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Topology Control

Topology Control. What’s topology control?. What’s topology control?. When nodes are deployed, how do they organize into a network? And how do they maintain this organization over the lifetime of the system? Neighbor-discovery protocol is important.

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Topology Control

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  1. Topology Control

  2. What’s topology control?

  3. What’s topology control? • When nodes are deployed, how do they organize into a network? And how do they maintain this organization over the lifetime of the system? • Neighbor-discovery protocol is important. • If neighborhood is sparse, use all neighbors. • What if neighborhood is dense? • Use a subset of neighbors. • How?

  4. Two Important Goals • Coverage: ensures critical events can be detected/monitored. • Connectivity: ensures data can be propagated over the network. • Tunable parameters: • Node mobility affects both coverage and connectivity. • Transmission power control. • Sleep schedules.

  5. Over-Deployed Networks • Redundant nodes. • Nodes are inexpensive. • Deployment is remote. • Position of sensors is not critical. • Advantages: • Longer lifetime. • Higher robustness. • Adjustable connectivity/coverage.

  6. Approaches to topology control • Adjust transmit power. • Turn nodes on/off. • Approaches that follow are sleep-based approaches that target connectivity.

  7. ASCENT

  8. ASCENT: scenario • Ad hoc deployment. • Energy limitations. • Arbitrarily large scale. • Unattended operation. • Assume CSMA.

  9. ASCENT: goals • Self-organization of nodes into topology that allows sensing coverage and communication under tight energy constraints.

  10. ASCENT: approach • Nodes turn themselves on/off depending on assessment of operating conditions. • Neighborhood density. • Data loss.

  11. State diagram After Tt Test Active After Tp: Nbors<NT And Loss>LT or Help After Tt: Nbors > NT or Loss > LT After Tp Passive Sleep After Ts

  12. In “test” state: • Signaling (e.g., neighbor announcements). • After Tt, goes to “active”. • Or, if before Tt, number of neighbors>NT or average data loss (Tt) > average data loss (T0), go to “passive”.

  13. In “passive” state: • After Tp, go to “sleep” or, • If neighborhood is sparse, loss > LT, or “help” from “active” neighbor, go to “test”.

  14. In “sleep”: • Turn off radio. • After Ts, go to “passive”.

  15. In “active”: • Node does routing and forwarding. • Sends “help” if data loss > LT. • Stays on until runs out of battery!

  16. Considerations • Why passive and test states? • Why once in active, a node runs until battery dies? • How to set parameters? • NT, LT. • Tt, Tp, Ts.

  17. Neighborhood and loss • Node is neighbor if directly connected and link packet loss < NLS. • NLS is adjusted according to node’s number of neighbors. • Average loss date uses data packets only. • Packet is lost if not received from any neighbors.

  18. Performance evaluation • Modeling, simulation, experimentation. • Metrics: • Packet loss. • Delivery ratio. • Energy efficiency. • Lifetime. • Time till 90% of transit nodes die.

  19. PEAS

  20. PEAS • Probing Environment, Adaptive Sleeping. • “Extra” nodes are turned off. • Nodes keep minimum state. • No need for neighborhood-related state. • PEAS considers very high node density and failures are likely to happen.

  21. Bi-modal operation • Probing environment. • Adaptive sleeping.

  22. PEAS state diagram Working No reply for probe Wakes up Probing Sleeping Hears probe reply. Sleep->Probe: randomized wake-up timer with exponential distribution.

  23. Probing • When node wakes up, enters probing mode. • Is there working node in range? • Broadcasts PROBE to range Rp. • Working nodes send REPLY (randomly scheduled). • Upon receiving REPLY, node goes back to sleep. • Adjusts sleeping interval accordingly. • Else, switches to working state. • Probing rate is adjusted over time based on the probe replies.

  24. Considerations • Probing range is application-specific. • Robustness (sensing and communication) versus energy-efficiency. • Location-based probing as a way to achieve balance between redundancy and energy efficiency. • Randomized sleeping time. • Better resilience to failure. • Less contention. • Adaptive based on “desired probing rate”.

  25. Evaluation • Simulations. • Simulated failures: failure rate and failure percentage. • Metrics: • Coverage lifetime. • Delivery lifetime.

  26. Cross-Layer Issues • Relationship to routing and to MAC. • Topology control <-> Routing: • Topology control provides network substrate for routing. • Topology control below routing layer. • Routing considers only “active” nodes. • Topology control <-> MAC: • Co-existence of MAC sleep schedules with topology control sleep schedules.

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