Topology Management

1. Topology Management EE206A (Spring 2003): Lecture #10

2. References • Mandatory Reading • Rong Zheng, Jennifer Hou, Lui Sha, “Asynchronous Wakeup for Power Management in Ad Hoc Networks,” ACM MobiHoc, June 2003.http://nesl.ee.ucla.edu/pw/ee206a/Zheng_MobiHoc03.pdf • C. Schurgers, V. Tsiatsis, S. Ganeriwal, and M. Srivastava, “Optimizing Sensor Networks in the Energy-Latency-Density Space,” IEEE Transactions on Mobile Computing, Janury-March 2002. http://nesl.ee.ucla.edu/pw/NESL/papers/2002/J18_2002_tmc.pdf • Recommended Reading • A. Cerpa and D. Estrin, “ASCENT: Adaptive Self-Configuring sEnsor Networks Topologies,” INFOCOM, June 2002.http://lecs.cs.ucla.edu/Publications/papers/ASCENT-Infocom-2002.ps • B. Chen, K. Jamieson, H. Balakrishnan, and R. Morris, “Span: An Energy-Efficient Coordination Algorithm for Topology Maintenance in Ad Hoc Wireless Networks,” ACM Wireless Networks Journal , Volume 8, Number 5, September, 2002.http://www.pdos.lcs.mit.edu/span/

3. Outline • Motivation • Algorithms • GAF • ASCENT • Span • STEM • Asynchronous Wakeup • Cone-Based • Conclusions

4. Topology Management Deciding on - which nodes turn on- when they turn on, and- at what Tx power …. So that desired network connectivity is maintained.

5. Motivation for Topology Control • High power • High interference • Low Throughput

6. Motivation for Topology Control • Low power • Low interference • High Throughput • Global Connectivity

7. Tradeoffs () • In sensor nodes, energy conservation desirable Energy conservation == longer network life • Energy   node density  • Energy latency  • Energy Tx power 

8. Energy Usage – Important Metric • Energy consumption of the radio dominates that of the sensors and CPU • perform event detection continuously • Observation: radios consume about the same energy in idle state than Tx and Rx state. • The only energy efficient mode of the radio is the sleep mode • put radio to sleep as often as possible

9. GAFGeography-informed Energy Conservation for Ad Hoc Routing Ya Xu, John Heidemann, Deborah EstrinUSC/ISI, UCLA Energy Node Density

10. Node Redundancy in Ad Hoc Routing

11. GAF: Geographic Adaptive Fidelity Energy  Density • Conserve traffic forwarding capacity • Divide network in virtual grids of size r <= radio_range/sqrt(5) • Each node in a grid is equivalent from a traffic forwarding perspective • Keep 1 node awake in each grid at each time • GAF reduces the energy by a factor M’ • This factor is a function of the average number of nodes in a grid: M Average number of neighbors of a node for uniformly random node deployment

12. GAF State Machine

13. ASCENTAdaptive Self-Configuring sEnsor Networks Topologies. Alberto Cerpa & Deborah EstrinUCLA Energy Node Density

14. Problem Description • Given an area A, a total number of nodes N distributed with a certain probability distribution  in A, we would like to find a subset of nodes that covers area A.

15. Problem Space • Goal: exploit the redundancy provided by high density to extend the system lifetime while providing a basic communication backbone that adapts to application needs • Problem: • few active nodes: • distance between neighboring nodes high  increase message loss • energy required to transmit the data over the longer distances will be prohibitive • too many active nodes: • at best, expending unnecessary energy; • at worst the nodes may interfere with one another by congesting the channel.

16. Neighbor Announcements Messages Help Messages Data Message Data Message Source Source Sink Sink Sink Source Passive Neighbor Active Neighbor (a) Communication Hole (b) Self-configuration transition (c) Final State ASCENT Basics • The nodes can be in activeorpassivestate. • Active nodes are part of the topology (or stay awake) and forward data packets (using an orthogonal routing mechanism). • Nodes in passive state can be sleeping or collecting network measurements. They do notforward any packets. • Each node measures the number of neighbors and packet loss locally. • Each node then makes an informed decision to join the network topology or to sleep by turning its radio off.

