfer without an immediate all the requirements and deadlines is essential for physical impact on the real world. In case of failure, the performance of the network. Because TTEth- the transmission can usually be repeated without ernet allows for complex topologies, the scheduling causing major difficulties. Therefore, the focus is involves additional complexity compared to bus or on the bandwidth and efficiency with only moder- passive star topologies of networks like FlexRay [5] ate demands on reliability. or CAN. In contrast, in industrial applications various control loops of physical devices are realized by the 1.1 TTEthernet Overview network, e.g. data from sensors are transferred to a processing unit which then sends commands to TTEthernet (TT stands for Time-Triggered) is an actuators. Any disturbance can, thus, have imme- extension of Ethernet for deterministic communica- diate effects on the real world with possible severe tion developed as joint project among the Vienna consequences. As jitter (variance in transmission University of Technology [6], TTTech and Honey- times) is detrimental to the function of the control well, and standardized as SAE AS 6802 [7] in 2011. loops, determinism is often required. In addition, TTEthernet operates at Level 2 of the ISO/OSI individual devices in the network are often limited model, above the physical layer of Ethernet. It re- in hardware and need to operate in demanding en- quires a switched network with fully duplex phys- vironments. ical links, such as Fast Ethernet physical link Due to these differences, different technologies 100BASE-TX or Automotive Ethernet standard and protocols were traditionally used for conven- 100BASE-T1, so that unpredictable conflicts, while tional computer networks and industrial networks. accessing a shared medium, are avoided. An exam- Ethernet has been used for home and office net- ple of the TTEthernet infrastructure is depicted in works while various Fieldbus networks (such as Fig. 1. CAN [2] or Profinet [3]) have been used for indus- TTEthernet specifies a protocol for clock syn- trial applications. However, with the increasing in- chronization and the rules for managing the traffic tegration of industrial systems, increasing demands on the network. After an initial startup phase when on the volume of data transferred and also the ma- the clocks of the devices in the network are synchro- turing and development of the Ethernet, there has nized for the first time, the operation of TTEther- 3 makespan does not participate in the optimization process. ic1 TT1 RC2 The schedule also provides temporal isolation 5 10 and enables fault tolerance. This is due to the critical gap ic2 TT2 TT3 TT4 fact that not only the sending of a frame is sched- 15 20 uled, but its reception is scheduled as well. If a TT ic 3 TT1 RC2 RC3 acceptance window 25 30 frame arrives outside the (i.e. Integration cycle Integration the time the frame is supposed to arrive consider- ic4 RC1 TT3 TT4 35 time [ms] 40 ing the synchronization inaccuracy), it is discarded by the receiver. This mechanism is called the tem- Figure 2: Example of communication on a link in poral firewall. one cluster cycle For traffic with less strict precision requirements, the RC traffic class can be used. This traffic class conforms to the ARINC 664p7 specification net is periodic. The clocks are being periodically [9] (also called AFDX). It offers greater flexibility synchronized to counter any possible clock drift because only the frame routing needs to be deter- when in steady operation. This period is called mined offline. The messages themselves are event- the integration cycle. The messages which follow a driven within some limitations. deterministic schedule are also periodic. The least The RC traffic represents the event-triggered common multiple of their period is called the clus- communication. Thus, it does not follow any sched- ter cycle. ule known in advance. It is organized in so-called TTEthernet integrates traffic of different time- virtual links. As stated in [10], a virtual link is an criticality levels into one physical network. There analogy to the ARINC 429 [11] single-source multi- are three traffic classes in TTEthernet. These drop bus. The virtual link determines the routing classes, ordered by decreasing priority, are Time- of the messages associated with it. Furthermore, Triggered (TT), Rate-Constrained (RC) and Best- there are two parameters: the maximum allowed Effort (BE) traffic. frame size and the bandwidth allocation gap, asso- The TT traffic class has the highest priority, and ciated with each virtual link. The bandwidth allo- sub-µs jitter can be achieved (depending on the net- cation gap represents the minimum allowed length work devices).