1. 1-1 Topology Control

2. 1-2 What’s topology control?

3. 1-3 What’s topology control? r When nodes are deployed, how do they organize into a network? And how do they maintain this organization over the lifetime of the system? r Neighbor-discovery protocol is important. r If neighborhood is sparse, use all neighbors. r What if neighborhood is dense? m Use a subset of neighbors. m How?

4. 1-4 Two Important Goals r Coverage: ensures critical events can be detected/monitored. r Connectivity: ensures data can be propagated over the network. r Tunable parameters: m Node mobility affects both coverage and connectivity. m Transmission power control. m Sleep schedules.

5. 1-5 Over-Deployed Networks r Redundant nodes. m Nodes are inexpensive. m Deployment is remote. m Position of sensors is not critical. r Advantages: m Longer lifetime. m Higher robustness. m Adjustable connectivity/coverage.

6. 1-6 Approaches to topology control r Adjust transmit power. r Turn nodes on/off. r Approaches that follow are sleep-based approaches that target connectivity.

7. 1-7 ASCENT

8. 1-8 ASCENT: scenario r Ad hoc deployment. r Energy limitations. r Arbitrarily large scale. r Unattended operation. r Assume CSMA.

9. 1-9 ASCENT: goals r Self-organization of nodes into topology that allows sensing coverage and communication under tight energy constraints.

10. 1-10 ASCENT: approach r Nodes turn themselves on/off depending on assessment of operating conditions. m Neighborhood density. m Data loss.

11. 1-11 State diagram Test Active Passive Sleep After Tt After Tt: Nbors > NT or Loss > LT After Tp: Nbors<NT And Loss>LT or Help After Tp After Ts

12. 1-12 In “test” state: r Signaling (e.g., neighbor announcements). r After Tt, goes to “active”. r Or, if before Tt, number of neighbors>NT or average data loss (Tt) > average data loss (T 0 ), go to “passive”.

13. 1-13 In “passive” state: r After Tp, go to “sleep” or, r If neighborhood is sparse, loss > LT, or “help” from “active” neighbor, go to “test”.

14. 1-14 In “sleep”: r Turn off radio. r After Ts, go to “passive”.

15. 1-15 In “active”: r Node does routing and forwarding. r Sends “help” if data loss > LT. r Stays on until runs out of battery!

16. 1-16 Considerations r Why passive and test states? r Why once in active, a node runs until battery dies? r How to set parameters? m NT, LT. m Tt, Tp, Ts.

17. 1-17 Neighborhood and loss r Node is neighbor if directly connected and link packet loss < NLS. r NLS is adjusted according to node’s number of neighbors. r Average loss date uses data packets only. r Packet is lost if not received from any neighbors.

18. 1-18 Performance evaluation r Modeling, simulation, experimentation. r Metrics: m Packet loss. m Delivery ratio. m Energy efficiency. m Lifetime. Time till 90% of transit nodes die.

19. 1-19 PEAS

20. 1-20 PEAS r Probing Environment, Adaptive Sleeping. r “Extra” nodes are turned off. r Nodes keep minimum state. m No need for neighborhood-related state. r PEAS considers very high node density and failures are likely to happen.

21. 1-21 Bi-modal operation r Probing environment. r Adaptive sleeping.

22. 1-22 PEAS state diagram Working Sleeping Probing No reply for probe Wakes up Hears probe reply. Sleep->Probe: randomized wake-up timer with exponential distribution.

23. 1-23 Probing r When node wakes up, enters probing mode. r Is there working node in range? m Broadcasts PROBE to range Rp. m Working nodes send REPLY (randomly scheduled). m Upon receiving REPLY, node goes back to sleep. Adjusts sleeping interval accordingly. m Else, switches to working state. r Probing rate is adjusted over time based on the probe replies.

24. 1-24 Considerations r Probing range is application-specific. m Robustness (sensing and communication) versus energy-efficiency. r Location-based probing as a way to achieve balance between redundancy and energy efficiency. r Randomized sleeping time. m Better resilience to failure. m Less contention. m Adaptive based on “desired probing rate”.

25. 1-25 Evaluation r Simulations. r Simulated failures: failure rate and failure percentage. r Metrics: m Coverage lifetime. m Delivery lifetime.

26. 1-26 Cross-Layer Issues r Relationship to routing and to MAC. r Topology control Routing: m Topology control provides network substrate for routing. m Topology control below routing layer. m Routing considers only “active” nodes. r Topology control MAC: m Co-existence of MAC sleep schedules with topology control sleep schedules.