VPX Ethernet Switches for Deterministic, Hard Real-Time Applications

Figure 3. Advanced Integrated Architectures with deterministic Gigabit-Ethernet VPX switches.
While the electrical and mechanical aspects of the VPX backplane standard are known at the component level to many engineers, its capabilities and support for the design of advanced integrated architectures with deterministic and hard real-time applications have been rarely discussed in technical publications and press. It is believed, even among market analysts, that the older VME variant is more suitable for real-time applications than VPX. However, the fact is that VPX relies on serial switched fabrics, quality-of-services, (QoS) and its real-time performance. The system based on VPX is as (hard) real-time as the underlying backplane “databus” technology. VPX switches with ARINC664 and SAE AS6802 services enable deterministic integration of many critical functions hosted on common embedded computing and networking resources. The comparison is provided in Table 1.

Deterministic Ethernet and Layer 2 QoS Enhancements

Figure 4. Deterministic Gigabit-Ethernet Switch for VPX by TTTech.
Layer 2 QoS enhancements provide traffic classes that can handle Ethernet traffic with well-defined temporal properties (latency and jitter control). The determinism of communication can be defined as a maximum point-to-point latency. For demanding hard RT behavior and processes with >Nx1000Hz sampling cycles, the determinism can be defined as a fixed latency, jitter controlled with μs-precision, and known message order. Another reason why the jitter should be controlled in complex integrated systems is the embedded virtualization.

In a distributed real-time computer with hosted hard RT functions, along with less critical functions for diagnosis, health management, and bulk data transfers (e.g. recording, A/V), the access to all resources shall be predefined in order to protect the performance for critical functions. The availability of fault-tolerant system time can simplify the virtualization.

ARINC664 is an Ethernet traffic class that provides defined maximum latencies for any periodic unicast/multicast data stream in the system. This is accomplished by per-stream traffic shaping and policing. The technology is used in integrated modular architectures for commercial aircraft and military transporters, such as the Boeing 787, Airbus A380, Airbus A350, Airbus A400M, and many others. All new aircraft use AFDX (ARINC664) networks to reduce SWaP.

SAE AS6802 is an Ethernet traffic class that provides fixed latencies for any periodic unicast/multicast data stream in the system, unaffected by other less critical traffic load. This service also provides a fault-tolerant distributed timebase used by different computing modules in the network, and/or Ethernet devices for scheduled forwarding of data.

With SAE AS6802, Ethernet gains strictly deterministic synchronous communication capability and can emulate circuit-switching communication in packet-switched Ethernet networks. Figure 2 shows the position of this service in the OSI layer model with relation to other Ethernet layers and applications. SAE AS6802 services do not depend on bandwidth or distance — they can operate at 0.1 to 10 Gbit/s or higher and can be used in large networks. Together with other QoS enhancements, Ethernet fully supports synchronous and asynchronous communication.

With SAE AS6802, system functions can be integrated on a common shared infrastructure and scheduled for all critical functions. All other bandwidth can be used for less critical applications.

Both ARINC664 and SAE AS6802 services do not modify operation of existing Ethernet services and are compliant with all standard Ethernet physical layers for backbone and backplane networks, including those described in VPX (VITA 46) and VITA 48. They are also compliant with higher OSI Layers 3-6.

Ethernet — As Deterministic as MIL-1553

With SAE AS6802 services, Ethernet networks can gain deterministic performance comparable to TDMA communication networks (e.g. MIL-1553 in synchronous communication mode, or Time-Triggered Protocol (TTP) – SAE AS6003), but at much higher communication speed, without bus controller and in complex switched architectures. It enables strictly deterministic communication, fixed latency, sub-μs-jitter, and predictable message order in redundant multi-hop networks. Layer 2 Quality of Service (QoS) enhancements, standardized as Time-Triggered Ethernet (SAE AS6802), guarantee deterministic computing and networking performance for advanced integrated systems. MIL-1553 operation can be emulated over an Ethernet network that implements SAE AS6802.

SAE AS6802 “Time-Triggered Ethernet” is used for human-rated space flight (NASA Orion), and evaluated for different aircraft and rotorcraft systems. It is used for the design of systems that utilize unified networking and support integration of hard real-time, real-time, and soft-time functions.

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