practical asynchronous neighbor discovery and rendezvous for mobile sensing applications prabal...
TRANSCRIPT
Practical Asynchronous Neighbor Discoveryand Rendezvous for Mobile Sensing
Applications
Prabal Dutta and David CullerComputer Science Division
University of California, Berkeley
ACM SenSys 2008
Outline Introduction Related Work Proposed Method Simulation Conclusion
Introduction Challenge for asynchronous neighbor discovery:
Awake infrequently Co-located discover each other Without any prior knowledge of their potential encounters Without external assistance
Introduction Two requirements
Low-power operation Active vigilance
These are at odds with each other since optimizing for one may come at the expense of the other.
Related Work Prior Work in asynchronous neighbor discovery:
Stochastic McGlynn and Borbash proposed “Birthday Protocols”
Quorum Tseng et al. propose a quorum-based protocol for multihop ad hoc
networks
Combinatorial For asymmetric duty cycles, the approach reduces to an NP-comple
te minimum vertex cover problem requiring a centralized solution.
Goal Discovery can be quite valuable in mobile networks
Discovery should be a fundamental and continuous service in both mobile and static networks
“Disco” addresses a more general set of neighbor discovery problems and avoids the need for a randomized protocol by using pairs of primes
Proposed method – using Chinese Remainder Theorem
Choosing a prime
Starting counting at reference period
Two nodes, i and j, pick two numbers, mi and mj , such that mi and mj are relatively prime (coprimes)
Example
Node i x ≡ 1 (mod 3)
Node j x ≡ 2 (mod 5)
prime
Starting counting period
Duty cycle
Duty Cycle (DC) = 1/mi
Node i x ≡ 1 (mod 3)
Node j x ≡ 2 (mod 5)
DC = 1/3 =33%
DC = 1/5 =20%
Coprimes are not enough The moduli cannot be chosen independently by the nodes
Such choices could lead to values of mi and mj that are not coprimes.
Restricting the moduli to coprimes is not scalable There are only a handful numbers that can satisfy both the target duty c
ycle and coprime requirement.
If mi = mj, then node i and j may never discover each if they wake up with the same period but different phase.
Example
x 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
ci - - 0 1 2 3 4 5 6 7 8 9 10 11 12 13
cj - 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Node i x ≡ 1 (mod 3)
Node j x ≡ 2 (mod 3)
Coprimes are not enough (cont.) To require each node i to pick two primes, pi1
and pi2, such that
pi1≠ pi2
ex: Duty Cycles are 97 and 103 (1/97 + 1/103 = 2%)
For every pair of nodes i and j, there will be at least one pair in the set {(pi1
, pj1 ), (pi1
, pj2 ), (pi2
, pj1 ), (pi2
, pj2 )} that are
relatively prime ex: (pi1
, pi2 ) = (30, 77) and (pj1
, pj2 ) = (35, 66)
x = 30k and x = 77k, for all k Z ∈ +
x = 35k + 1 and x = 66k + 1, for all k Z∈ +
Choosing Primes
Low discovery times are possible: one of the primes is very close to the reciprocal of the duty cycle the other prime is a much larger number
The limit of the ratio between the auspicious and unfortunate worst-case latencies is
Node i : (53, 883)
Node j : (97, 103)
883 × 103 = 9094953 × 97 = 5141
Slot Non-Alignment To maximize the likelihood that overlapping slots
result in discovery, Disco transmits a beacon at both the beginning and end of a slot.
Duty Cycle form Discovery Latency Maximum discovery latency: tdisco,
Without loss of generality, p = pi1 = pj1
The minimum duty cycle, DC, must satisfy the following inequality
The minimum required beacon rate:
Simulation Study
Programming language nesC
Operating System TinyOS
Empirical Sensor Node Telos Wiress sensor node
Discover Latency Comparison
Discovery Latency: A Deeper Look
prime pairs: (23,157), (29, 67), (31, 59), (37, 34)
Impact of Duty Cycle Asymmetry
Empirical Evaluation
Conclusion This paper presents a practical solution to the low-
power asynchronous neighbor discovery problem.
This simple protocol achieves discovery faster than other discovery protocols for a given duty cycle, allows nodes to independently select their own duty cycle.
Thank you