17. after Tt Test Active • neighbors < NT • and • loss > LT • loss < LT & help neighbors > NT (high ID for ties); or loss > loss T0 after Tp Sleep Passive after Ts State Transitions NT: neighbor threshold LT: loss threshold T?: state timer values (p: passive, s: sleep, t: test)

18. Energy Savings Analysis NT: # nodes that have radio on. Tp: passive state timer Ts: sleep state timer = Tp/Ts =PowerSleep/PowerIdle = 0.004

19. Energy Savings Energy Savings (normalized to the Active case, all nodes turn on) as a function of density. ASCENT provides significant amount of energy savings, with a factor of 5 for high density scenarios.

20. SpanAn Energy-Efficient Coordination Algorithm for Topology Maintenance in Ad Hoc NetworksBenjie Chen, Kyle Jamieson, Hari Balakrishnan, & Robert MorrisMIT Energy Node Density

21. Span • A power saving technique for multi-hop ad hoc networks that reduces energy consumption without significantly diminishing the capacity or connectivity of the network • Adaptively elects coordinators from all nodes in the network • Non-coordinators remain in power saving mode and periodically check to see if they should wake up and become coordinators

22. Goals • Ensure that enough coordinators are elected so that every node can reach one coordinator • Rotate the roles of coordinator amongst all nodes • Minimize the number of coordinators (without suffering from significant capacity loss or increased latency) • Decisions based on local information only

23. Coordinator Announcement • Eligibility Rules: if two neighbors of a non-coordinator cannot reach other within two coordinators, the node should be a coordinator

24. Randomized Back-off Delay • Announcement contention is resolved by delaying announcements with a randomized back-off delay • Er = energy remaining at node • Em = maximum amount of energy • Ci = number of new connections through node i • Ni = number of neighbors for node i • R = random number in [0, 1] • T = round-trip delay for packet over wireless link

25. Coordinator Withdrawal • Each node periodically checks whether it should withdraw as a coordinator • A node withdraws if all of its neighbors can reach each other directly or with other coordinators • For fairness, after some period of time, a coordinator withdraws if all neighbors can reach each other via other neighbors, even if these are not coordinators (allows neighbors to act as coordinators)

26. Results

27. STEMSparse Topology and Energy Management Curt Schurgers, Vlasios Tsiatsis, Saurabh Ganeriwal, Mani SrivastavaUCLA STEMEnergy Latency STEM + GAF Energy  Density Latency

28. Energy Conservation Strategy • Observe: Most of the time, the network is only monitoring, waiting for an event to happen • New strategy: put nodes to sleep and only wake them up when they need to participate in data forwarding Nodes have their radio in sleep mode to conserve energy Nodes turn on their radio, when they need to communicate

29. Listen TRx T Sleep time How can a Sleeping Node be reached? zzzzzzz • Solutions • Each node wakes up occasionally to listen for wake up messages • Low duty cycle paging – save energy.

30. A B Active mode Interference Polling mode C Sleep mode D f1 Sleep Active time Listen f2 Sleep time Implementation Issues • Problem: Wake up messages can collide with ongoing data Tx • Solution: Use separate channel for wake up messages.

31. Wakeup plane: f1 Data plane: f2 Principle of STEM • Low duty cycle paging channel to wake up a neighboring node • Use separate radio for the paging channel to avoid interference with regular data forwarding • Trades off energy savings for setup latency

32. Power f1 Tx Time Power f2 Tx /Rx Sleep Initiator node Target node Rx Power f1 Sleep Time Power f2 Tx /Rx Sleep High Level Operation of STEM

33. Detailed Operation of STEM Initiator node f1 B1 B2 1. beacon received Train of beacon packets TRx 2. beacon acknowledge f1 Target node

34. STEM: Wake up Messages • STEM-B: • Send wake packets (Beacons) to target node • Smaller Latency time (TS) than STEM-T • ACK sent back to initiator Node also turns on its data radio if it hears a collision while listening • STEM-T: • Send a Tone • All nodes hearing the tone wake up • No ACK

35. STEM-B T (s) Energy versus period for STEM-B STEM-B (collisions) STEM-T STEM-T STEM-B (no collisions) T (s) T (s) Energy versus period for STEM-T Average setup latency per hop versus Wake up period STEM: Energy Analysis • fw: wakeup frequency •  = fw . tburst •  = T / TRx •  = Pwakeup / Pdata

