SAE International provides some of key system architecture, design, and networking standards, as well as reports and recommended practices for commercial and military avionics, such as SAE ARP 4754, ARP4761, IEEE1394b (SAE AS5643), MIL1553 (AS15531, SAE AS5652), Generic Open Architecture (AIR5314), and others. All of those standards help reduce development and integration time, decrease life-cycle costs, provide a framework for technology insertion, and accommodate expansion of existing critical embedded systems with new capabilities, according to Dr. Mirko Jakovljevic, who works in the area of time-triggered technology for TTTech Computertechnik AG and serves as Chair of an SAE subcommittee.
Time-triggered protocol (TTP) is one of the technologies for design of open time-triggered architectures (TTAs) and already is used in distributed and modular aircraft systems such as the Boeing 787, Airbus A380, and Lockheed Martin F-16. Jakovljevic said TTAs help fight system complexity and help reduce life-cycle costs in safety-critical, fault-tolerant, and distributed onboard systems for A&D (aerospace and defense) applications. By providing a means for unambiguous system interfacing and true modularity, TTAs improve commonality, enable design of open systems, and support the use of COTS components.
SAE is currently in the process of standardizing TTP communication protocol and TTP physical layer. The main objective of the standard, AS6003, is to protect past system investments and simplify design of open, distributed, and truly modular A&D systems and architectures, said Jakovljevic. The standard is being developed by the AS-2D Time-Triggered Systems and Architectures subcommittee of the SAE AS-2 Embedded Computing Systems Committee.
TTTech has developed a new network technology called TTEthernet, combining commercially available Ethernet infrastructure with TTTech's time-triggered services. TTEthernet provides a set of time-triggered services implemented on top of standard IEEE802.3 Ethernet. Those services are designed to enable design of synchronous, highly dependable embedded computing and networking, capable of tolerating multiple faults. With TTEthernet, robustly partitioned multimedia data streams, critical control data, and standard LAN messages can operate in one network without unintended congestions or interactions, according to Jakovljevic. "This enables handling of mixed level criticality functions in complex Ethernet-based networks, effectively circumventing limitations of Ethernet technology for design of advanced integrated systems," he said.
TTEthernet has been selected for use in NASA's space program and will be standardized in SAE A
S-2. NASA signed a Space Act Agreement with TTTech in March 2009 to support the standardization activities. Other key OEMs, integrators, and suppliers from aerospace and defense industry will be involved in the standardization activity.
"Today, telecom, automotive, and industry automation companies adopt synchronous system-integration technologies due to high determinism, hard real-time capability, and efficient resource use," said Jakovljevic. "Synchronous network technologies allow the highest, hard real-time quality of service for critical embedded systems. Aerospace and defense industries will most probably follow those trends as high-volume industries drive the development of electronics today."
TTEthernet's scalability enables deployment in high-volume cross-industry applications, which can benefit from the most advanced validation and verification procedures used in aerospace applications, Jakovljevic noted.
"In terms of system integration," he said, "key NASA tenets are to address architecture [ground and flight systems] as one complex interdependant system. This requires more than a standard avionics databus or a high-bandwidth Ethernet network, as systems are more integrated and more interdependent then ever before. Critical control data, audio, video, and maintenance data could be handled in one system, relying on system-integration technology for synchronization and robust partitioning."
TTEthernet is developed as an additional time-triggered service to standard Ethernet to make it suitable for highly-critical embedded applications. Before TTEthernet, Ethernet was considered incapable for handling mixed criticality distributed functions over one network, so it enjoyed limited application in most critical systems.
Time-triggered services facilitate design of time-driven architectures, Jakovljevic said. "Historically, major aerospace control and avionics systems have been designed to be synchronous and strictly deterministic—dependant only on the progression of time and proactively reacting to external events—not dependant on the number and frequency of external events. TTEthernet enables very predictable operation and reaction times, but also permits handling of synchronous audio/video streams with jitter below a few microseconds."