Asynchronous Hard Real Time Signals Transmission in Embedded Networks

Asynchronous Hard Real Time Signals Transmission in Embedded Networks

Liudmila Koblyakova (St. Petersburg State University of Aerospace Instrumentation, St. Petersburg, Russia), Yuriy Sheynin (St. Petersburg State University of Aerospace Instrumentation, St. Petersburg, Russia) and Elena Suvorova (St. Petersburg State University of Aerospace Instrumentation, St. Petersburg, Russia)
DOI: 10.4018/IJERTCS.2014100102
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Nowadays in the aerospace industry the router-based onboard embedded networks gradually replacing bus-based networks because they are already not satisfy the aerospace performance requirements. The SpaceWire, GigaSpaceWire and SpaceFibre standards are developing to meet the increasing aerospace requirements. The important requirement for any aerospace embedded onboard network is a transmission of control information and system signals in hard real time. These signals can be synchronous and asynchronous, periodic and aperiodic, with or without acknowledges. The distributed interrupt mechanism is used for asynchronous signal transmission and it is included into the second edition of SpaceWire standard. The Time-code propagation mechanism is used for synchronous signal transmission in SpaceWire. The broadcast messages mechanism is used for transmission of different system signal in SpaceFibre but it does not quite meet the requirements of hard real time. In this paper the authors consider the asynchronous signals transmission with and without acknowledges. The aims of this paper are following: 1) theoretically investigate the distributed interrupt mechanism; 2) to prove its properties; 3) to specify parameters and limitations; 4) to derive the time characteristics. For these purpose the authors developed the analytical model which describe the distributed interrupt propagation mechanism in terms of the graph theory.
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1. Introduction

1.1. Architectures of Onboard Systems

The amount of information processed on a board of the spacecraft are increasing, algorithms of onboard systems becomes more complicated. The operating conditions of the spacecraft are very difficult: overload when starting, temperature changing, radiation and inability to repair a satellite on orbit require reliability and survivability of onboard equipment [Parkes, 2011]. The critically important requirement to onboard control system of spacecraft is ability to operate in hard real time mode, [Parkes, 2005], [Raszhivin, 2014]. Last decades in the design of spacecraft onboard system the architecture dominates, where the data and control traffic are functionally separated. The data exchange is performing by data exchange bus which specialized for every concrete case. For example, the CAN network was developed for car onboard network in 1987, and in 1994 CAN became the international industrial standard, [Bosch, 1991], [Road Vehicles-Controller Area Network, 2003], [Fuehrer, 2001]. CAN network allows low traffic load and can be used in application which are not sensitive to data transmission delays [Zhang, 2010]. PCI Express is a computer bus which uses the program model of PCI bus and high performance physical protocol based on serial data transmission. It is targeted for using like a local bus and sends all control information, including interrupts, throw the same links, which used for data transmission [PCI SIG, 2006]. In avionics the ARINC 664 part 7 (AFDX) standard is used, which main aim is to design a deterministic data transmission network which can be used by necessary for fly control systems [ARINC Specification 664P7], [Rao, 2012]. The task of control information transmission is usually assigned to standardized communication channel which done by MIL-STD-1553В standard [MIL0STD-1553 Protocol tutorial, 2010]. All of these standards don’t support hard real-time signaling with microsecond’s delivery latency. For hard real-time signaling the sideband signals on a separate proprietary wiring are used in embedded onboard networks.

Such principle of information and control system design has several disadvantages [Yuasa, 2012], for example: the need for synchronization of data and control traffic; small number of nodes in the control network and low speed of transmission between them; fundamental impossibility of rapid rerouting of data traffic. Also it is a bus based systems, which already do not correspond to performance requirements. So the new solutions in the field of spacecraft system architecture are required. The new solution proposes to combine the control and data transmission function within the same communication network. The best suited standards for it are SpaceWire [Standard ECSS-E-50-12C, 2008], GigaSpaceWire [Yablokov, 2013], [Matveeva, 2014] and SpaceFibre [Parkes, 2013], [Parkes, 2013], [Parkes1, 2014], [Parkes2, 2014], which were specifically tailored to the aerospace industry.

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