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Deterministic Version of Ethernet Offers Real-Time Performance at Low Risk

Leveraging the low-risk established infrastructure of Ethernet has many benefits. But by adding deterministic capability to the technology, military system designs reap the best of both worlds.

DR. MIRKO JAKOVLJEVIC, MARKETING MANAGER, AEROSPACE TTTECH

Keywords in this Article:

  • Ethernet
  • Avionics
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Ethernet enjoys tremendous success in a variety of different commercial and industry applications. It has limitations as a deterministic communication interface when applied to critical embedded systems. With SAE AS6802 (TTEthernet) Layer 2 Quality of Service (QoS) enhancements, Ethernet can be used in strictly deterministic embedded applications for time-, safety- and mission-critical systems. In addition, it enables unified Ethernet communication in shared networks without traffic congestion for critical data. In other words, that means a blend of hard real-time, rate-constrained and best-effort communication.

The use of Ethernet in multi-decade programs helps to minimize risks of technology and component obsolescence in aerospace and defense applications. There are no other system integration technologies on the market with such continuing industry momentum and guaranteed growth.  The capability of system integration technologies to ensure predictable (deterministic) interaction between functions under different workloads is essential for design of sustainable integrated and modular systems. Determinism at network level influences electronics platform complexity, software design methodology, application design, system integration, reuse, upgrades, obsolescence management and certification. Therefore, Ethernet with deterministic communication enables design of less complex software-centric systems at lower lifecycle cost. 

Deterministic QoS Enhancements

Ideally, the determinism of communication is defined as full control of jitter, constant message latency and repeatable message order. Obviously, Ethernet as a packet switched network was not designed with those requirements back in the early 1980s. Fortunately Ethernet is not a monolithic standard. Rather it is a family of frame-based networking LAN/MAN technologies and can be extended by additional Quality of Service (QoS) enhancements to satisfy different industry-specific requirements. By using those, otherwise transparent, network services, distributed applications can advance their real-time capability and deterministic operation.

VLAN and IEEE802.1Q for example are typical widely used QoS enhancements, which, in conjunction with limited bandwidth use, support more predictable network operation. But those still don’t provide any absolute guarantees for temporal performance and determinism. They offer “more deterministic” communication but don’t represent a real advantage for critical embedded systems that require predictable communication in all system operation scenarios.

In commercial avionics, ARINC664-P7 (Avionics Full Duplex Ethernet or AFDX) QoS enhancement has been added to standard Ethernet switches to enable redundant rate-constrained communication with defined maximum latency. AFDX networks are used in commercial programs such as Airbus A380 and Boeing 787 as a backbone network for integrated avionics, and in military avionics (Airbus A400M) and rotorcraft cockpits for display integration. With a known network traffic profile, virtual link prioritization, defined switch buffer dimensioning and decent network calculus and configuration tools, very deterministic operation with respect to maximum latency can be ensured for all end-to-end virtual links (VLs) in AFDX networks. VLs enable point-to-point communication among different functions in the switched network system. Configured VLs have guaranteed bandwidth use and periodicity with defined maximum latency. ARINC664-P7 standard describes traffic policing and shaping required to ensure planned AFDX network performance.

While ARINC664 communication is very deterministic, it is asynchronous and relies on statistical bandwidth multiplexing. It offers very limited control of jitter and message order. As usual, jitter can be reduced by limiting the number of hops (number of switches on the path between two end systems) and maximum bandwidth use. In order to minimize communication jitter (in µs) and keep the latency constant, a different type of QoS enhancement with fault-tolerant synchronization capability is required: a strictly deterministic SAE AS6802.

