Post on 18-Jan-2016
description
A Mini-survey of Dealing with Faults in Wireless Sensor Networks
Qi Han
Motivation
Current WSNs exhibit high loss rates In indoor environments,
• half of the links: 10% packet loss• A third links: 30% packet loss
At MAC layer, link-layer re-transmissions are unable to mask this loss
Assuming packet loss rateis p, then the probability that a message is successfully receivedAcross n hops is (1-p)n
Understanding packet delivery performance In dense wireless sensor networks J. Zhao and R. Govindan, SenSys 2003 (Best Paper Award)
Taming the underlying challenges of reliable multihop routing in sensor networksA. Woo, T. Tong and D. Culler, SenSys 2003
Strategies to deal with faults
MAC layer Apply ARQ
Network layer: Select high quality paths for data transmission Multi-path routing
• Braided Diffusion• GRAB (Gradient Broadcast)
Transport layer Downstream data delivery
• PSFQ, GARUDA Upstream data delivery
• TAG, ESRT
Braided Diffusion
How to perform energy-efficient and robust dissemination of data from sources to sinkstradeoff between resilience and energy
consumed Based on directed diffusion:
Construct dissemination path from multiple sources to multiple sinks on-demand
D. Ganesan, R. Govindan, S. Shenker, D. EstrinMobiHoc 2001 and MC2R 2002
Directed Diffusion
a) Source periodically broadcasts events at a low rateb) Sink sends a reinforcement message to one of its neighborsc) The message is propagated to the source, hop by hopd) When a node on the reinforced path fails, the sink re-initiates
reinforcement
Drawback: A periodic low-rate flooding scheme notifies the sink and other nodes of available alternate paths --- Consumes energy
Disjoint Multipath
Localized algorithm:-using local information- use two kinds of reinforcements
Braided Multipath
- Alternate paths in a braid are partially disjoint from the primary path
Failure Models (used for evaluation) Isolated failures:
capture independent node failures Patterned failures:
capture geographically correlated failures
Gradient Broadcast
The sink builds a cost field Cost at a node: minimum energy
overhead to forward a packet from this node to the sink along the path
The cost field gives the global direction towards the sink implicitly
• At each hop, only nodes that have costs smaller than the sender can forward the packet
F. Ye, G. Zhong, S. Lu, L. X. ZhangACM WINET 2005
Credit-based Forwarding Mesh
Limit the ‘width’ of the forwarding mesh More than enough paths of
decreasing cost exist A source assigns a credit to
the packets it sends out• Credit: An extra budget that
can be used to send a packet to the sink along a path
• The amount of credit controls the redundancy of the mesh
Reliable Downstream Sensor Data Delivery Data flows from sink to sources for the purpose of
control or management PSFQ
Assumptions: message loss occurs due to the poor quality of wireless links
Hop-by-hop recovery: node In-sequence forwarding
GARUDA: reliable delivery • To all sensors, • To a sub-region, • To minimal sensors to cover the sensing field• To a certain percentage of the sensors
Reliable Upstream Sensor Data Delivery Data flows from sources to sink TAG (Tiny Aggregation):
A node switches its parent in two cases:• Each node monitors the quality of the link to each
of its neighbors by tracking the proportion of packets received from each neighbor
• When a node observes that it has not heard from its parent for some fixed period of time, it assumes that its parent has failed
ESRT: Event-to-Sink Reliable Transport in WSN
Event!
