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Sense-and-Respond Systems and Play-Back Buffers

Sense-and-Respond Systems and Play-Back Buffers. Vincenzo Liberatore Division of Computer Science. Research supported in part by NSF CCR-0329910, Department of Commerce TOP 39-60-04003, NASA NNC04AA12A, and an OhioICE training grant. Sense-and-Respond. Computing in the physical world

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Sense-and-Respond Systems and Play-Back Buffers

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  1. Sense-and-Respond Systems and Play-Back Buffers Vincenzo Liberatore Division of Computer Science Research supported in part by NSF CCR-0329910, Department of Commerce TOP 39-60-04003, NASA NNC04AA12A, and an OhioICE training grant.

  2. Sense-and-Respond • Computing in the physical world • Components • Sensors, actuators • Controllers • Networks Control Playback

  3. Sense-and-Respond • Enables • Industrial automation [BL04] • Distributed instrumentation [ACRKNL03] • Unmanned vehicles [LNB03] • Home robotics [NNL02] • Distributed virtual environments [LCCK05] • Power distribution [P05] • Building structure control [SLT05] • Merge cyber- and physical- worlds • Networked control and tele-epistemology [G01] • Sensor networks • Not necessarily wireless or energy constrained • One component of sense-actuator networks Control Playback

  4. Flow Sensor data Remote controller Control packets Timely delivery Stability Safety Performance Information Flow Control Playback

  5. Playback Buffers [Infocom 2006] • Main objective • Smooth out network non-determinism • Related to • Multimedia buffers • TCP RTO Control Playback

  6. Packet generation Packet arrival Multimedia Play-Back Sequence number Play-back time Control Playback

  7. Related Work • Multimedia buffers • Important source of inspiration • Physics versus multimedia quality • Playback delay computed in advance • Affects control signal computation • Round-Trip Times • TCP RTO • Another source of inspiration • Upper bound on RTT • Large time-out cost • Conservative estimate Control Playback

  8. Algorithm Control Playback

  9. Main Ideas • Predictable application time • If control applied early, plant is not in the state for which the control was meant • If control applied for too long, plant no longer in desired state • Keep plant simple • Low space requirements • Integrate Playback, Sampling, and Control Control Playback

  10. Algorithm • Send regular control • Playback time • Late playback okay • Expiration • Piggyback contingency control Control Playback

  11. X X Deadwood packets • Old • Received after the expiration time • Out-of-order • Later control more appropriate for current plant state • Would get us into a deadlock • New packet resets the playback timer • Keep resetting until no signal applied • “Quashed” packet • Discard! controller plant Playback delay Control Playback

  12. Countermand control • Scenario • Packet i+1 overtakes packet I • ti+1 << ti • Likely caused by delay spike • New signal countermands previous one controller plant ti Playback delay ti+1 Control Playback

  13. Playback Delays (I) • Modular component • Compute playback delay t and sampling period T • Use short term peak-hopper [EL04] • Original peak-hopper for TCP RTO • Too conservative for networked control • Aggressively attempt to decrease t time Control Playback

  14. Playback Delays (II) • Aggressively attempt to decrease T • Add upper bound on playback delay t • Avoid dropping deadlock packets • Bound t ≤ T+RTT • Caps t and T • Must estimate lower-bound on RTT • Use symmetric of peak-hopper • Add negative variability estimate to compensate for short-term memory Control Playback

  15. Playback Delays (III) Calculate current RTT variability Positive variability coefficient Negative variability coefficient if then Update min RTT estimate Age min RTT estimate Calculate  Control Playback

  16. Playback Delays (IV) if then Attempt to avoid quashed packets else Decrease sampling period Control Playback

  17. Control Pipes • Bandwidth and delays • t is playback delay • T is sampling period • 1/T proportional to bandwidth • Control pipe • T«t • Multiple in-flight packets • Pipe depth • Bound by constraint t ≤ T+RTT • Keep pipe predictable Control Playback

  18. Observer • Estimate future plant state • Plant sample current state, including local variables • Keep log of outstanding control packets • Assumption on packet delivery • Future packet delivery is uncertain • Purge from log • Old packets • Packet that should be overtaken by new control • Countermands signals generated when delay spike is transient • Out-of-order packets Control Playback

  19. Evaluation Control Playback

  20. Network Model • Simulated network • Losses: Gilbert model • Delays • Shifted Gamma distribution • Heavy tail • Low probability of out-of-order delivery • Correlate delays to introduce delay spikes • Wide-area implementation • Use RT scheduling whenever possible • Use otherwise unloaded machines • RT made little difference • Host worldwide, heterogeneous conditions Control Playback

  21. Plant • Scalar linear plant • Plant state x(t) • Input u(t) (control) • Output y(t) • Disturbances v(t), w(t) • Akin to white noise • Deadbeat controller • Aggressive Control Playback

  22. Metrics • Metrics • Root-mean square output • Output: 99-percentile • Comparison • Open-loop plant u(t)=0 • Proportional controller (no buffer) • Proportional controller with constant delays Control Playback

  23. Plant output Open Loop Play-back Control Playback

  24. Packet losses Figure 8 Control Playback

  25. t ≤T+RTT Sampling period Root-mean-square error Imperfection of the control pipe Control Playback

  26. Other Research inSense-and-Respond Control Playback

  27. Bandwidth Allocation • Definition • Multiple sense-and-respond flows • Contention for network bandwidth • Desiderata • Stability and performance of control systems • Must account for physics • Efficiency and fairness • Fully distributed, asynchronous, and scalable • Dynamic and self-reconfigurable Control Playback

  28. Problem Formulation • Define a utility fn U(r) that is • Monotonically increasing • Strictly concave • Defined for r ≥ rmin • Optimization formulation Control Playback

  29. Conclusions (I) • Sense-and-Respond • Merge cyber-world and physical world • Critically depends on physical time • Playback buffers integrated with • Sampling (adaptive T) • Control (expiration times, performance metrics) • Packet losses • Reverts to open loop plant (contingency control) Control Playback

  30. Conclusions (II) • Playback delay t • Adapts to network conditions • Sampling period T • Avoids imperfection of control pipe • Simulations and emulations • Low variability around set point • Robust Control Playback

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