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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

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Background

• Smart wireless sensor networks call for inter-node 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

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The core theme: a tradeoff

routingdebate

We want small state!!

We want small stretch!!

• State: the routing table size describing the network topology

• Stretch:path length found by the routing algorithm

optimal path length

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avg/worst-case stretch

state

Design space

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geographicrouting

Shortest-pathrouting

O(1)

O( )

O(n)

hierarchicalrouting

Virtual-coordinaterouting?

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Goals

• Small stretch– Efficient usage of the wireless resources.– Constant bound for worst-case stretch and near-optim

al 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( ) bound– Reasonable control traffic to maintain the state

• Practical– Don’t assume perfect radios– No GPS or preconfigured physical locations

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S4 routing algorithm in a nutshell

• Theoretical foundation on compact routing [SPAA’01]– Worst-case routing stretch is 3– O( ) state per node

• Node classification– beacon nodes

• 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.

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Radius=2 hops

Dest

Beacon 2

Beacon 1

Beacon 3

Source

Example

Rules:• Inside cluster: route

on the shortest path• Outside cluster: route towards the beacon

closest to the dest

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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

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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

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4

3

6

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Dest

source#1 asks for help from neighbors.

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

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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

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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

S4 has smaller avg. stretch and variation.

# of beacons = lossless link with contention and collision

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routing state per node

•Routing state of S4 increases at the scale of O( );•The amount of state is evenly distributed between beacon and non-beacon nodes.

BVR

S4

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Stretch under irregular topologies

The stretch of S4 is not affected by the irregular topology, even for those worst cases.

BVR

S4

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Distance-guided local failure recovery

DLF greatly increases the success rate of S4 under node failures.

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Testbed Deployment

• 42 mica2 motes– 915MHz radios– 11 of them (called gateway motes) are connect

ed to MIB600 Ethernet board, powered by the adapters

– 31 of them are powered by batteries• Reduce power level to create multi-hop top

ology– A link between two nodes exists if the packet d

elivery 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 nodesSources 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( )

• 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

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3-stretch guarantee

dist<= |BD|+|SB| (shortcut) <= |BD| + (|BD|+|SD|) (triangle inequality) = |SD| + 2|BD| <=|SD| + 2|SD| (cluster definition) <=3|SD|

B S

D

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Control traffic overhead

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Link quality over time

Real world is tough: unstable, asymmetric links do exist

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stretch comparison

High-level simulation: 3200 nodes, high density

For average cases, S4 has routing and transmission stretches close to optimal, consistently smaller than

BVR.

nK nK

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Transmission Stretch in TOSSIM simulation

nK

BVR: stretch increases when the simulation is more realisticS4: no change

BVR

S4

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Topology

A link between two nodes exists if the packet delivery rates of both directions are above 30%