Practical Asynchronous Neighbor Discovery and Rendezvous for Mobile Sensing Applications

1. Practical Asynchronous Neighbor Discovery and Rendezvous for Mobile Sensing Applications PrabalDutta and David culler 6th ACM conference on Embedded network sensor systems (2008) JiheeRyu , Keunchan Park, Jaehyun Han, and Kihoon Cha

2. Contents • Introduction • Problems & Solution Approach • Related Work • Simplified Algorithm • Disco Example • Design • Why two primes are chosen? • Pair of Primes • Slot non-alignment • To Avoid Choosing The Same Pair • Discussion • Pros & Cons

3. Introduction • Mobile object interacts with other mobile and static objects • 3 common patterns – talking, docking, and flocking • Neighbor Discovery • A sensor node discovering another (one-hop) sensor node • Rendezvous • Delivering messages to previously discovered neighbors Docking Flocking Talking

4. Introduction(cont’) • Characteristics of Sensor Network • Nodes may have different capabilities • Energy • Functionality • Mobility • Nodes operate at different (asymmetric) duty cycle • Duty Cycle (DC): The fraction of time that a system is in an "active" state • Nodes starts asynchronously • No synchronization

5. Problems & Solution Approach

6. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 28 25 26 27 29 30 31 32 33 34 35 36 Related work m t • Quorum [Tseng02] • Listen during a row • Transmit during a column • Global agreement on duty cycle T m L R • Birthday [McGlynn01] • Randomly choose to listen, transmit or be idle (sleep) • Unpredictable

7. Simplified algorithm • Two nodes, i and j • reference period number, x • start their counters ci and cj • at arbitrary times, say x = 1 and x = 2 • increment counters with equal period Tslot • and wake up at some relatively prime intervals, say mi = 3 and mj = 5 • Dark cells indicate times when node i and j turn on radios • Both nodes are awake at times x = 7 and x = 22 • x= 7+15k, k  Z+ • Disco uses two primes/node to ensure pair-wise relative primes

8. Disco Example • Node i is awake at times: • 5, 10, 15, 20, 25, 30, 25, and • 7, 14, 21, 28, 35,… • Node j is awake at times: • 1, 6, 11, 16, 21, 26, 31, and • 1, 8, 15, 22, 29, 36, … • Nodes i and j are both awake at: • 15, 22 • Two primes per node ensures even if both nodes pick same primes, discovery will occur

9. Why two primes are chosen? • The prime cannot be chosen independently by the nodes • Restriction of the coprimes to expression their desired duty cycle • (1, ½, 1/3, … 1/k, where k  Z+) • If prime of node i equals prime of node j, then nodes i and j may never discover each other if they wake up with the same period but different phase

10. Pair of Primes • To Overcome Previous Limits • Choosing two primes, p1 and p2 • Such that p1≠ p2 and the sum of their reciprocals (approximately) equals the desired duty cycle • Simple Validation • Ensuring a coprime pair from any two nodes • A coprime pair is exist in the set of probable pairs • E.g. node i picks (3, 5) and node j picks (5, 7) •  {3,5} / {3,7} / {5,5} / {5,7} 3 rendezvous cases • E.g. node i picks (30, 77) and node j picks (35, 66) •  {30,35} / {30,66} / {77,35} / {77,66}  0 rendezvous cases

11. Slot non-alignment • In practice, slots will rarely be aligned becausenodes are… • run independently • do not adjust clock skews • To discover other node, • transmits a beacon at both the beginning and end of a slot when beaconing Slot Node i Node i Beacon Node j Node j

12. To Avoid Choosing The Same Pair • Assumption • There are classes which do not need to discover each other • Deterministic algorithm • An ordered list of possible prime pairs are generated • A prime pair’s position indicate a particular class’s modulo • Example • Target duty cycle = 1%, Number of classes = 3

13. Glossary • Balanced primes • the intra-node primes are approximately equal (e.g.37 and 43). • Unbalanced primes • the intra-node primes are significantly different (e.g.23 and 157). • Symmetric pairs • both nodes choose the identical pair of primes • Asymmetric pairs • both nodes choose a different pair of primes • Discovery Latency • the delay between the moment two nodes are within communications range to the moment when they first discover each other

14. Implementation • t_slot = 10 ms • An Application can control • Parameters • Ex) DC • The beaconing policy • Listening • Beaconing Slot Beacon Power

15. Beacon Rate Adaptation • By the implementation, beacon mode can be changed • Sending beacons during their normally scheduled rendezvous slots with nodes in their neighbor table to ensure that routing links remain connected and synchronized, but they would not send other beacons • Sharing neighbors information, new node candiscover neighbor more faster than expected

16. Impact of Duty Cycle Asymmetry • Suggests that more powerful beacons combined with less powerful mobile tags is feasible and beneficial Mobile (Low DC) Mobile (Low DC) Mobile (Low DC) Static (High DC)

17. Predictable 2%

18. Robustness to Clock Skew • Over longer timescales, nodes that have significantly different skews are likely to have difficulty with rendezvous • but asynchronous discovery should still work Node i Node j Node k

19. Pros • Algorithm is simple • A solution to the low-power asynchronous neighbor discovery problem in mobile sensing applications • It is possible to make decision independently at node sides

20. Cons • Why they use a slot numbers at evaluation? • Different discovery time for different nodes • Restriction of possible number of frequency

21. Discovery Rate • Small time slot makes neighbor discovery faster

22. Why they use a slot numbers at evaluation?

23. Different discovery time for different nodes 29, 67 23, 157 23, 157 29, 67

24. Restriction of possible number of frequency • 1% duty cycle : 21 pairs • 5% duty cycle :4 pairs