Time-Triggered Ethernet

In advanced integrated modular avionics and vetronics architectures with many hard real-time constraints, strict determinism is required to simplify sensor fusion and distributed payload processing, enhance resource use, and advance design of integrated modular avionics. With new services, Ethernet networks can gain the deterministic performance comparable to TDMA communication networks (Time-Triggered Protocol (TTP), or MIL 1553 in synchronous communication mode), but at much higher communication speed and with variable Ethernet packets. Layer 2 Quality of Service (QoS) enhancements standardized as TTEthernet (SAE AS6802) guarantee deterministic computing and networking performance for time-, mission- and safety-critical systems. This Ethernet service allows strictly deterministic communication, fixed latency, sub-µs-jitter and predictable message order in redundant multi-hop networks. 

With SAE AS6802, Ethernet gains strictly deterministic synchronous communication capability and can emulate circuit-switching communication in packet-switched Ethernet networks. The image shows the position of this service in the OSI layer model with relation to other Ethernet layers and applications (Figure 1). 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. This is a revolutionary approach to Ethernet networking, which represents a significant upgrade for its packet-switching capabilities to cover strictly deterministic and mixed time-criticality communication in critical embedded applications.

Figure 1
Layer 2 QoS enhancements for deterministic Ethernet communication.

TTEthernet Switch Capabilities

At a minimum, TTEthernet switches implement standard Ethernet and SAE AS6802 functionality. TTEthernet switches behave just like any other Ethernet switch, and work with any standard physical copper or optical layers compliant with IEEE802 standards. AS6802 as a Layer 2 QoS enhancement is transparent to all higher network layers, protocols and applications. When configured, AS6802 services allow strictly deterministic communication for all higher layers and applications. 

As SAE AS6802 facilitates robust TDMA bandwidth partitioning and synchronous communication, it enables simple integration of the ARINC664-P7 standard on the same switch. SAE AS6802 can seamlessly integrate with ARINC664-P7 network design processes, because it can be modeled as microsecond jitter, fixed latency ARINC664-P7 traffic. Therefore, advanced TTEthernet switches support both synchronous (time-driven / time-triggered) and asynchronous (rate-constrained, best-effort) communication in complex Ethernet networks. In other words, audio/video, critical control systems and standard LAN applications can share common Ethernet networking resources.

Synchronous time-triggered communication will be intact, even if the network bandwidth is completely overloaded and asynchronous communication drops packets due to congestion. Figure 2 shows an example of robust bandwidth partitioning among different virtual links with mixed time-criticality QoS using full Ethernet bandwidth. Due to its ARINC 664-P7 capability and DO-254 Level A compliance, TTEthernet switches are suitable for application in commercial aircraft systems, and offer an extended set of networking capabilities for time-, mission- and safety-critical applications.

Figure 2
Shown here, pipes represent traffic classes with different time-criticality and robust isolation among them. All Ethernet traffic—represented by colored pipes—flows through standard Ethernet physical layer.

System architects have full control over network bandwidth use, determinism and timing for critical functions, while being able to execute existing Ethernet applications with less demanding timing requirements. The bandwidth configured for synchronous time-triggered messages, but not used by the application, is dynamically released. This means each synchronous message that is not sent will release the bandwidth for asynchronous network traffic immediately.

Integrated vehicle architectures and countermeasures profit from hard real-time capability and deterministic response on quick external events, while maximizing bandwidth use. The properties of TTEthernet are used in highly available energy production programs and industrial distributed control systems (DCS). Safety-critical automotive applications take advantage of TTEthernet switches to continuously process large amounts of video data to detect obstacles and prevent traffic accidents.

Integrated Architectures with VPX

Ethernet is widely used in backbone networks, where its high-bandwidth throughput remains its key strength. TTEthernet switches enable design of deterministic backbone networks, but also close the gap between backbone and backplane networks, due to its high QoS capabilities. 

With TTEthernet switches, synchronous time-triggered communication is congestion-free, hard real-time and independent of the network traffic load. Therefore, TTEthernet switches can be used for both high-speed backbone and switched fabrics—such as an OpenVPX or ATCA backplane—where it uses lanes reserved for Gigabit Ethernet. Control-plane applications in VPX typically use single or dual-star (redundant) topology with switched Gigabit Ethernet, which are also supported by TTEthernet switches. Depending on application, TTEthernet can be used for control plane, data plane and some utility plane applications (synchronization) in OpenVPX-based systems. 

