Friday, November 11, 2011

What is SAE AS6802 "Time-Triggered Ethernet"?

The World of Ethernet: OEM-specific or Industry-Specific Ethernet Standards

Unlike monolithic standards such as CAN, Firewire, ARINC429 and MIL-1553, Ethernet comprises a family of standards. It is good policy to create IEEE802-style amendments in order to maintain compatibility with other Ethernet LAN networks services and traffic classes. Ethernet, as a family of frame-based standards, can be enhanced to gain new capabilities and penetrate into different applications. Fortunately, Ethernet as a standard has managed to adapt to new needs, without changing its essence.

Unfortunately, real-time Ethernet standard efforts typically lead to “New Unique Industry-Specific Ethernet” with limited Ethernet compatibility. In the best case, these standards can retain full compatibility by using standard Ethernet devices, but at the cost of real-time performance. In most cases, industry-specific real-time Ethernet standards have both limited real-time performance, and can be used only in closed systems.

Ideally, Ethernet can be extended by new services, support standard LAN capabilities, and provide different applications with different time-, safety- and mission-criticality requirements. This could help to position Ethernet as a deterministic unified network, and replace many industry-specific Ethernet modifications.

Of course, industry-specific protocol services or modifications will always be present, but their scope can be minimized.

SAE AS6802 and Ethernet Standard
Early in the standard development process at SAE AS-2D committee, we talked about “TTEthernet” as a standard we are working on. In the meantime, it turned out that the SAE AS6802 “Time-Triggered Ethernet” (informally “TTEthernet”) standard is in fact a Layer 2 QoS enhancement and protocol service, similar to other IEEE802.1 protocol services, such as VLAN.
Network devices implementing SAE AS6802 protocol service can extend Ethernet capabilities by using formally verified distributed fault-tolerant synchronization, robust TDM-style bandwidth partitioning, and synchronous communication with fixed latency and ┬Ás-jitter.
With capabilities described in SAE AS6802, Ethernet networks can efficiently and natively handle both synchronous and asynchronous communication, and enable robust separation of different traffic classes in time, whereas the properties of time-critical traffic cannot be affected by varying network workload or asynchronous traffic. The fact that we can integrate different traffic classes with different flavors of determinism and predictability leads to Ethernet as a “deterministic unified network”, that can handle any critical application, and still run standard LAN applications.

Overview: SAE AS6802 Standard Content 

Time-Triggered Ethernet functionality described in the SAE AS6802 standard is a Layer 2 Quality-of-Service (QoS) enhancement for Ethernet networks. It provides the capability for deterministic, synchronous, and congestion-free communication, unaffected by any asynchronous Ethernet traffic load. This occurs via a fault-tolerant, self-stabilizing synchronization strategy, which helps to establish temporal partitioning and ensures isolation of the synchronous time-critical dataflows from other asynchronous Ethernet dataflows. 

By implementing this standard in network devices (network switches and network interface cards), Ethernet becomes a deterministic network which can be shared by low-latency, low-jitter, and non-time-critical applications. This means that distributed applications with mixed time-criticality requirements (e.g., real-time command and control, audio, video, voice, data) can be integrated and coexist on one Ethernet network.