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2006/9/14 Kim Young Hoon

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A Topology Control Approach for Utilizing Multiple Channels in Multi-Radio Wireless Mesh Networks (broadnet2005)Mahesh K. Marina, Samir R. Das

2006/9/14

Kim Young Hoon

- Introduction
- Problem Formulation
- Channel Assignment Algorithm
- Simulations Results
- Conclusions

- Introduction
- Problem Formulation
- Channel Assignment Algorithm
- Simulations Results
- Conclusions

- Wireless mesh networks
- Wired router nodes/wireless nodes
- No need of infrastructure
- Wider coverage

- Mesh networks with multi-hop extension of 802.11 standard
- Different from 802.11 LANs: risk of disconnection
- All nodes use same channel in mesh for connectivity

- Inefficient Utilization of Available Channels
- Need to use Multiples Channels

- Single radio for multiple channels?
Possible. But need of ...

- Dynamically switch between channels
- Tight time synchronization among nodes
- Slow switching
reducing synchronization requirements and overhead,

increasing end-to-end delay

- Slow switching
- Require MAC or hardware modification

- Therefore, Multiple radios per node
- Effective use of given channels
- Overcoming the deficiencies of single radio

- What happens in multi-radio mesh network?
- Disconnection between nodes can happen
- Node’s transmission is interfered by other nodes’

- So, a key issue in multi-radio mesh network architecture is Channel Assignment Problem.

- Channel Assignment Problem
- has to balance between connectivity and interference
- is viewed as topology control problem (adjustable links between nodes in wireless)
In this paper, authors proposed base channel assignment

to obtain an initial, well-connected topology.

- Introduction
- Problem Formulation
- Channel Assignment Algorithm
- Simulations Results
- Conclusions

- Channel Assignment Problem belongs to the class of NP-complete
Proof flow)

Channel Assignment Problem

Topology Control Problem

Topology Control’s Target

Reducing Interference

Channel Assignment Problem

Optimization Problem

Optimization Problem

Decision Problem

Showing that decision problem is in NP-complete (using minimum edge coloring)

For more detail, see 2nd part of the paper

- CLICA (Connected Low Interference Channel Assignment)
- Polynomial time heuristic
- Order nodes by their degree of flexibility
- degree of flexibility: amount of freedom when choosing channel

- Greedily assign channel between nodes

Coloring uncolored links with a common color to already assigned radios

Coloring nodes which have no available radios

Greedily coloring uncolored links

Initial order: a-d-c-b

- Starting from a, assign channel a-b, b’s priority bumps up
- Assign channel b-c, and c-d in similar manner
- Node a and d have a common channel, so assign that to a-d

<1>

<5>

b

a

c

<2>

<6>

<4>

d

<7>

<3>

Node a and d has two radios

- Starting from a, assign channel a-b, b’s priority bumps up
- Assign channel b-c, and c-d in similar manner
- Node a and d have additional radios, so assign different channel to a-d

<1>

<5>

b

a

c

<2>

<6>

<4>

d

<7>

<3>

- Each coloring decision is made in a greedy fashion
- Locally optimal choice

- Theorem 2: CLICA algorithm yields a connectivity preserving color assignment

- Introduction
- Problem Formulation
- Channel Assignment Algorithm
- Simulations Results
- Conclusions

- Graph-based simulations
- Interference and capacity properties of topologies generated by different channel assignment algorithms
- Compared with CCA (Common Channel Assignment – assign same set of channelr to all nodes)

- Measure:
- Maximum link conflict weight – network wide interference
- Maximum number of concurrent transmissions – total one-hop capacity

- Interference and capacity properties of topologies generated by different channel assignment algorithms

- CCA
- CCA interference performance is unaffected by the number of channels
- CCA capacity performance shows a linear growth

- CLICA
- As the number of radios increases, interference goes up and capacity shows marginal perfromance
- Minimum interference doesn’t match maximum capacity
DUE TO HEURISTIC NATURE

- Ns-2 simulations
- Evaluating the performance of CLICA
- Aggregate throughput and average delay
- 50 nodes with 250m TX range in 1000m x 1000m
- 550m interference range
- 802.11 physical layer model in ns-2
- Fixed data rate of 2Mbps

- The authors
- have formulated base channel assignment as a topology control optimization problem
- solved the channel assignment (radio-channel mapping) problem in greedy way (called CLICA)
- shows the interference-reducing results by simulations