Distributed Channel Assignment in Multi-Radio 802.11 Mesh Networks

Bong Jun Ko (IBM T.J. Watson Research)Vishal Misra (Columbia University)

Jitendra Padhye (Microsoft Research)Dan Rubenstein (Columbia University)

Wireless Mesh Networks

WMN: Multi-hop wireless network infrastructure for local/residential area networks.

Goal: better channel utilization higher network capacity. For scalability and adaptability, light-weight distributed solution is desirable.

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Our PhilosophyWhy focus on channel assignment?

Decouple channel assignment and end-to-end routing.Routing protocols adapt to dynamic traffic load, link quality, and

even channel configuration (e.g., MR-LQSR1) .Channel assignment focuses on quickly-stabilizing channel

configuration based on physical topology.More scalable than centralized, joint-optimization approaches.

There are K channels, and assume (for now) every node can transmit and receive from all channels simultaneously.

Approach: For each node, minimize the number of other interfering nodes on the same channel.

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Node x’s interference range

1. R. Draves et al., “Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks”, Mobicom 2004.

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Distributed Greedy Channel Selection

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Q : Will this process converge?

Let each node select its own channel.Whenever it is needed, each node changes to a channel that mi

nimizes the number of other nodes on the same channel in the interference range.

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Q : Will this process converge?

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YES!Proof :

•N(x): # of nodes on the same channel for node x.• xN(x) decreases monotonically.

Let each node select its own channel.Whenever it is needed, each node changes to a

channel that minimizes the number of nodes on the same channel in the interference range.

Distributed Greedy Channel Selection

>(-1) (+1)

Local optimization improves global optimization metric – Self-stabilizing!

Experience with 802.11 Mesh NetworksPractical limitations

Current 802.11 transceivers can send or receive through only one channel at a time.

Neighboring nodes need to be at the same channel. Conflicting goals: connectivity vs better utilization.

Multi-radio stations1 common, default

channel for all nodesVariable channels

selected by channel assignment algorithm

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•Links of variable channels: express way •Links of common channel: local roads

Performance Evaluation

Experiments on a 14-node testbed. A default channel from 802.11a Variable channels from 802.11g Interference range : 3 hops

Routing protocol : MR-LQSR (Multi-Radio Link Quality Source Routing) Aware of multi-radio, multi-channel environment Preference given to channel-diverse paths

Measure end-to-end throughput of multiple, concurrent TCP flows with random source-destination pairs

Compare to samech : all nodes are assigned the same channel for additional radio

rand : each node is assigned a channel uniformly at random for additional radio

Testbed

Individual TCP Throughput

•CDF of all TCP flow throughputs in all experiments.•Flows over longer paths benefit the most.

Aggregate TCP Throughput

•Measured average TCP throughput of all flows in each experiment, and took median value of 5 experiments.•50% higher than samech / 20% higher than random.

Conclusion

Developed a fully-distributed, self-stabilizing channel assignment algorithm for multi-hop wireless networks.

Experiments on multi-radio 802.11 mesh network testbed. Our mechanism shows improvements in network throughput

by 50% and 20% compared to homogeneous and random assignments, respectively.

Open ProblemsTheoretical running time and bounds of the distributed gr

eedy algorithm?Formal time-scale decomposition in radio resource contro

l (e.g., channel, power, rate, route control).

Thank You

Backup slides

Other Results

Channel Utilization (in %) of 802.11g channels

Protocol Dynamics

samech rand DA10.1 15.1 22.7

# msgs 90.0Bytes 2080Time (sec)

32.4

Changes 0.22Requests 0.70

Dealing with Delay and Asynchrony

Solution : a 3-way handshake protocol for distributed mutual exclusive operation.

REQUEST → ACCEPT or REJECT → UPDATE or ABORT

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3-way Handshake Protocol

REQUEST

3-way Handshake Protocol

REQUEST specifies: Intended channel change Perceived channels of other nodes

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ACCEPT

3-way Handshake Protocol

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UPDATE

3-way Handshake Protocol

When a node ACCEPTed a REQUEST, it “freezes” its channel until corresponding response (UPDATE or ABORT) is received.

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xy

3-way Handshake Protocol

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yREQUEST

3-way Handshake Protocol

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REJECT

3-way Handshake Protocol

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ABORT

3-way Handshake Protocol

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yREQUEST

3-way Handshake Protocol

REQUEST

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REJECT

3-way Handshake Protocol

ACCEPTx

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Break ties by predefined order of nodes- if x < y, y will be accepted to change.

yABORT

3-way Handshake Protocol

UPDATE

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