A sensor node
A sensor node that can sense the event
Sink wants reliable event detection with minimum energy expenditure
[Y. Sankarasubramaniam, O. B. Zkan, I. F. Akyildiz, ACM MobiHoc 2003]
[Slides modified based on the class presentation of A. Abouzeid from RPI]
Problem Definition
Motivating application: Reliable detection/estimation of event features based on the
collective reports from a number of sensors, not on individual sensor reports
The sink must decide on the event feature every time units Definition
The reliability of even feature is measured by the number of received data packets
• Observed event reliability ri • Desired event reliability R
Problem statement: (congestion solution) Model any increase in source information as a increase in
the sensor reporting rate f To configure the reporting rate f of source nodes so as to
achieve the required event detection reliability R at the sink with minimum resource utilization
Evaluation Environments-- to study the relationship between f and r
ns-2 simulator 200 sensor nodes 100m x 100m area 40m transmission range 30 byte packets 65 packets buffer size 10 sec decision interval (τ)
Effect of varying sensor reporting rate f on the event reliability r
network gets congested sooner with increasing number of source nodes
r linearly increases with f until f=fmax, then drops After fmax, it is wavy
with increasing n, the drop in r is more significant
This confirms the need for a reliable transport solution with a congestion control mechanism
CongestedNot Congested
Lower reliability than required
Higher reliability than required
OOR
Five characteristic regions
Goal: To stay in
OOR where energy
expenditure is optimal
R
r
Main Idea of ESRT
Sink Based on current state Si, calculates a updated
reporting frequency fi+1, broadcasts it to sensor ndoes
• Si {(NC,LR),(C,LR)}: aggressively update f to reliably track event ASAP (Primary objective: reliably detect event)
• Si{(NC,HR),(C,HR)}: decrease f conservatively (Secondary objective: conserve energy)
Sensors Listen to the sink broadcast at the end of each
decision interval and update f Deploy a local congestion detection support
mechanism
ESRT Actions
Network State
Action
(NC,LR) Multiplicatively increase f,
Achieve required reliability ASAP
OOR f remains unchanged
(NC,HR) Decrease f conservatively
Cautiously reduce energy consumption while not compromising reliability
(C,HR) Decrease f carefully but aggressively to (NC,HR) to relieve congestion
Then, follow (NC,HR) behavior
(C,LR) Decrease f exponentially to relieve congestion ASAP
i
ii
ff
1
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1(21
i
ii
ff
i
ii
ff
1
)/(1
kii
iff
Stability of ESRT
ESRT converges to OOR from any of four initial states {(NC,LR), (NC,HR), (C,HR), (C,LR)}
From (NC,HR), ESRT stays in the state until converges to OOR Convergence time depends on ε – smaller ε
causes longer convergence time
Congestion Detection Congestion status is required at the
sink to determine the network state Based on expectation of buffer
overflow at sensor nodesDuring a single interval, f and n do not
change much If pending congestion (bk+b>B) is
detected CN bit is set in event reports
From (NC,LR)
Reaches OOR in two intervals
From (NC,HR)
ESRT stays in (NC,HR) until
reaching OOR in five intervals
(C,HR) to (NC,HR) then OOR
(C,LR) to (NC,LR) then OOR
Power savings from (NC,HR)
Reporting rate gets reduced conservatively
while maintaining reliability
What I like about the paper
Collective reliability• Individual sensor ID is not necessary• Each source attaches event ID
Biased implementation• Almost entirely in sink
What I dislike about this paper Sink must broadcast the updated reporting
frequency at high energy so that all sources can hear it Ongoing event transmission would be
disrupted Regulating all sensors to have the same
reporting rate may not work well with heterogeneous sensors
Assuming that sensors report periodically may not be true for all applications
Congestion in WSN not just caused by frequent sensor reporting
My class project- reliable upstream data delivery
Acquisitional query How to interpret the answer A
Is A based on a very incomplete subset What about the remaining sensors
Query may specify its reliability requirements(how good it wants A to be) Percentage of sensors (e.g. A is based on
reports from 80% of the sensors) Recall (answer-set/exact-set)
Quality Metrics
Sensors of interest N
Answer Nyes
Missing at most N-Nyes-Nno
NnoAnswer Nyes
Answer Nyes
recall r=Nyes/(N-Nno)
Problem Description
Given: 1 sink (where continuous queries are injected) and n sensors with constant failures in the sensor network
Objective: minimize energy consumption
s.t. r>=R, where r=Nyes/(N-Nno)
Issues to Address
How to distribute rq from the sink to intermediate nodes ?
Given the assigned rq’, what should each
intermediate node do after finishing the transmission of the reports from all children? Go to sleep immediately Re-transmit immediately Stay awake in case re-transmission is
requeted
Our Approaches
To come
Evaluation Methodology
ns-2 simulator Comparison against
Plain collection (CSMA/CA)Link layer acknowledgement
(CSMA/CA + ack/re-transmission)Multi-path routing