Performance Evaluation of Deterministic Communication in the Railway Domain
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Performance evaluation of deterministic communication in the railway domain Daniel Onwuchekwa Roman Obermaisser Chair for Embedded Systems Chair for Embedded Systems University of Siegen University of Siegen Holderlinstr.¨ 3, 57076 Holderlinstr.¨ 3, 57076 Siegen, Germany Siegen, Germany Abstract—This paper presents an in-depth physical evalu- function vehicle bus (MVB). The initial version of the TCN ation that studies the inclusion of deterministic and mixed- standard (IEC 61375-1, IEC 61375-2-1ff) did not take care criticality features on the train communication networks. Cur- of applications having large bandwidth, such as surveillance rently, the railway industry uses networks with multiple back- bones. The most prominent are communication setups with cameras and onboard entertainment. The data rate of this the WTB/MVB(Wire train bus/ multifunctional vehicle bus) earlier TCN version tops at 1 Mbit/s between the train vehicle and the ETB/ECN (Ethernet Train Backbone/ Ethernet Consist (Train bus) and 1.5Mbit/s within a vehicle (Vehicle bus). network). Where the WTB/MVB is used to host safety-criticality Therefore, this resulted in the development of additional parts applications, the ETB/ECN is used for non-safety criticality to the IEC 61375-family. The improved standard defined a applications, since the ETB/ECN is not able to provide deter- minism. Nevertheless, the main reason for the addition of the faster TCN (IEC61375-2-5) capable of handling over 100 Ethernet-based communication is the high bandwidth availability, Mbits/s, and conformant to IEEE 802.3. This standard is to satisfy the increasing data traffic generated by applications referred herein as the ETB/ECN and is based on a hierarchical on the train. However, these mixed networks create greater model, comprising the Ethernet Train Backbone (ETB) and the complexity, increased cable length, weight and maintenance cost Ethernet consist network (ECN). as a result of two different topologies. This paper proposes and evaluates TTEthernet as means to provide determinism and However, the railway industry still uses the WTB/MVB mixed criticality platform for the train communication network for safety functions, such as traction control, brakes, and (TCN). A fault injection approach is adopted for the evaluation, doors. While the ETB/ECN bus is used for high bandwidth and the performances of ETB/ECN profile and TTEthernet are functions such as for passenger’s comfort (entertainment) and observed under a babbling idiot failure mode. The ETB/ECN passenger information (e.g. audio message) [3], this presents profile is used as a baseline to reveal specifically the rate- constrained (RC) traffic type behaviour of TTEThernet. a mixed network with two backbones, one for ETB/ECN and Index Terms—Fault Injection. Deterministic communication. the other WTB/MVB. Apart from the increase in complexity Train communication network. TTEthernet. Performance evalu- brought by this solution, there is an extended cable length and ation. Mixed-criticality. Virtual links. an increase in maintenance cost resulting from two different network topologies. Recently, a European railway project I. INTRODUCTION titled ”SAFE4RAIL” [4] began to address these problems. The current electronic systems found in trains consists of SAFE4RAIL proposed the use of a deterministic platform applications running at different safety-criticality levels. Such for the train communication network. The determinism and applications include propulsion control, door controls, diag- bounded jitter feature of certain networks termed ”determin- nostics, seat reservations, and so on. These applications host istic” provides a way to support mixed criticality traffic. It multiple embedded devices situated in a distributed manner is expected that future communication system in the railway in the train vehicles. A real-time communication system is industry will utilise this mixed criticality feature of deter- necessary to ensure efficient communication between these ministic networks to reduce wiring/weight, complexity, and embedded devices, hence the development of the Train Com- maintenance cost. Deterministic platforms such as TSN [5] munication Network (TCN) standard [1], which provides the and TTEthernet [6] were considered during the SAFE4RAIL guideline for exchanging information between devices on rail- project. This is due to the anticipated worldwide acceptance, way vehicles. The establishment of a common standard for the integrability and its cost-effectiveness, which are critical con- electronic coupling of the vehicle’s electronic equipment was siderations for the railway vendors. The proposed integrated the motivating factor behind the TCN development [2]. Before modular platform for the train control and management system the development of this standard, the railway communication (TCMS) in SAFE4RAIL was based on such a deterministic system was mostly proprietary solution, and tailored to meet communication platform. specific requirements of railway operators and vendors. In this work, we evaluate the application performance of TCN functions on a hierarchical model consisting of two TCN-TTEThernet (TCN based on a TTEthernet profile). The network buses, the Wire Train Bus (WTB) and the Multi- results of the evaluation herein provide a means to compare and the medium access control algorithm. However, despite the importance of this evaluation, the target network (WTB/MVB) is foreseen to be replaced in modern trains, primarily because of its low bandwidth availability. In addition, the work used a simulation-based approach as opposed to a physical hardware Fig. 1. Train composition setup for the train communication system, in contrast to the strategy adopted in our work. The work in [8] did not also consider the performance evaluation under faulty scenarios, the performance of TTEthernet to other deterministic and non- which is valuable in ascertaining the robustness of the network. deterministic platforms quantitatively. Our work evaluates the The propulsion control system in trains over Ethernet was application behaviour in the presence of network faults, to evaluated in [9]. The work supports the view that the legacy aid the implementation of adequate fault coverage. Under Ethernet-based standard is not able to guarantee the fixed and a network flooding failure scenario, we observe both TCN- sustained data rate required for safety criticality functions such ETB/ECN (TCN based on ETB/ECN profile) and TCN- as propulsion control and braking controls, this was a key TTEthernet. To the best of our knowledge, this is the first motivation for studying the performance of the propulsion performance evaluation that uses a fault injection approach control over Ethernet. The work in [9] employed co-simulation for TCN-TTEthernet. The fault injection framework used was using a MATLAB/SIMULINK based toolbox; the TrueTime designed in [7], and is capable of injecting, observing, and Toolbox. Although the view that acknowledges the insuffi- analysing various network faults such as corruption, omissions ciency of legacy Ethernet for safety criticality functions aligns and network flooding. The fault injection framework provides with ours, nevertheless we propose the use of a deterministic the platform to evaluate the actual hardware behaviour in Ethernet (TTEthernet). Furthermore, the work in [9] did not the presence of faults as opposed to evaluation using models consider performance evaluation under failure scenarios. provided in mathematical or formal methods. For this reason, Additionally, more works on performance evaluation carried fault injection is used to evaluate the performance of the out for train communication include [10] for train communi- implemented train communication system. The framework can cation system using radio network and [11] for mmWave in thus be used, to test hardware implementation of TCN based 5G Train communications. However, performance evaluation on several deterministic platforms, to ensure conformance to under failure scenarios was not within the scope of these standards, and to verify proper functionality. works. Our work, therefore, provides the following contributions. • Provides the baseline for future comparison of TCN- III. OVERVIEW OF TCN AND THE TTETHERNET TTEThernet to other deteministic networks. A. Overview of TCN • Exposes the impact of failure scenarios on the safety and A comprehensive description of TCN based on WTB and performance of applications over TCN. MVB (TCN-WTB/MVB) is giving in [1] and [2]. TCN • Proposes and illustrates the use of fault injection frame- comprises the train bus that connects vehicles and the vehicle work for performance evaluation of TCN. bus that connects the pieces of equipment within the vehicle. • Exposes the efficacy of TTEthernet as a suitable under- The former is referred to as the WTB and the latter the lying network for TCN, specifically at layer 2 of the OSI MVB. The WTB uses either jumper cables or automatic stack. couplers to interconnect vehicles. The twisted shielded-wire The terminologies used to describe the train system in this is the preferred media for message transmission, because of work are illustrated in Figure 1. The car is the smallest its reliability feature in the railway use-case [2]. The WTB can unit from the physical composition. However, from a network function at a length up to 860 meters and is capable of carrying perspective, multiple cars can constitute a consist. A train fleet data at 1 Mbps. WTB transmits data using the Manchester