Fault Tolerant Routing in Tri-Sector Wireless Cellular Mesh Networks

Yasir Drabu and Hassan Peyravi

Kent State University

Kent, OH - 44240

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Agenda

Introduction Wireless Network Overview

Problem Definition Proposed Solutions

Shortest Path Routing Fault Tolerant Routing

Conclusion

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Intro - Wireless Network ArchitecturesPoint to Multipoint Multipoint to MultipointPoint to Point

Dedicated links Currently Most Common Our Focus

Topology Reliability Adaptability Scalability Routing Complexity

P to P High Low None None

P to M Low Low Moderate Moderate

M to M High High High High

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Wireless Mesh Networks (WMN)

Structured, energy rich wireless multi-hop networks: Wireless Client

Mobile, no routing, limited power

Wireless Router Low mobility, routing and

power rich Wireless Gateway

Access to wired network. Solves – “Last Mile

Connectivity”

INTERNET

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Problem Definition

How do you route packets in a Wireless Mesh/Multi-Hop network?

Given: Faulty wireless – multi path fading, selective

fading, noise etc. Shortest path may not be the best alternative. Multiple hops/ multiple channel – radio limitations,

channel allocation problem etc.

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Background Many wireless routing algorithms:

Pro-active (DBF) , reactive (DSR, TORA) and hybrid (ZRP)

None are very fault tolerant and very focused on energy poor applications

Few provide fault tolerance Agarwal 2004 (Stony Brook research lab) – build routing

using spanning trees then re-associate to different root when link fails. Slow, high message complexity, order of seconds.

However not much work done on using topological properties of wireless network

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Proposed Honey Comb WMN Model

No wired backbone for each node Place wireless elements on the edge instead

of the center in a typical network Uses more nodes with lower power for better

coverage and higher throughput

Modified to Honey Comb

Typical Cellular Network Proposed Honeycomb Model

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Honey Comb Network ComparisonCellular Network:

Central Base Station Omni-directional antenna

Advantages+ Established Technologies+ Fewer Base Stations

Limitations– High power consumption– Limited coverage– Lower bandwidth– No Fault tolerance– Expensive to deploy and

maintain due to wired back bone infrastructure.

Honeycomb Network: BS as the edge Directional antennas

Advantages+ Lower power per node+ Better coverage+ Higher throughput+ Fault tolerance+ Wireless interconnect, cheaper

to deploy when wired infrastructure is factored in

Limitations– More complex hardware– More nodes for same area

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Proposed Tri-Sector Node Model

N

1200

Wireless Router

Four Radios Three directional antennas for

communication with other routers

One omni-directional for wireless clients

The directional antenna can be on the same channel as they are spatially multiplexed. Using different channels on

different lobes will add to the complexity of the problem.

Omni-directional antenna is on a separate channel to minimize interference.

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Earlier routing in Honeycomb Network Honeycomb routing

was introduced in [Stojmenovic:97]

Issues: Uses (x, y, z) co-

ordinates to route. No consideration for link

failure.

Src: Stojmenovic:97

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Honeycomb Brick Representation

Two dimensional representation of honeycomb

Each node can be represented by a co-ordinate (x,y)

They have 25% smaller degree than regular grid meshes.

stretch

stretch

Isomorphic pruned 2Dsquare mesh

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Shortest Path Routing Algorithm

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Fault Tolerance In Brick Networks

Link faults common in wireless networks

How do we handle a fault in a mesh network? Localized Temporal Routing

Temporal Routing Based on Final direction of packet Position of fault Number of faults

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Fault Tolerant Routing Algorithm

Fault detection: Physical layer or the Medium Access Layer detects the fault.

Fault avoidance: Once a fault has been detected, the algorithm goes into recovery mode. Exploited topological properties to define alternate path.

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Fault Routing – Single and Multiple failures

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Limitations

Fault tolerance is a trade-off between delay and deliverability. More hops introduce delay.

Model needs ground up deployment Topological Rigidity

Cannot be deployed on all terrains

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Conclusions

Contributions: Modeled fault tolerant network topology Efficient addressing scheme Shortest path routing algorithm Developed fault tolerant routing which can handle

multiple faults. Future Work:

Gateway Placement Resource allocation (channel assignment)

Questions?

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Wireless Multi-hop Networks

Type of multi-hop networks (Application Level Classification): Ad hoc

Limited power, high mobility, relatively small. Primary application – file sharing and collaboration.

Sensor Networks Very low power, low bandwidth, large networks. Primary application – Data accusation and sensing.

Wireless Mesh Networks (WMN) Power rich, structured, high throughput Primary application – access network to end users.

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Routing Challenges in WMN

Time varying link behavior Shortest Path not always the best route Using spanning trees do not exploit the natural

robustness of a WMN. Exploit alternate routes to make WMN fault tolerant How to achieve load balancing.

How to maintain alternate routes? How to choose one route over the other? On what

basis/metrics?