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SEP: A Stable Election Protocol for clustered heterogeneous wireless sensor networks
Ibrahim Matta
Joint work with
Georgios Smaragdakis and Azer Bestavros
SANPA 2004Second International Workshop on Sensor and Actuator Network Protocols and Applications
August 22, 2004
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 2
Heterogeneous WSN
o Nodes have different energy levels
o Initial setting, after some operation time, or after re-energizing some sensors
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 3
Routing
o Goal:prolong network lifetime/coverage
o Direct transmission to sinko Min-energy routingo Sensing process can become
biased
o LEACH – Low Energy Adaptive Clustering Hierarchy [Heinzelman et al., 2000]
sink
Cluster-head node
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 4
Selecting Cluster-heads in LEACHo Node i chooses random number, s, between 0 and 1o If s < T(i), node i becomes a cluster head in current
round where:
where: P = desired percentage of cluster headsG = set of nodes that have not been a cluster head in the last 1/P rounds
o Each node is elected cluster-head once every 1/P rounds (epoch length)
o On average, n x P nodes elected per roundn n = total number of nodes
{=?
× G i if
)modr(P -1P
otherwise 0P1
)i(T
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 5
Rotating Cluster-heads
Round r Round r + 1
When all nodes start with the same energy level (i.e., homogeneous setting),nodes “randomly” die and within a short period
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 6
But in a heterogeneous environment…
o The result of initial setting or evolution of the sensor network’s operationn “advanced” node has α times more energy than “normal”
node
o Once the first node dies, feedback and cluster-head election stays unreliable for a long timen advanced nodes don’t get elected as often as they should
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 7
Naïve Modification to LEACH
o View network as homogeneousn Every “virtual” node is “normal” with unit energyn We have n + α X m x n = n (1+ α X m) “virtual” nodeso m = fraction of nodes that are advanced
o Extend epoch length to (1+α x m)/P roundso Hope is:n Each normal node is elected cluster-head once every
epochn Each advanced node is elected cluster-head (1+α)
times every epocho This ensures well balanced energy consumption
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 8
Problem with Naïve Solutiono Probability of electing an advanced node is not
weighted by its additional energyn advanced nodes may be underutilizedn Lifetime is prolonged, but time until the first node
dies can still be short!
51P,3a,5
1m,5n ====
A
N N NN
“A” node elected with probability 4P -1
P)A(T
× =
N Nr=0 1 2 3 4 5 6 7
N N
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 9
Our SEP (Stable Election Protocol)
o Idea: force each advanced node to be elected every sub-epoch of length (1+α x m)/P /(1+α) rounds
o Probability of a normal node getting elected as cluster-head is Pnormal
o Probability of an advanced node getting elected as cluster-head is Padvanced
o Average number of nodes elected per round = nxP
{=
×+=
?× normal
normalP1
normal
normal G i if )modr(P -1
P
otherwise 0
normal
)i(T
ma1P
P
{=
+×+
=
?× advanced
advancedP1
advanced
advanced G i if )modr(P -1
P
otherwise 0
advanced
)i(T
)a1(ma1
PP
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 10
Numerical Example
N NA
51P,3a,5
1m,5n ====
NN
{=
=×+
=×+
=
?× normal
normalP1
normal
normal G i if )modr(P -1
P
otherwise 0
normal
)i(T
81
5/1315/1
ma1P
P
{=
=+=+×+
=
?×
advanced
advancedP1
advanced
advanced G i if )modr(P -1
P
otherwise 0
advanced
)i(T
84
)31(81
)a1(ma1
PP
N Ar=0 1 2 3 4 5 6 7
N A N A N A
Our SEP Scheme
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 11
Performance Measures
o Stability Periodn Time until death of the first node
o Network lifetimen Time until death of the last alive node
o Number of cluster-heads per roundn Nodes which will directly send aggregated
information to the sink
o Number of alive nodes of each type per roundo Throughputn Rate of data reporting to cluster-heads and to sink
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Simulation Model
o Two networksn 100 nodes uniformly distributed over 100mx100mn 900 nodes uniformly distributed over 300mx300m
o Sink is in the center of the sensor fieldo Message size = 4000 bitso Initial energy for a normal node = 0.5 Jouleso Energy consumed depends on radio
characteristics
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 13
Results for LEACH
o LEACH does not take full advantage of the extra energy of advanced nodes
o Normal nodes die very fastn Sensing field becomes
sparse very fastn Election process
becomes unstable fasto Throughput is low
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Results for LEACH (cont’d)
o Advanced nodes die very slowlyn They are not elected
as often as they shouldo Throughput is low
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 15
Results for SEP
o SEP takes full advantage of the extra energy of advanced nodes n Stable region increases by
up to 26% over LEACHn Higher throughput
o Advanced nodes follow the death process of normal nodesn Smaller unstable region
9/6/2004 I. Matta - SEP: A Stable Election Protocol for clustered heterogeneous WSN 16
Sensitivity of SEP
o Performance depends on the product m x α
o FAIR: ideal distribution of extra energy
o Gain in stability period (over system with no extra energy, i.e., m=0 & α=0) is maximized under SEPn SEP > LEACHn SEP > FAIR for up to 75%
relative extra energy
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Conclusiono SEP is heterogeneous-aware
n Cluster-heads elected based on node’s energy relative to that of others
o SEP is scalable and dynamicn Even normal node can be electedn No global knowledge required at every roundn No prior distribution of energy levels assumed
o Study SEP for more than 2 hierarchical levels and more than 2 types of nodes
o Implementation in Berkeley/Crossbow moteso Deployment issues
n Dynamic updates of weighted election probabilitiesn Integration with power-aware MAC protocolsn …
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