This means all functions and modules connected to backplane and backbone networks operate as if connected directly to a large fault-tolerant Ethernet backbone. By allowing robust TDMA partitioning of networking resources, the system designer can determine the level of integration/interaction or isolation among different functions. This enables design of innovative architectures and distributed platforms that can host many distributed functions using shared computing/networking resources for advanced integrated system architectures.

In TTEthernet networks, it is possible to emulate reflective memory by using a periodic global data exchange with applications that are synchronized to the global timebase generated at the network level by SAE AS6802 services. From the application perspective, a distributed application possesses a private conflict-free shared memory. 

By using this approach, we can scale down or up with integration of functions, without influencing other existing functions in the system. Also, distributed applications do not need to know about underlying architecture or topology.

Sensor Fusion and Smart Sensors

Sensor fusion and distributed payload processing can be executed without fear of unintended interactions with other system functions. Voting on data from synchronous sources simplifies redundancy management and application software design. Obsolescence management, modernization and upgrades with new DSP processors and applications are simplified, as the behavior of already integrated functions will not change and cause new system integration or timing issues. Critical hard real-time functions will not be influenced by other less critical distributed functions. Sensor front-end data can be streamed to platform systems or common core computing systems, with exact latency and no jitter, independent of network load. This also means that processing functions do not require spatial proximity to a specific sensor, and can be placed anywhere in the system. 

With TTEthernet it is possible to design complex architectures consisting of smart sensors and actuators, and without any dedicated control unit. In this case temporal system behavior is defined at the network layer, and all control loops use limited available processing capability within sensors and actuators. Smart sensors and actuators designed around standard Ethernet-based controllers can be integrated and synchronized to other endsystems by using a software-based AS6802 protocol layer.

A 35-Year Ethernet Legacy

Ethernet was designed as an asynchronous best-effort technology 35 years ago, but with SAE AS6802 QoS enhancements it becomes a strictly deterministic unified network technology covering different aerospace and defense applications. TTEthernet enables integration of hard real-time, synchronous and asynchronous applications in N-redundant architectures, and allows mixing of applications with mixed time-criticality levels in backplane or backbone networks. With available Layer 2 QoS enhancements, today’s Ethernet is a deterministic technology, suitable for design of critical embedded systems, payloads and by-wire applications. This confirms a wise old saying of the networked world: Never bet against Ethernet.

TTEthernet Networks: Operation Principles

The SAE AS6802 standard describes synchronous startup and recovery, and media access by asynchronous and synchronous messages. Synchronization capability is essential for deterministic operation of TTEthernet networks. The system time is created by the execution of distributed clock synchronization algorithms defined in SAE AS6802. The synchronization is based on continuous adjustments of local clocks in the system, via asynchronous messages, with precision better than 1µs. It uses a robust two-step synchronization algorithm that can operate at distances of up to 60 miles with hundreds or thousands of switches and end systems.

The TTEthernet network timebase is designed to be resistant against multiple faults and complicated fault scenarios, and does not rely on a single master or prioritized best master search algorithms used in IEEE1588. The TTEthernet synchronization approach guarantees continuous system synchronization in case of faults. Synchronization algorithms defined in SAE AS6802 are formally verified by aerospace, automotive and defense experts. Every TTEthernet switch and end system has a configuration that tells it how to schedule incoming synchronous traffic based on system time. It will forward asynchronous packets at time periods where outgoing port bandwidth is free and no synchronous packets are being sent.

TTEthernet switches can be used for different applications in SWaP-optimized integrated modular architectures, high-integrity by-wire systems, distributed payload processing, display integration, core computing and mission systems, stores, countermeasures, distributed power generation, and other applications taking advantage of deterministic unified Ethernet networking and full line-speed synchronous switching.

TTTech Computertechnik AG
Vienna, Austria.
+43 1 585 34 34-899.

[www.tttech.com]

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