1. CMPE 259: Sensor Networks Topology Control Manikandan Punniyakotti

2. Paper-1: PEAS A Robust Energy Conserving Protocol for long-lived Sensor Networks Fan Ye, Zhong, G, Cheng, J, Songwu Lu, Lixia Zhang Proceedings of the 23rd International Conference on Distributed Computing Systems, 2003.

3. Basic ideas in the paper Maintain a necessary set of working nodes and turn off the redundant ones Sleeping nodes wake up once in a while and probe their neighborhood – replace any failed working nodes Nodes depend only on local information * no per-neighbor state * no information about topology of network * no lifetime estimation of neighbors

4. Comparison with other work Protocols designed for ad-hoc networks assume dynamic changes in connectivity but not frequent node failures GAF, SPAN, ASCENT, AFECA: They either depend on predictability of individual nodes’ lifetime or require each node to maintain state of all its neighbors On the contrary, PEAS just assumes that density of deployed nodes >> density of working nodes and that nodes may fail unexpectedly

5. PEAS : Design Probing EnvironmentAdaptive Sleeping

6. Probing Environment Sleeping Initial state Sleep for an exponentially distributed duration – using a PDF Probing To detect working nodes Send PROBE Receive REPLY? Sleep! No REPLY? Work! Working Work until energy drained or node failure Probing Range Probing Rate PDF used

7. Things to note Multiple working nodes in range = Collision of REPLIES. Wait for a RANDOM time before sending REPLY Once REPLY is received, go to sleep for another random time. But will change depending on information in REPLY message. Initial decides how fast network has enough working nodes during boot-up. Probing range determines redundancy of working nodes; specified by application.

8. Design Rationale All nodes are equal (don’t choose based on energy/degree) Number of nodes more important than capability of nodes PEAS maintains enough working nodes = proper sensing/communication Location based probing = Working nodes are appropriately placed Node’s location decides if it should be turned on or not Sleeping time of node is randomized Wake ups are spread over time Synchronized periodic wakeup assumes reliable & predictable environment Reduces collisions caused by synchronized wakeups

9. Adaptive Sleeping To adjust of each sleeping node (as their no. vary) To keep the aggregate rate at about a desired rate given by the application Counter N (how many probes) : time when N is set to 0. Threshold K (no. of probes) Calculate New Aggregate using: Send and in REPLY Nodes calculate their new rate using New sleep duration by

10. Explanation Probings from different sleeping neighbors construct a Poisson Process with parameter given by: Calculating directly using the above equation is difficult as a working node should keep a per- neighbor state of A working node cannot know when it has collected all from its neighbors If a neighbor fails while asleep, working node cannot determine if it has failed or it has a very long sleeping period

11. Explanation… should be close enough to. Hence K should be large enough. Using Central limit theorem, K > 16 gives 99% accuracy that the measured value has only 1% error compared with real value. Authors use K=32 to accommodate the random wait time before REPLY and communication & processing latencies The new rate calculated will be approximately equal to the desired rate as shown

12. Discussion Compensate Packet Losses through multiple PROBEs and REPLIES (Authors use 3 PROBES & REPLIES) Working node hearing a REPLY is put to sleep. The one that had been working for a long time is preferred. Probing nodes with more than 1 working neighbors hear multiple replies; Uses the lowest probing rate. Nodes with fixed transmission power use a threshold to filter out Deployed nodes should be dense enough everywhere. Even distribution not necessary but preferred.

13. Simulation Implement in PARSEC, hardware parameters similar to Berkeley Motes Power consumption: Tx 60 mW, Rx 12mW, idle 12mW, sleep 0.03mW Initial energy is in range of 54 ~ 60 Jules Sensing & max Tx range is 10m Communication Capacity: 20Kbps Size of PROBE and REPLY: 25bytes Area of Simulation: 50 x 50 sq. meter Nodes are uniformly distributed and stationary Source & sink placed at opposite corners of the field Source generates data report every 10s GRAB forwarding protocol is used Initial probing rate = 0.1 wakeup/sec Probing range is 3m Desired aggregate probing rate is 0.02 wakeup/sec No. of nodes: 160, 320, 480, 640

14. Results

15. Energy for 1 wakeup = 0.00316 Joule ( Includes energy spent for 3 Probes + 3 replies + 100ms idling during which working nodes randomly back off beforing replying)

16. With Node Failures * 480 node network * Failure rate of 5.33 to 48 failures per 5000s in steps of 5.33 * The maximum failure rate case had about 38% node failure * Coverage life time dropped by 12% to 20% only. * Data delivery lifetime also dropped by 20%. * No. of wakeups also decrease as failure rate increases (less sleepers for high failure rates). * Energy overhead is constantly less than 0.25%

17. Paper-2: EECDS Energy efficient distributed connected dominating sets construction in wireless sensor networks Zeng Yuanyuan, Xiaohua Jia, He Yanxiang IWCMC '06 Proceedings of the 2006 international conference on Wireless communications and mobile computing

