realistic and efficient multi-channel communications in...

23-05-2011 | Real Time Networks

Realistic and Efficient Multi-Channel Communications in WSN

Miji Kim

Andreas Schädeli

Silvia Dorotheea Rus

Cievoloth Gilber Coca Olmos


Introduction

Problem description

Interference problems

Time synchronization problems

Description of solution

TCMP

Channel assignment

Test settings

Results

Performance Evaluation

Strengths and drawbacks

Paper Analysis

Conclusion

Content


Introduction

WSN: ad-hoc, multi-hop routing

Single-Channel vs. Multi-Channel communication

Single-Channel problem: Interferences

Multi-Channel problem: Synchronization


Only use orthogonal channels (no interference)

We are not alone!

Introduction


Channel Detection (CD)

Channel Assignment (CA)

Data Communication (DC)

Tree Based Multi-Channel Protocol (TMCP)



Find usable channels

2 motes sample the link quality of each channel in different positions

Select non-adjacent channels (eliminates inter-tree interference)

Communication Range (CR)

Interference Range (IR)


Pass through all frequencies..

Then find theones that suit

us!IR= 1,5 * CR



Channel Assignment (CA) Partition the network into k subtrees

Assign a unique channel to each subtree

Minimal intra-tree interference


Now, we areall partitionedinto subtrees.



Channel Assignment (CA) Partition the network into k subtrees

Assign a unique channel to each subtree

Minimal intra-tree interference


And we all chosea channel with

minimal interference!



Channel Assignment (CA)

Data Communication (DC) No time synchronization needed

Base-station is equipped with

multiple tranceivers

Data reception and

dissemination is possible


Communication can go on!


Goal

Minimize intra-tree interferences.

Assumption

The network is static

Define

Node interference

Number of nodes by which the reception

can be disturbed at a given node

Tree interference

Maximum interference value among all

non-leaf nodes

Minimum interference Channel Assignment


Algorithm

All interference values are known

Build shortest-path tree

Channel allocation

From top to bottom level of the tree

Process nodes with fewer parents first

Connect the node to the subtree where

adding the node brings the

least interference to the subtree

Select parent with least

interference value among

all parents

Assign nodes with large interference

as leaves

Minimum interference Channel Assignment


Graph simulation in Java

200mX200m field 250 nodes uniformly distributed Communication range 10 to 35m Interference range is 1.5 times the communication range.

Since no other PMIT algorithms exists

Prim's algorithms to construct spanning trees (base schema with a single channel)

Eavesdropping channel assignment(used by node-base protocols MMSN).

lowest bound Maximum interference value/number of channels

Test Settings


PMIT algorithm 1/3 of Prim’s interference

PMIT vs Eavesdropping.- when density is high PMIT has less interference.

Reason: Eavesdropping.- not enough channels for

nodes within two hops neighbors

PMIT looks for the local optimal.

higher density PMIT assign nodes with large interference as leaves.

PMIT - Lower Bound.

Algorithm is scalable with different densities.

With 8 channels Eavesdropping has 18% less interference. (no explanation why)

Results PMIT Performance


Simulation - Same settings as before

TMCP vs. Spanning tree routing (protocol with a single channel).

Different network workloads

TMCP splits heavy workload into different trees.

More tolerant to systems load variations than spanning tree.

TMPC performance is unstable.

Flow congestion occurs on sub trees

TMCP Performance Evaluation


Performances very close when number of channels is small.

TMCP achieves 10% higher throughput with less than 5 channels.

When the number of channels increases MMSN outperforms TMCP.

TMCP Performance Evaluation


Strengths The algorithm doesn't do channels switching so doesn't need time

synchronization Every node keeps the shortest path to the base station, partitioning algorithm

doesn't require extra transmission and no extra energy consumption. Interference can be reduced by setting nodes with higher interference rate

as a leaf nodes

Drawbacks

The algorithm is not dynamic, what if we add a new node? The solution is not necessarily the best one since is an heuristic, maybe a

better one can be implemented (i.e. combination between tree base and node base).

(hardware Limitation) base station with multiple radio transceiver limitation Intra sub tree interference(proven to be a problem even in the test flow

congestions in sub trees).

Strengths and Drawbacks of TMCP


Paper Analysis

Strengths

Few unstated assumptions

Presents realistic scenarios

Algorithm well understandable

Weaknesses

No details about other protocols

Very optimistic conclusion based on few results

Orthographic faults and bad graph labeling in one case


What the paper says:

“It can be seen that without time errors, the node-based scheme always has higher packet reception ratios than the single channel scheme.”

What the figure says:

Paper Analysis


Tree-based algorithm improves the throughput of the WSN.

Performed tests are not necessarily the best, Some real tests should be performed(existents are not relevant).

Maybe a combination of “tree-based – Node–based” could be implemented to improve the sub-tree flow congestion problem.

Fast Data Collection in Tree-Based Wireless Sensor Networks Paper makes comparisons of TMCP with other algorithms.

Conclusions


Thank you for your attention!

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