Rendezvous Planning in Mobility- assisted Wireless Sensor Networks Guoliang Xing; Tian Wang; Zhihui...
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Transcript of Rendezvous Planning in Mobility- assisted Wireless Sensor Networks Guoliang Xing; Tian Wang; Zhihui...
Rendezvous Planning in Mobility-assisted Wireless Sensor
Networks
Guoliang Xing; Tian Wang; Zhihui Xie; Weijia Jia
Department of Computer Science City University of Hong Kong
Agenda
• Motivation
• Problem formulation
• Rendezvous planning algorithms– Optimal algorithm under limited mobility– Heuristic under unlimited mobility
• Protocol design
• Performance evaluation
Challenges for Data-intensive Sensing Applications
• Many applications are data-intensive – Structural health monitoring
• Accelerometer@100Hz, 30 min/day, 80Gb/year
– Micro-climate and habitat monitoring• Acoustic & video, 10 min/day, 1Gb/year
• Most sensor nodes are powered by batteries• A tension exists between the sheer amount of
data generated and the limited power supply
Mobility-assisted Data Collection
• Mobile nodes move close to sensors and collect data via short-range communications
• Number of wireless relays is reduced
• Mobile nodes are less power-constrained– Can move to wired power sources
Mobile Sensor Platforms
• Low movement speed (0.1~2 m/s)– Increased latency of data collection– Reduced network capacity
Networked Infomechanical Systems (NIMS) @ CENS, UCLA
Robomote @ USC [Dantu05robomote]
XYZ @ Yale http://www.eng.yale.edu/
enalab/XYZ/
Rendezvous-based Data Collection
• Some nodes serve as “rendezvous points” (RPs)– Other nodes send their data to the closest RP– Mobiles pick up data from RPs and transport to BS
• In-network caching + controlled mobility– Mobiles can collect a large volume of data at a time– Mobiles contact static nodes at RPs at scheduled
times and disruptions to network topology are reduced
mobile node
rendezvous point
Rendezvous-based Data Collection
source node
The field is 500 × 500 m2 The mobile moves at 0.5 m/s
It takes ~20 minutes to visit six randomly distributed RPs
It takes > 4 hours to visit 200 randomly distributed nodes.
Assumptions
• Only one mobile is available
• Average speed of mobile is v m/s
• Mobile picks up data at locations of nodes
• Data collection deadline is D seconds– User requirement: “report every 10 minutes and
the data is sampled every 10 seconds”– Recharging period: e.g., Robomotes powered
by 2 AA batteries recharge every ~30 minutes
Geometric Network Model• Transmission energy is proportional to distance• Base station, source nodes and branch nodes
are connected with straight lines
a multi-hop route is approximated by a straight line
Source nodes
Source nodes
approximated data route
real data route
Non-source nodes
Branch nodes
Rendezvous points
a branch node lies on two or more source-to-root routes
The Rendezvous Planning Problem
• Choose RPs s.t. the data collection tour of mobile node is no longer than L=vD
• Total network energy of transmitting data from sources to RPs is minimized
• Joint optimization of positions of RPs, motion path of mobile, and routing paths of data
Illustration of Problem Formulation
Objective: minimize length of routes from sources to RPs
Constraint: mobile tour is no longer than L=vD
The problem is NP-hard
Source nodes
Rendezvous points data route
branch nodes
Rendezvous Planning under Limited Mobility
• The mobile only moves along routing tree– Simplifies motion control and improves reliability
XYZ @ Yale
An Optimal Algorithm
• Sort edges in the descending order of the number of sources in descendents
• Choose a subset of (partial) edges from the sorted list whose length is L/2
• The mobile tour is the pre-order traversal of the chosen edges
• Set the intersections between the tour and the routing tree as RPs
2
3
Illustration
• All edges have a length of 50m• Deadline = 500 s, v = 0.5 m/s• L = 0.5 m/s x 500 s = 250 m
Correctness• Edges chosen are connected
Optimality• A tour can cover at most L/2 edges• L/2 mostly 'used' edges are chosen
# of sources in the descendents
1
1
1 1
3
A Heuristic under Unlimited Mobility
• Add virtual nodes s.t. each edge is no longer than L0
• In each iteration– Choose the RP candidate x with the max utility defined by c(x)– Remove RPs with zero utility
• Terminate if all sources become RPs or no more RPs can be chosen without violating the constraint of L
c(x) = the increased length of the mobile tour
the decreased length of data routes
obtained by running a Traveling Salesman
Problem solver
Agenda
• Motivation
• Problem formulation
• Rendezvous planning algorithms– Optimal algorithm under limited mobility– Heuristic under unlimited mobility
• Protocol design
• Performance evaluation
Initialization• Mobile computes locations of RPs • Find real nodes around the computed RPs
– Find the nodes along the routing tree– Mobile travels to RPs and discover real nodes
Source nodes
Source nodes
Rendezvous points
approximated data route
real data route
Non-source nodes
Handling Unexpected Delays
• Movement of mobile node is subject to various delays– Obstacles, mechanical failures…
• RPs should cache data as long as possible without violating the deadline
• Mobile node may adjust motion path online e.g., skips some of the RPs
Simulation Settings
• 100 sources are randomly distributed in a 300m X 300m field, base station is on the left corner
• Each source generates 2 bytes/second, delivery deadline is 20 minutes
• Implemented USC model [Zuniga et al. 04] to simulate lossy links on Mica2 motes
• Baseline algorithms– NET: collect data via the routing tree without using mobile nodes– Sector: mobile moves on a sector of 45o
– RP-CP: the optimal algorithm with limited mobility– RP-UG: the utility-based heuristic– RP-SRC: choose a subset of sources as RPs
Conclusions
• Proposed a rendezvous-based data collection approach– In-network caching + controlled mobility
• Developed two rendezvous planning algorithms– An optimal algorithm under limited mobility– A efficient heuristic under unlimited mobility
• Designed the rendezvous-based data collection protocol