Realistic and Efficient Multi-Channel Communications in Wireless Sensor

Network

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Reference

• Yafeng W., Stankovic J.A., Tian H, Shan L, “Realistic and Efficient Multi-Channel Communications in Wireless Sensor Networks,” In proceedings of INFOCOM 2008, Phoenix, AZ, USA, April 13-18, 2008.

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Outline

• Introduction• Experiments on Multi-Channel Reality• Tree Based Multi-Channel Protocol (TMCP)• Minimum Interference Channel Assignment

Problem• Performance Evaluation• Conclusion

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Introduction

• Wireless sensor network (WSN)

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(Base Station)

Introduction• Current WSN hardware such as Micaz and Telos

provide multiple channels– Improve network throughput– Provide reliable and timely communication services

• Recently MAC layer multi-channel protocols are proposed– To assign different channels to two-hop neighbors (hidden

terminal problem)and coordinate channel switching

– Also called node-based schemes– E.q., MMSN(INFOCOM2006), TMMAC(ICC2007), MCMAC(CIT2006).

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Introduction

• Practical issues for node-based scheme– A large number of channels are needed in dense

networks.– Require precise time synchronization at nodes.– Channel switching delay and scheduling overhead.– Complex.

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Outline

• Introduction• Experiments on Multi-Channel Reality• Tree Based Multi-Channel Protocol (TMCP)• Minimum Interference Channel Assignment

Problem• Performance Evaluation• Conclusion

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Experiments on Multi-Channel Reality

• Number of available channels– Adjacent Channel Interferences– Interferences with 802.11 network

• Impact of time synchronization errors

• Experiment setup– CC2420 radio chip used in Micaz motes– 16 non-overlapping channels, with 5MHz spacing

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Adjacent Channel Interferences

• Place three Micaz motes in a line– One transmitter, one receiver, and one jammer– The jammer’s transmission is synchronized with

the transmitter.

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Transmitter : channel 11Receiver : channel 11

Jammer : channel 12 (adjacent) channel 13 (2 channel

away)

Adjacent Channel Interferences

• Adjacent channel interference can cause collisions and packet losses.

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Interferences with 802.11 networks• Put 8 pairs of Micaz motes closely in a department

office with 802.11 networks– Each pair uses unique channel and all 8 channels are non-

adjacent.– Multi-channel protocols must have capabilities to work

well with a small number of available channels.

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Impact of Time Synchronization Errors

• Put 5 Micaz motes on a line• Node-based scheme

– Each node with unique channel and all are synchronized.

– A time period is divided into 2 time slots• 1st time slot

– Nodes in odd positions send packets – Nodes in even positions receive packets

• 2nd time slot is vice versa

1 2 3 4 5

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Impact of Time Synchronization Errors

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Outline

• Introduction• Experiments on Multi-Channel Reality• Tree Based Multi-Channel Protocol (TMCP)• Minimum Interference Channel Assignment

Problem• Performance Evaluation• Conclusion

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TMCP

• To overcome two problems in practical networks– The small number of available non-adjacent

channels.– Unavoidable time errors.

• Data collection traffic – Multiple information flows generated at sensor

nodes converge to the base station.

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TMCP• Main idea

– Partition the whole network into multiple vertex-disjoint subtrees all rooted at base station

– Allocate different channels to each subtree.– Forward each flow only along its corresponding

subtree.• 3 components

– Channel detection (CD)– Channel assignment (CA)– Data communication (DC)

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TMCP

• CD finds available non-adjacent channels – Two motes are used to sample the link quality, and

we selected good link qualities with non-adjacent channel.

– Assume we have k channels at this point.

• CA partitions the whole network into k subtrees and assigns one unique channel to each subtree– Intertree interference is eliminated– Intra-tree interference is minimized

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TMCP

• DC manages the data collection through each subtree– Assume the base station is equipped with multiple

radio transceivers.– Without time synchronization

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Outline

• Introduction• Experiments on Multi-Channel Reality• Tree Based Multi-Channel Protocol (TMCP)• Minimum Interference Channel Assignment

Problem• Performance Evaluation• Conclusion

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Model and Problem Definition

• The goal is to minimize intra-tree interferences.

• Assume that a sensor network is a static.– Interference value: int(u) = the number of other

node by which the reception at u can be distrurbed

• The intra-tree interference value of a tree T is defined as– int(T) = max{int(u): u is a non-leaf of T}

2008/9/1 20* M. Burkhart, P. V. Rickenbach, R. Wattenhofer, and A. Zollinger, “Does topology control reduce interference,” in ACM MobiCom, 2004.

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The intra-tree interference value of the tree is 4.

PMIT Algorithm• Apply Breadth-First search algorithm from the base

station to construct a fat tree.– Nodes keep height and have multiple parents on the fat

tree.– The tree is a shortest path tree.

• Execute the channel allocation one-by-one level from top to bottom on the fat tree– For each node, choose an optimal tree and add this node

to bring the least interference to this tree.– Selects a parent which has the least interference value.– Nodes with fewer parents first, because they are less free

to choose channels.

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Greedy PMIT

• K = 3

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Evaluation of the PMIT Algorithm

• Simulations parameters– 200m x 200m field– 250 nodes are uniformly distributed– Communication range is 10~35m – Interference range is 1.5 times as the

communication range

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Evaluation of the PMIT Algorithm

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Prim’s algorithm constructs “Minimum Spanning Tree” (single channel)

Eavesdropping is a node-based protocol (multi-channel)

Lower bound : maximum interference value among all nodes divided by k channels

Outline

• Introduction• Experiments on Multi-Channel Reality• Tree Based Multi-Channel Protocol (TMCP)• Minimum Interference Channel Assignment

Problem• Performance Evaluation• Conclusion

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Performance Evaluation of TMCP

• Simulation parameters– 200m x 200m field– 250 nodes are uniformly distributed– Communication range is 10~40m – Interference range is 1.5 times as the

communication range– 50 Many-to-one CBR streams

• 40 packets / second

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Performance with different node density

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workload

workload workload

Performance with different workload

Performance comparison of TMCP and MMSN• 50 CBR streams• Node density: 38• The radio range: 40m

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Evaluation in a Real Testbed

• Experiment setup– A real testbed with 20 Micaz motes.– Four motes are laid closely together to act as a

base station with four transceivers.

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• TMCP effectively reduces interferences and mitigates congestion at nodes.• TMCP works well in a real sensor network.

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Saturated data rate (reception radio above 80%)

Outline

• Introduction• Experiments on Multi-Channel Reality• Tree Based Multi-Channel Protocol (TMCP)• Minimum Interference Channel Assignment

Problem• Performance Evaluation• Conclusion

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Conclusion• This paper studies how to efficiently use multiple

channels to improve network performance in WSNs.• TMCP

– Work with a small number of channels.

– Work without the need of time synchronization.– Decrease potential radio interferences.

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