18. Dominating set (DS): A subset of nodes such that each node in graph is either in the subset or adjacent to at least one node in the subset Connected DS: A DS that is a connected sub- graph CDS is a good candidate for the back-bone. Non- back-bone nodes turned off when not transmitting Goal is to keep the no. of nodes in CDS minimum MCDS is NP-hard Basic Idea of the Paper

19. Two Phases MIS construction Add minimal number of connectors 1.MIS: Maximal Independent Set 2.MIS is a DS. Hence to make sure it is connected we have to add minimal number of connectors (non-MIS nodes)

20. MIS Construction 4 States: White, Transition, Grey, Black 4 message types: Black msg, Grey msg, Inquiry, Reply INQUIRY is to know the weights and states of neighbors Weight is calculated based on the battery power and effective degree of a node (no. of neighbors in white and transition state Nodes in transition state compete to become black nodes. Send INQUIRY to neighbors and start a timer Note: Each grey node keeps a list of all its black node neighbors

22. Set of Black Nodes = MIS Set of black nodes is an independent set Black nodes cannot have an edge between them (according to the algorithm that proceeds layer by layer) It is an MIS: Algorithm ends with grey and black nodes only All grey nodes have at least one black neighbor. Hence coloring a grey node black is not possible.

23. CDS Construction MIS node can be in 3 states: Black, B-transition, Blue Non MIS node can be in 3 states: Grey, G-transition, Blue 3 types of messages: BLUE, INVITE, UPDATE INVITE: sent by MIS node to a non-MIS node to invite to become a connector UPDATE: sent by a non-MIS node to notify its neighbors about its current state Here weight is based on battery power and no. of black neighbors

25. Simulations Network size: 100-300 nodes, in steps of 50 nodes Area: 160 x 160 square field Tx range: 30-50m All time-outs are 100ms New back-bone construction is done when energy level falls to 50%

27. Paper-3: A3 A3: A Topology Construction Algorithm for Wireless Sensor Networks Wightman, P.M, Labrador, M.A IEEE Global Telecommunications Conference, 2008. IEEE GLOBECOM 2008

28. Basic Idea Just like EECDS, A3 also makes use of a sub- optimal Connected Dominating Set to construct the back-bone Turn off other nodes when they don’t have data to transmit Makes use of a weighted distance-energy metric to trade-off length of braches (distance) for robustness and durability of tree (energy) Unlike other CDS based schemes, A3 has a low linearly bounded worst-case msg complexity

29. Topology Control’s Two Components Topology Construction Finds a reduced topology while preserving properties like coverage, connectivity etc Topology Maintenance Changes the reduced topology when it can’t provide the service any longer These two mechanisms work iteratively until the network energy is depleted. This kind of iterative operation will increase the lifetime of sensor network

30. Advantages of A3 Completely Distributed Does not need location information No synchronization scheme needed Simple and low computational overhead Energy efficient CDS construction is done in 1 phase and node contests/two-hop information query overhead are absent Low linearly bound message complexity

31. A3 Algorithm Assumes no knowledge of position or orientation of nodes But, nodes determine how far a node is based on the strength of the signal received Works in 2 moments: * Neighborhood discovery * Children selection & second opportunity

32. Neighborhood Discovery Sink sends HELLO msg and starts a timer Uncovered nodes that receive it, mark themselves as covered and choose the sender as parent and send back a PARENT RECOG msg (This contains a metric based on strength of the received signal & remaining energy in the node) Already covered nodes ignore the HELLO msg If Parent node does not receive a PR msg back before the timer expires, it goes to sleep

33. Children Selection After the time-out, parent sorts the list of its neighbors (who replied) in descending order based on the selection metric Parent broadcasts a Children Recognition msg with this sorted list Candidate nodes that receive this msg, set a time-out period proportional to their position in the list. Candidate nodes wait for a sleep msg from their brothers. On reception of a sleep msg, node goes to sleep Nodes that didn’t receive this sleep msg (since they were outside the range of the well-qualified brother) will start their own process of finding children

34. Second Opportunity Sometimes, a node put to sleep may be a bottle- neck access to a section of the network All nodes start a timer at the reception of sleep msg. When this timer expires, they wake up and send a HELLO msg looking for uncovered children

35. THEOREM: If the initial graph is connected, all nodes will finish in any of the final states: Active or Sleeping Assume there exists an uncovered node. All covered nodes are forced to send a HELLO msg and hence all covered nodes will explore all their edges finding uncovered nodes. But we assumed there exists an uncovered node. This means it does not have an edge to any of the covered nodes. Hence the given graph is not connected. Contradiction.

36. Selection Metric When nodes receive HELLO msg, they calculate the selection metric by X: candidate node Y: Parent node : Weight of rem energy : Remaining Energy in X : Max initial energy : Weight for distance from Parent : Received signal strength from Parent : Min RSSI to ensure connectivity

37. Evaluation 3 sets of experiments * Changing the node degree * Changing the node density * Ideal grid topologies Changing node degree is through changing the CTR Metrics: 1)Number of active nodes 2)Number of msgs used to build CDS 3)Amount of energy spent for that process

40. A3 covered same or more area with lesser overhead and resource utilization