deterministic ethernet - sae as6802 (ttethernet)
DESCRIPTION
http://deterministic-ethernet.blogspot.com/2011/06/why-deterministic-why-ethernet.htmlTRANSCRIPT
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Deterministic Ethernet: SAE AS6802 "Time-Triggered Ethernet"
Mirko Jakovljevic, TTTech / SAE AS-2D
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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
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What is „Time-Triggered“?
Predictable (deterministic) communication◦ Fixed latency, µs-jitter, TDMA
System timebase available, but no master clock(alignment of local clocks in µs!)Autonomous network operation◦ based on scheduled communication and system time
Examples:◦ FlexRay◦ TTP (SAE AS6003)◦ TTEthernet (SAE AS6802)
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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
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Deterministic Ethernet: SAE AS6802 "Time-Triggered Ethernet“
Capabilities
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Capabilities: DistributedFault-tolerant Synchronization
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
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Capabilities: Robust Partitioning forDeterministic Unified Ethernet
Mechanisms:◦ Switch knows the traffic schedule for synchronous (TT) traffic◦ Switch knows about properties of time-sensitive traffic and
possible time-violationse.g. for AFDX / ARINC664 (e.g. rate constrained – BAG, periodicity)
◦ Switch knows when the best effort (asynchronous) traffic can be scheduled to prevent violation of temporal constraints for RC and TT
Synchronous (TT)
Time-Sensitive Traffic (RC)
Asynchronous (Priority-Driven and/or Lossless)
Output Port
Configuration for TT Traffic
Configuration for RC Traffic
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Capabilities: Deterministic Unified Ethernet
What if synchronous links (VLs) are reserved, butthe message is not sent?◦ … ECU / LRU is not installed◦ … Function is
currently inactive
Dynamic Bandwidth Release: immediate availability for asynchronous traffic
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Comparison with other Ethernet standards
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Ethernet in Critical EmbeddedSystems
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
IEEE DCB
IEEE AVB
ARINC 664 Part 7
SAE AS6802 (TTEthernet)
Deterministic,Time-critical,
Safety-critical,Mission-critical
Lossless,Loss-sensitive,Time-sensitive,Mission-Critical
Time-Critical,Mission-Sensitive
IEEE1588
IEEE1588
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Positioning: SAE AS6802 forDeterministic Unified Ethernet
QoS
in s
hare
d E
ther
net n
etw
orks
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Positioning: SAE AS6802 forDeterministic Unified Ethernet
QoS
in s
hare
d E
ther
net n
etw
orks
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XXXMixed Criticality TTEthernet Switch
X--IEEE802.3 switchXX-Plain TTEthernet Switch
-X-TTEthernet Switch (synch. AS6802 comm. only)
-XXRC Switch w/ A/V and hard-RT
--XRC Switch
BETT(SAE AS6802)
RC (ARINC664)
Used QoS Protocol Services
TTEthernet Switch
Services
TTEthernetSwitch Behavior
TTEthernet Switch withSAE AS6802 / ARINC664-P7
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Positioning: SAE AS6802 forDeterministic Unified Ethernet
QoS
in s
hare
d E
ther
net n
etw
orks
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SAE AS6802 and RT-Ethernet
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SAE AS6802 in IEC61850 Context
GOOSE Messages (low-latency, critical) can besent via time-triggered messgaes
Data
Data
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Impact on System Design
Mirko Jakovljevic, TTTech / SAE AS-2D
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Network capabilities impact architecture and application design◦ the number of ECUs, wiring and weigth◦ use of network bandwidth
and computing resources◦ level of functional integration◦ system and application complexity
cost of SW/HW design and integration
◦ maintenance, reuse and upgrades◦ obsolescence management◦ certification◦ …
Why DeterministicTime-Triggered Networks ?
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DistributedDistributedIntegrated Modular Integrated Modular
Architectures (DIMA)Architectures (DIMA)
Many high- and low-levelfunctions share distributedcomputing and networking
resources
Integrated Modular Integrated Modular Architectures Architectures
(e.g. IMA)(e.g. IMA)
Many higher level functions on one processing module
Additional low-level functions via networked RDCs
FederatedFederated
Computing Resources (one function per box)
Networking Resources (physically separatedunidirectional links) Integ
ration Lev
el: Very
High
WholeSys
tem is
an IM
A!!!
Integration Level: High
Integration Level: Low
??HowHow DIMA DIMA coverscovers
thisthis gapgap??
Integration Level: Very Low
Federated Architectures
StatisticalMultiplexing
TDMA Multiplexing
Advanced System Integration
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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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Q&AVisit Deterministic Ethernet / LinkedIn Group (!!)
Mirko Jakovljevic, TTTech / SAE AS-2D
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