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SAE AS6802: „Time-TriggeredEthernet“ (TTEthernet)
Public specification available since 2008IP designed to DO-254/DO-178B Level A◦ formal proof of protocol algorithms
used for IEC61508 SIL-rated systemsStandardization at SAE◦ SAE standardization project started in 2009◦ SAE Standard release in November 2011◦ http://standards.sae.org/as6802
System timebase available, but no master clock(alignment of local clocks in µs!)Autonomous network operation◦ based on scheduled communication and system time
A family of frame-based standards forLAN/MAN networks by IEEE802◦ Standard physical medium◦ Set of medium access control rules with fair
arbitration◦ Variable size packets in Ethernet format
What is Ethernet, really?
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SAE AS6802: „Time-TriggeredEthernet“ (TTEthernet)
The objective was to:
◦ make Ethernet viable for any time-, safety- and mission-critical embedded system application (SAE AS6802 is a Layer 2 protocol service)◦ … and retain asynchronous communication capabilitiesThe objective was NOT to:
◦ create yet another real-time industrial Ethernet network, that is incompatible with IEEE802.3 or standard Ethernet LAN
In fact it is a service …◦ compare it to deterministic VLAN messaging with fixed
end-to-end latency and µs-jitter!
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SAE AS6802: A Layer 2 QoS EnhancementTime-Triggered Ethernet (TTEthernet)
Allows integration of low-latency, low-jitter and standard LAN applications
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Time-triggered extensions (Layer 2) implemented in an Ethernet switch enable deterministic unified networking◦ Synchronous startup◦ Synchronous recovery◦ Robust fault-tolerant
distributed clock◦ Foundation for synchronous
communication and robust bandwidth partitioningin Ethernet networks
SAE AS6802: A Layer 2 QoS Enhancement
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Summary:System Designer Perspective
SAE AS6802 is just another Ethernet serviceAdd SAE AS6802 service to Ethernet switch◦ capabilities to design Ethernet-based
time-, safety- and mission critical system !Add other comptible IEEE802.1 Layer 2 servicesto gain specific capabilities and support(deterministic) unified networking◦ VLAN(IEEE802.1Q), DCB, IEEE1588/IEEE802.1AS,
Rate-contrained services with traffic shaping, etc.Use Ethernet as a deterministic network capableof handling different types of time-critical traffic
Robust algorithm based on exchange of asynchronous IEEE802.3 messagesSynchronizes local clocks – system time (!)◦ no wall clock (external time source - e.g. GPS) requiredFail-operational: ◦ tolerates multiple faults◦ tolerates byzantine
synchronization faults◦ no search for best master
(distributed clock!)Provides defined worst-case synchronous startup& recovery time (in ms)
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Capabilities: Synchronous Communication
System time available on switches and endsystems◦ Scheduled traffic can have fixed latency and µs-jitter◦ Switch knows when the message is forwarded
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Capabilities: Deterministic Unified Ethernet
Synchronous and Asynchronous Traffic
„Virtual links“ are forwarded through 100BASE-TX, 1000BASE-CX, 1000BASE-SX or other Ethernet physical layer connections
Resource Use & Complexity ReductionMaximize use of network bandwidth and computing resourcesfor critical embedded functions◦ Reduce uncertainity, jitter and unintended system statesImprove functional alignment (and separation!)◦ Simplified sensor fusion◦ Simplified distributed processing◦ Simplified redundancy management
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Integrating many functions on fewECUs
Emulate distributed computer / reflective memoryIntegrate critical and non-critical functions
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SAE AS6802 in Critical Applications
Robust TDM-style bandwidth partitioning(Hard) Real-time operationStrictly deterministic system operationPredictable computing and networking resourcesharingAlignment of different functions accross thesystemsDesign of advanced system architectures forminimizing SWaPSystem complexity reduction(system state explosion!)
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SAE AS6802 in Critical Applications
If we managed to go from federated to integratedarchitectures …◦ Read: time/space partitioning at module level… we can manage transition to „distributed AND integrated“ architectures◦ Read:
time/space partitioning at module level plus time partitioning at network levelsystem-level partitioning (of all computing and networkingresources)Predictable virtualization of complex embedded systems
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New activities on SAE AS-2D agenda?
SAE AS6802 (TTEthernet)◦ Scheduling and system design◦ Application Handbook◦ Complexity reduction◦ Linear Deterministic Networks◦ Closer work with IEEE?
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