Communications for Integrated Modular Avionics Richard L. Alena [email protected] John P. Ossenfort IV, SAIC Kenneth I. Laws, QSS Andre Goforth NASA Ames Research Center Moffett Field, CA 94035 Fernando Figueroa, NASA Stennis Space Center TABLE OF CONTENTS Abstract—The aerospace industry has been adopting avionics architectures to take advantage of advances in computer engineering. Integrated Modular Avionics (IMA), 1. INTRODUCTION ..................................................... 1 as described in ARINC 653, distributes functional modules 2. INTEGRATED MODULAR AVIONICS...................... 2 into a robust configuration interconnected with a “virtual 3. NETWORKS FOR AEROSPACE ............................... 4 backplane” data communications network. Each avionics 4. MISSION COMPUTER DESIGN............................... 8 module’s function is defined in software compliant with the 5. LAB EVALUATION............................................... 12 APEX Application Program Interface. The Avionics Full- 6. CONCLUSIONS..................................................... 16 Duplex Ethernet (AFDX) network replaces the point-to- REFERENCES........................................................... 17 point connections used in previous distributed systems with BIOGRAPHY ............................................................ 18 “virtual links”. This network creates a command and data path between avionics modules with the software and 1. INTRODUCTION network defining the active virtual links over an integrated physical network. In the event of failures, the software and Recent advances in software architecture and data network can perform complex reconfigurations very communications can benefit modern aerospace avionics quickly, resulting in a very robust system. systems by increasing capability and improving reliability. In particular, network-oriented standards offer high In this paper, suitable architectures, standards and bandwidth and flexible support for modern devices and data conceptual designs for IMA computational modules and the types. Specifically, advances in computational capability, virtual backplane are defined and analyzed for applicability software interfaces and time deterministic networks can to spacecraft. The AFDX network standard is examined in improve system architecture and facilitate software detail and compared with IEEE 802.3 Ethernet. A reference development for modern aerospace avionics systems. The design for the “Ancillary Sensor Network” (ASN) is ARINC 653 “Avionics Application Software Standard outlined based on the IEEE 1451 “Standard for a Smart Interface” operating system specification incorporates Transducer Interface for Sensors and Actuators” using real- partitions for separating critical and non-critical functions. time operating systems, time deterministic AFDX and Its Integrated Modular Avionics (IMA) concept relies on wireless LAN technology. Strategies for flight test and functional isolation between operating system partitions to operational data collection related to Systems Health limit propagating failure modes within avionics software; Management are developed, facilitating vehicle ground and to simplify software validation and verification (V&V). processing. Finally, a laboratory evaluation defines IMA replaces point-to-point cabling with a “virtual performance metrics and test protocols and summarizes the backplane” data communications network. The network results of AFDX network tests, allowing identification of connects software-configurable computing modules that can design issues and determination of ASN subsystem adapt to changes in operating modes or respond to an scalability, from a few to potentially thousands of smart and 12 avionics system fault. There is a potential path between any legacy sensors. of these modules, with the software and network defining the active Virtual Links to support effective partitioning. In the event of failures, the system can quickly reconfigure its software functions (in pre-determined ways), resulting in a very robust system. 1 U.S. Government work not protected by U.S. copyright. 2 IEEEAC Paper #1230, Version 1.3, Updated December 27, 2006 1 next generation of commercial aircraft. Although space- IMA requires robust yet flexible data communications. This qualified parts are not currently available, use of AFDX in paper focuses on the Airbus Avionics Full-Duplex Ethernet certain roles aboard spacecraft can simplify cabling, leading (AFDX) and ARINC 664 Aircraft Data Network Part 7 to a significant reduction in cable weight and volume. Other time-deterministic network Standards in order to determine significant advantages include simplified software design completeness of the Standards, maturity of commercial and network management, ease of reconfiguration and products, acceptance and adoption by the aerospace extension, and compliance with industry network standards. industry, and appropriate role on-board spacecraft. AFDX uses the physical layer widely used in the computer industry The team from Ames Research Center has significant as specified in the IEEE 802.3 Switched Ethernet Standard. experience in spacecraft data systems and communications, This is a “star” topology, with the switch forming the center, providing key analysis for the Space Station Freedom routing data only to specified destinations and thereby Command and Data Handling System. In 1995, the Wireless minimizing network contention. The AFDX switch enforces Network Experiment aboard Shuttle and Mir led to the timing and policies, limiting fault propagation and ensuring adoption of wireless network technology for the time determinism. International Space Station (ISS). Subsequently, the team developed the Databus Analysis Tool, a MIL-STD 1553B Dual-redundant AFDX networks are used for reliability. data-bus monitor space-qualified during STS 85 and used The physical medium is generally shielded twisted pair for ISS Control Momentum Gyro checkout during STS 92. copper cables, although optical fiber can also be used. Bandwidth is 10 Mbits per second (Mbps), 100 Mbps, and even 1000 Mbps. AFDX creates Virtual Links (VLs), which 2. INTEGRATED MODULAR AVIONICS are point-to-multipoint time-division multiplexed connections. Also, time-synchronous delivery of critical The aerospace industry has been moving from “distributed” data streams can be guaranteed, with a timing jitter of at and “federated” avionics architectures to “modular” most 0.5 msec for any VL. AFDX uses the concept of architectures, and is adopting other advances in computer Bandwidth Allocation Gap (BAG) to distribute the data engineering such as networks and Internet Protocol (IP). stream into timed packets, with the BAG value determining These advances promise to increase the performance and the time interval between packets on a given VL. reliability of avionics systems while simplifying the development and certification of flight software and Primary benefits of AFDX are a reduction of cable weight avionics hardware. and volume, an increase in bandwidth and data volume, and greater flexibility in software design and V&V. AFDX The International Space Station (ISS) exemplifies a complements ARINC 653 software architectures, serving as distributed architecture. The ISS avionics system is built the virtual backplane. AFDX provides compatibility with with a large number of relatively small Orbital Replaceable network-oriented data communication formats. AFDX can Units (ORUs)—identical in concept to aircraft Line- support additional sensors for flight test and monitoring Replaceable Units (LRUs)—interconnected with multiple capability upgrades. The Ancillary Sensor Network is MIL-STD 1553B data buses.[1] The units typically have proposed for such use, to support up to 10,000 sensors at very specific functions that do not change over time high data rates and resolutions, supporting smart sensors (although a unit’s operating mode within a redundant compliant with the IEEE 1451 Standard, simplifying configuration may change). An example of federated calibration and maintenance. architecture is the Boeing 767 aircraft, which has dedicated LRUs responsible for specific functions such as engine The Ancillary Sensor Network (ASN) was prototyped using control interconnected by ARINC 429 data buses. up to four end stations. A total of 120 VLs were used to move large amounts of data in a time-deterministic manner Such architectures have several drawbacks. The multiplicity suitable for sensor-data acquisition. Packet-to-packet of specific modules requires a large and complex “bandwidth allocation gap” (BAG) timing was measured for development effort, plus the maintenance of a similar every packet generated by the traffic generator according to multiplicity of spare parts. Each component on a recipes that simulated various target functions. Average synchronous data bus must interoperate with all the other packet-to-packet timing for each VL was determined, along components, so software changes in one may require with standard deviation and minimum and maximum corresponding changes in the others. Experience has shown packet-to-packet timing intervals. These were compared to such systems to be slow as well as difficult to develop and the AFDX jitter specification of 0.5 msec maximum. Lost, to upgrade. On the plus side, the architecture can be fairly missing, or delayed packets were detected and reported. robust. Each module can be designed to function independently despite failures at other points in the system.