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Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

Computer Engineering. and Networks Laboratory . Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus. Federico Ferrari PhD Defense October 18, 2013 — Zurich, Switzerland. Cyber-Physical Systems (CPSs).

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Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

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  1. Computer Engineering and Networks Laboratory Enabling Dependable Communication in Cyber-Physical Systems with aWireless Bus Federico Ferrari PhD Defense October 18, 2013 — Zurich, Switzerland

  2. Cyber-Physical Systems (CPSs) • Tightly integrate physical processes, computation, and communication • Safety-critical control loops • Sensors gather data from the environment • Actuators react according to a control law Physical processes Communication Computation Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  3. Dependability Gap in Current CPSs • Safety-critical CPS application • Most of the existing CPS communication protocols operate in a best-effort manner Infrastructure control … Environmental monitoring and control Medical systems Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  4. Communication Challenges in CPSs • Tight physical integration → Severe constraints • Resource-constrainedwireless embedded devices [Tmote Sky] Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  5. Communication Challenges in CPSs • Tight physical integration → Severe constraints • Resource-constrainedwireless embedded devices • Multi-hop network topologies that vary over time Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  6. Communication Challenges in CPSs • Tight physical integration → Severe constraints • Resource-constrainedwireless embedded devices • Multi-hop network topologies that vary over time • Operate for consecutive months/years How to design efficient protocols that provide also deliveryguarantees? Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  7. Looking for Inspiration:Safety-Critical Wired Embedded Systems • Based on time-triggered, shared buses • Time-Triggered Protocol (TTP)[Kopetz et al., FTCS 1993] • FlexRay[FlexRay Consortium, 2005] • Successfully employed in automotive, avionics Can we apply similar networking designs to CPSs? Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  8. Our Wireless Bus Conjecture • A time-triggered communication infrastructure for multi-hop low-power wireless networks • Common notion of time • Communicate as if connected by a shared bus It is possible to enable dependable yet efficient communication in CPSs by employing a wireless bus Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  9. Building a Wireless Bus Safety-critical CPS application Dependability gap Multi-hop low-power wireless network Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  10. Building a Wireless Bus Glossy Chapter 2 [IPSN 2011] Global time synchronization One-to-all communication Multi-hop low-power wireless network Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  11. Building a Wireless Bus Glossy [SenSys 2012] Chapter 2 [IPSN 2011] LWB Chapter 3 j,k,l j,k,l Adaptive scheduling Time-triggered operation × j,k,l Global time synchronization One-to-all communication j,k,l Low-Power Wireless Bus Multi-hop low-power wireless network Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  12. Building a Wireless Bus Glossy [SRDS 2013] Chapter 4 Virtus [SenSys 2012] Chapter 3 LWB [IPSN 2011] Chapter 2 Safety-critical CPS application Failure management Delivery guarantees j,k,l j,k,l j,k,l j,k,l Adaptive scheduling Time-triggered operation × j,k,l j,k,l Global time synchronization One-to-all communication j,k,l j,k,l Virtus Low-Power Wireless Bus Multi-hop low-power wireless network Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  13. Glossy: Objectives Chapter 2 [IPSN 2011] Chapter 3 LWB Glossy VIRTUS Chapter 4 [SRDS 2013] [SenSys 2012] • Fast and reliable flooding of messages • Accurate global time synchronization • Hide complexity of multi-hop networks Safety-critical CPS application Failure management Delivery guarantees Adaptive scheduling Time-triggered operation Global time synchronization One-to-all communication Multi-hop low-power wireless network Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  14. Challenges for Efficient Flooding How to relay packets efficiently and reliably? • Avoid aggressive, uncoordinated broadcasts • Typical approach:Coordinate packet transmissions • CF [Zhu et al., NSDI 2010] • RBP [Stann et al., SenSys 2006] • Maintain topology-dependent state initiator Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  15. Glossy Flooding Architecture • All receiving nodes relay packets synchronously • Simple, but radically different solution • No explicit routing • No topology-dependent state • Key Glossy mechanisms • Start execution at the same time • Compensate for hardware variations • Ensure deterministic execution timing initiator Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  16. Propagation in Glossy • A relay counter c is set to 0 at the first transmission • A node increments c before relaying the packet Proc. Proc. Proc. Proc. Rx Rx Tx Tx Tx Tx initiator Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Proc. Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx Rx Rx Tx Tx t c = 1 c = 2 c = 3 c = 4 c = 5 c = 0 t c = 1 c = 2 c = 3 c = 4 c = 5 c = 0 (In this example a node transmits at most twice) Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  17. Time synchronization in Glossy • Estimate the relay length during propagation • Compute a common reference time Proc. Proc. Rx Tx Tx initiator Proc. Proc. Proc. Proc. Proc. Proc. Rx Tx Rx Tx Rx Tx Rx Tx Rx Tx Rx Tx Proc. Proc. Proc. Proc. Proc. Proc. Rx Tx Rx Tx Rx Tx Rx Tx Rx Tx Rx Tx Proc. Proc. Proc. Proc. Proc. Proc. Rx Tx Rx Tx Rx Tx Rx Tx Rx Tx Rx Tx t c = 1 c = 2 c = 3 c = 4 c = 5 c = 0 Reference time Constant relay length Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  18. Glossy: Main Evaluation Findings • A few ms to flood packets to hundreds of nodes • Reliability > 99.99 % in most scenarios • Synchronization error < 1 µs even after 8 hops Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  19. LWB: Objectives Chapter 3 Chapter 4 [IPSN 2011] Chapter 2 [SenSys 2012] Glossy Chapter 4 VIRTUS LWB [SRDS 2013] [SRDS 2013] Glossy Chapter 2 [IPSN 2011] LWB Chapter 3 VIRTUS [SenSys 2012] A concrete wireless bus that: • Adapts to varying conditions and demands • Efficiently supports a wide range of scenarios • Delivers messages with high reliability Safety-critical CPS application Failure management Failure management Delivery guarantees Delivery guarantees Adaptive scheduling Adaptive scheduling Time-triggered operation Time-triggered operation Global time synchronization Global time synchronization One-to-all communication One-to-all communication Multi-hop low-power wireless network Multi-hop low-power wireless network Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  20. LWB Design Principles • Bizarre idea: broadcast-only communication! • Multi-hop wireless network → Shared bus • Synchronized, time-triggered operation • Collision-free and efficient bus accesses • Centralized scheduling • A host node orchestrates all communication Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  21. Time-Triggered Operation in LWB • LWB operation is confined to rounds • A round consists of non-overlapping slots • Each slot corresponds to adistinct Glossy flood Round period T t n1 n1 n1 n2 n3 Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  22. Centralized, Adaptive Scheduling • Demand response scheduling at the host • Example scheduling policy • Minimize energy while providing enough bandwidth • Ensure fair allocation of slots Host Demand Response Low-Power Wireless Bus Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  23. LWB Activity during a Round • Schedule: sent by the host H, also for time-sync • Data: messages transmitted by senders S1, S2, etc. • Requests: competed by senders to join LWB T t Host:compute schedule … • H S1 S2 not allocated Requests Schedule Data Data Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  24. Additional LWB Mechanisms Support for nodesjoining and disconnecting Host failover policy LWB Optimizations forenergy efficiency Prompt adaptationto traffic changes Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  25. LWB: Main Evaluation Findings(4 testbeds, 7 state-of-the-art protocols, 256 runs, 838 hours) The same LWB prototype: • Is efficient under a wide range of traffic loads • Supports mobile nodes with no performance loss • Is minimally affected by interference or failures Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  26. Reliability and Energy Efficiency with Many-to-Many Communication 90 nodes • Varying senders • 8 receivers • LWB outperforms state of the art • Reliability • Energy efficiency Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  27. Virtus: Objectives Chapter 3 VIRTUS LWB [IPSN 2011] Glossy Chapter 2 Chapter 4 Virtus [SenSys 2012] [SRDS 2013] Chapter 4 Glossy Chapter 2 [IPSN 2011] LWB Chapter 3 [SRDS 2013] [SenSys 2012] • Provide guarantees on message delivery • In the face of communication failures • In the face of node crashes • Keep overhead low compared with LWB Safety-critical CPS application Failure management Failure management Delivery guarantees Delivery guarantees Adaptive scheduling Adaptive scheduling Time-triggered operation Time-triggered operation Global time synchronization Global time synchronization One-to-all communication One-to-all communication Multi-hop low-power wireless network Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  28. Key Virtus Mechanisms • Guarantee virtually-synchronous executions • All nodes see the same events in the same order • Delivered messages • Joining and failing nodes • Atomic multicast • Deliver messages reliably and with total order • Group management • Share information on currently active nodes (Formally proven) Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  29. New Interactions in Virtus • View: set of active nodes, sent by the host H • Ack: receivers R1, R2, etc. buffer received data and send the content of their buffers T t Host:compute schedule … … • H • H S2 R1 S1 R2 not allocated and update view Requests View Schedule Ack Ack Data Data Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  30. Virtus Efficiency 90 nodes • 45 senders • Varying receivers • Virtus provides delivery guarantees while outperforming existing best-effort solutions Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  31. Conclusions [SRDS 2013] Chapter 4 Virtus [SenSys 2012] Chapter 3 [IPSN 2011] LWB Glossy Chapter 2 Wireless bus:delivery guarantees and efficiency • Novel solutions • Narrows the current dependability gap in CPSs Safety-critical CPS application Failure management Delivery guarantees • First to provide virtual synchrony to CPSs • Efficient support for multiple traffic patterns Adaptive scheduling Time-triggered operation • Multi-hop broadcasts have become cheap! Global time synchronization One-to-all communication Multi-hop low-power wireless network Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

  32. Chapter 4 Chapter 3 LWB [SRDS 2013] [SenSys 2012] Chapter 2 Glossy Virtus [IPSN 2011] Safety-critical CPS application Failure management Delivery guarantees Adaptive scheduling Time-triggered operation Global time synchronization One-to-all communication Multi-hop low-power wireless network Enabling Dependable Communication in Cyber-Physical Systems with a Wireless Bus

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