36. GAF: Geographic Adaptive Fidelity Energy  Density • Conserve traffic forwarding capacity • Divide network in virtual grids of size r <= radio_range/sqrt(5) • Each node in a grid is equivalent from a traffic forwarding perspective • Keep 1 node awake in each grid at each time • GAF reduces the energy by a factor M’ • This factor is a function of the average number of nodes in a grid: M Average number of neighbors of a node for uniformly random node deployment

37. STEM vs. GAF STEM Curve of comparable energy savings Leverage latency  Leverage density GAF

38. Combining STEM and GAF • As in GAF, 1 node is active in each grid • the grid can be considered a virtual node • Observe: In GAF, the leader has to keep its radio on all the time • Absence of traffic in the monitoring state not exploited • This virtual node runs the STEM protocol • Requires changes in leader election scheme

39. Relative energy saving versus density for TGAF = 5 hours Leader election overhead for STEM-T hours GAF STEM-B/T + GAF = 4 = 6 = 8 STEM-T + GAF = 92   STEM-T + GAF = 92 = 4, 6, 8   STEM + GAF: Energy Analysis • GAF leader election overhead increases with  •  affects choice of STEM-B or STEM-T STEM alone Energy Savings (compared to no topology management) as a function of density. STEM + GAF providesenergy savings, with a factor of 100 for practical settings

40. Asynchronous Wakeup for Power Management in Ad Hoc Networks Rong Zheng, Jennifer Hou, Lui ShaUIUC Energy Node Density

41. Transmit Receive Idle Sleep 1400mW 1000mW 830 mW 130 mW Wakeup mechanism • Why? • Idle radio power consumption is comparable to Rx, Tx consumption • Objective: Maintain network connectivity while reducing idle power consumption • 3 categories • On-demand wakeup • Scheduled rendezvous • Asynchronous wakeup (like scheduled rendezvous without time-sync) • Paper focus: Node on/off schedule problem can be formulated as block design problem

42. Asynchronous Wakeup Mechanism Sleep time­ ÞEnergy¯ BUT Neighbor Discovery Latency ­ • Nodes follow an a listen/sleep cycle • Problem: • Each node follows a listen/sleep schedule of T slots • Objective: Minimizeku, kv (energy minimization) • Given: T = schedule duration • Constraint: m = number overlapping slots (delay constraint) • Symmetric design (ku=kv) • Asymmetric design ((ku¹kv) active slots node U ku=4 T-1 0 kv=3 node V

43. 124 235 346 457 561 672 713 1 2 3 4 5 6 7 slot Solutions • Symmetric Design • Every node chooses the same number of active slots AND circular shifts of the same schedule: • Asymmetric design • The asynchronous wakeup schedules should be generated every time there is a topology change Þinefficient (depends on the topology) • Could be used for heterogeneous networks (powerful/constrained nodes) Necessary condition: Examples of schedules: (T, k, m)=(7,3,1) (T, k, m)=(73,9,1) T: schedule length k: active slots m: overlap slots Necessary condition:

44. I T Implementation • Slot alignment needs time-sync and slots may shift • Introduce a protocol that allows a node to: • Discover the state of the neighbor nodes (listen/sleep) • Keep track of the neighbor schedules so that a link can be formed • Nodes learn other nodes’ schedules and transmit only when recipient is active Frame structure Node U Slot duration Schedule duration Node V U can hear V’s beacon V can hear U’s beacon

45. Active Mode PS Mode Wakeup Schedule Tx/Rx awake idle sleep Power Management Policy Power Management with Asynchronous Wakeup • Problem: If nodes use the listen periods for packet transmissions then bandwidth is reduced • Solutions: • Slot-based power management • Slot-by-slot link reservation when buffer exceeds threshold • On-demand power management • PS Mode -> Active : communication events • Active -> PS mode : soft state timers

46. Performance Evaluation – Neighbor Discovery

47. Performance Evaluation – Neighbor Discovery

48. Network Statistics - Static Networks

49. Network Statistics – Mobile Networks

50. Summary of Asynchronous Wakeup • Theoretical results about an optimal slotted listen/sleep schedule • Asynchronous wakeup protocol design • One signaling layer • Power management techniques: • Slot-based • On demand