S4: Small State and Small Stretch
Routing for Large Wireless Sensor
Networks
Yun Mao2, Feng Wang1, Lili Qiu1,
Simon S. Lam1, Jonathan M. Smith2
Univ. of Texas at Austin1, Univ. of Pennsylvania2
1
Background
• Smart wireless sensor networks call for internode communication
– In network processing
– In network storage
• Challenges for a point-to-point routing
protocol in wireless sensornets
– Limited resources: Scalability
– RF phenomena: Efficiency, Resilience
2
The core theme: a tradeoff
We want small
state!!
We want small
stretch!!
routing
debate
• State: the routing table size describing the network topology
• Stretch:
path length found by the routing algorithm
optimal path length
3
Design space
state
O(n)
Shortest-path
routing
hierarchical
routing
O( n )
O(1)
?
Virtual-coordinate
routing
geographic
routing
avg/worst-case stretch
4
Goals
• Small stretch
– Efficient usage of the wireless resources.
– Constant bound for worst-case stretch and near-optimal
for average cases
• Small state
– Memory size is increasing, but still limited
• 0.5KB (WeC)  1KB(Dot)  4KB (Mica, Mica2)  10KB (telos) 
64KB (iMote)
– O( n) bound
– Reasonable control traffic to maintain the state
• Practical
– Don’t assume perfect radios
– No GPS or preconfigured physical locations
5
S4 routing algorithm in a nutshell
• Theoretical foundation on compact routing [SPAA’01]
– Worst-case routing stretch is 3
– O( n) state per node
• Node classification
– beacon nodes
• n nodes
– regular nodes
• Know how to route to the beacons
• Node clusters
– Each regular node d has a cluster, in which each node
knows how to route to d.
– Radius is the distance to the closest beacon.
– Different from hierarchical routing.
6
Example
Beacon 1
Source
Beacon 3
Radius=2 hops
Dest
Rules:
• Inside cluster: route
on the shortest path
• Outside cluster:
route towards the beacon
closest to the dest
Beacon 2
7
Protocol Design Challenges
• How to maintain routing state inside a cluster?
– Flooding is expensive
• How to maintain routing state for beacon
nodes?
– Unreliable broadcast may affect routing stretch
• Routes to beacons may not be optimal.
• Unnecessarily long radius
• How to provide resilience against node/link
failure?
– Transient failure
– During routing state convergence
8
Key components of S4
• Disseminate routing states inside the clusters:
Scoped Distance Vector (SDV)
– <d, nexthop(d), seq(d), hop(d), radius(d)>
– Incremental update
• Inter-cluster routing: Resilient Beacon Distance
Vector (RBDV)
– Passively listen to further broadcasts of neighbors
– Re-broadcast if overhearing too few broadcasts within a
certain time.
• Failure handling
– Distance Guided Local Failure Recovery (DLF)
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Distance-guided local failure recovery
6
4
source
Dest
5
#1 asks for help from neighbors.
1
2
3
The nodes closer to dest reply earlier.
Priorities are estimated from SDV & RBDV.
#3 suppresses unnecessary packets.
#1 chooses the best neighbor to forward.
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Other design issues
•
•
•
•
Location Directory
Beacon node maintenance
Link quality estimation, neighbor selection
Please refer to the paper for details
11
Evaluation
• Methodology
– High-level simulation with ideal radio model
• No loss, no contention, circle communication range
– TOSSIM packet level simulation
• Lossless and lossy link with contention
– Mica2 test bed evaluation
• Real environment, unpredictable obstacles
• Use Beacon Vector Routing (BVR) [NSDI 2005] as
benchmark
– Virtual coordinate approach
– A similar goal: practical
– Code available
12
Questions to answer
• Does S4 achieve small stretch?
– routing stretch and transmission stretch
– Average case vs worst case
•
•
•
•
Does S4 achieve small state?
How does S4 perform under failure?
How well does S4 work in a real testbed?
Many others in the paper..
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Routing/transmission stretch in TOSSIM
# of beacons = n
lossless link with contention and collision
n
S4 has smaller avg. stretch and variation.
14
routing state per node
BVR
S4
•Routing state of S4 increases at the scale of O( n );
•The amount of state is evenly distributed between beacon and
non-beacon nodes.
15
Stretch under irregular topologies
BVR
S4
The stretch of S4 is not affected by the irregular
topology, even for those worst cases.
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Distance-guided local failure recovery
DLF greatly increases the success rate of S4 under node failures.
17
Testbed Deployment
• 42 mica2 motes
– 915MHz radios
– 11 of them (called gateway motes) are connected
to MIB600 Ethernet board, powered by the
adapters
– 31 of them are powered by batteries
• Reduce power level to create multi-hop
topology
– A link between two nodes exists if the packet
delivery rates of both directions are above 30%
– The network diameter is around 4 to 6 hops.
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ACES Building 5th Floor NW @ UT Austin
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Routing success rate
6 random beacon nodes
Sources are randomly chosen from all nodes.
Destinations are randomly chosen from 11 “gateway” nodes.
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Routing under node failures
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Summary
• Key properties:
– avg stretch ~ 1; worst-case stretch <=3
– State ~O( n )
• Key components
– Scoped distance vector (SDV)
– Resilient beacon distance vector (RBDV)
– Distance guided local failure recovery (DLF)
• Extensive simulation and experimental results
• Limitations and Future work
– ETX aware
– Rapid mobility
http://www.cs.utexas.edu/~lili/projects/s4.htm
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Backup slides
23
3-stretch guarantee
B
S
D
dist<= |BD|+|SB|
(shortcut)
<= |BD| + (|BD|+|SD|) (triangle inequality)
= |SD| + 2|BD|
<=|SD| + 2|SD|
(cluster definition)
<=3|SD|
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Control traffic overhead
25
Link quality over time
Real world is tough: unstable,
asymmetric links do exist
26
stretch comparison
High-level simulation: 3200 nodes, high density
K n
K n
For average cases, S4 has routing and transmission
stretches close to optimal, consistently smaller than
BVR.
27
Transmission Stretch in TOSSIM simulation
BVR
K n
S4
BVR: stretch increases when the simulation is more realistic
S4: no change
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Topology
A link between two nodes exists if the packet delivery rates
of both directions are above 30%
29