Design of a reliable communication system for grid-style traffic light net-

works

Junghoon LeeDept. of Computer science and statistics

Jeju National UniversityRep. of Korea

Song Han, Aloysius K. MokDept. of Computer sciences

University of TexasAustin, Texas, USA

Performance Evaluation

Background and Related Work

Conclusion

Introduction

Channel schedule and routing

Contents

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Feedback control loop

consists of geographically distributed components. needs reliable and timely communication. can install wireless process control protocol. WirelessHART standard has Clear Channel Assessment and provides predictable network access.

Controller Actuators

Sensors

Controlledprocess

Current state

Current state

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Controlled process

Jeju area is located in the southernmost part of Korea.

It hosts many pilot projects for the purpose of testing a sys-tem before its deployment.

Vehicular telematics system

Smart grid system

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Traffic light network

The traffic light is secure place for wireless nodes. Traffic lights form a grid topology in urban area. Process control applications can run on this network.

Global control system

Level 1 network

Level 2 network

Performance Evaluation

Routing and Scheduling Scheme

Conclusion

Introduction

Background and Related Work

Contents

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WirelessHART standard

IEEE 802.15.4 2.4 GHz radioband physical link supports mesh networking directly. 16 frequency channels with 5 MHz guard time Channel blacklisting and hopping The slot-based access scheme with time synchronization The size of a single time slot : 10 ms central network manager schedule provisioning and updating CCA (Clear Channel Assessment) to avoid interference

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Slot organization

Setting the channel each time slot begins. 10 ms slots have CCA (Channel Condition Assessment). CCA and channel tuning just take several bit time. additional CCA and channel tuning for a slot

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Protocol stack and mesh

From Jianping Song et al.’s paper in RTAS 2008 [5]

Performance Evaluation

Background and Related Work

Conclusion

Introduction

Routing and Scheduling Scheme

Contents

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Motivation

The standard does not define what to do when the CCA result is not good.

If the CCA result is not good, can it be possible to take another route?

Primary and secondary receivers can solve this prob-lem.

Two receivers wait for the message and the sender se-lects the receiver (channel) according to CCA result.

The transmission paths can be easily split and merged over the grid topology.

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System Model

Traffic Light Network

The traffic lights, installed in the intersections, form a grid network in the Manhattan-style road network.

Each node can exchange messages directly with its verti-cal and horizontal neighbors, but not its diagonal one, considering the directional antenna.

Control Loop

① reading state variables from sensors, ② deciding the control action, and ③ sending the value of control vari-ables to the actuators

①, ③ : the network and the communication schedules

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System Model

The controller node is located at the left-top. For N0,0 -> N1,1, H0,1->V1,1 and V1,0, H1,1 paths are available. N0,0 performs split and N1,1 runs merge op. over 2 slots. CCA result is always right.

Controllernode

Controllernode

Four 4 4 gridsⅹ

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Split-merge operation

sender

Primary receiver

secondary receiver

receiver

Primary sender

secondary sender

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Sample slot allocation table

Split op.

Merge op.

slot allocation for a 3 3 grid and downlink.

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Route decision

Each rectangle can be considered to be a virtual link. Run the shortest path algorithm. The virtual link can be mapped to 2 slot transmissions.

[….Si,j,......]Primary [ …, Hi,j+1,Vi+1,j+1,…]Secondary [ …, Vi+1,j,Hi+1,j+1,…] F1 substitution

[….Si,j,......]Primary [ …, Vi+1,j,Hi+1,j+1,…]Secondary [ …, Hi,j+1,Vi+1,j+1,…] F2 substitution

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Virtual link model

Success probabilities for the two paths in a rectangle, F1 and F2 Select the bigger of the two as the primary path Set the error rate of the virtual link (1 – F1) or (1- F2) Run the Dijkstra’s shortest path algorithm

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Routing and Scheduling Scheme

Background and Related Work

Conclusion

Introduction

Performance Evaluation

Contents

<19>

Performance Evaluation

Simulation Environment

using SMPL which provides discrete event scheduling For simplicity, only the downlink graph was considered. 500 sets of link error rates are generated. The success ratios are averaged.

Main metrics

success ratio according to slot error rate, hop count. success ratio for the different routing schemes, dimen-

sion additional receive ratio effect of node failures

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Performance Evaluation

Each link has the same error rate

Guilbert-Elliot error model

End-to-end messages to each node a round Overall success ratio a large performance

gap on the high slot er-ror rate

7.9 %

Effect of slot error rate

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Performance Evaluation

11.4 %7.8 %

5.5 %

Success ratio classified by hop counts

hop by hop success ratio no improvement for the

1 hop nodes The 6 hop node has 3

split-merge operations. SM shows stable suc-

cess ratio for the hop count change.

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Performance Evaluation

Improvement by the routing scheme

6.69 %

Each link has its own error rate.

Enhance performance by the routing scheme based on the virtual link model

just 3.6 % loop length overhead

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Performance Evaluation (4/4)

Effect of grid dimension

9.0 % average slot error rate

is set to 0.1. A larger grid has more

rectangles. Average hop length in-

creases. Performance gap in-

creases for a larger grid.

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Performance Evaluation (4/4)

Overhead analysis Additional receive due

to the split operation total slots needed in a

control round

All nodes in the same row and column have the same SM opera-tions

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iinn

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in

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Performance Evaluation (4/4)

Gap for diagonal nodes and other nodes

Compare with a link disjoint path

Each link has the same error rate

Difference in average success ratio is less than 6 %

For the diagonal case, the difference reaches up to 35 %

Performance Evaluation

Routing and Scheduling Scheme

Background & Related Work

Introduction

Conclusion

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Conclusions

Summary

Process control for grid topology traffic light network Attempts two paths according to CCA by split-merge SM operation can be modeled as a virtual link Shortest path-based routing and slot allocation Enhances delivery ratio by up to 7.9 % on average

Future Work

Apply the split-merge operation for data collection and multicasting

Thank You

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Performance Evaluation (4/4)

Effect of node failures

1 node failure makes 4 links unconnected

A non-fringe node fail-ure makes it difficult to run the SM operation.

outperforms grid for all node failure range.

4.7 %