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 Objective : Maximize throughput. 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.

Our Philosophy Why 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. x x Node x’s interference range 1. R. Draves et al., “Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks”, Mobicom 2004.

Distributed Greedy Channel Selection Let each node select its own channel. Whenever it is needed, each node changes to a channel that minimizes the number of other nodes on the same channel in the interference range. Q : Will this process converge? x x y y

> Distributed Greedy Channel Selection YES! (+1) (-1) 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. Q : Will this process converge? YES! Proof : N(x): # of nodes on the same channel for node x. xN(x) decreases monotonically. x x y > (-1) (+1) Local optimization improves global optimization metric – Self-stabilizing!

Experience with 802.11 Mesh Networks Practical 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 stations 1 common, default channel for all nodes Variable channels selected by channel assignment algorithm 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 Problems Theoretical running time and bounds of the distributed greedy algorithm? Formal time-scale decomposition in radio resource control (e.g., channel, power, rate, route control).

Thank You

Backup slides

Other Results Channel Utilization (in %) of 802.11g channels samech rand DA 10.1 15.1 22.7 Protocol Dynamics # msgs 90.0 Bytes 2080 Time (sec) 32.4 Changes 0.22 Requests 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

3-way Handshake Protocol x

3-way Handshake Protocol x REQUEST REQUEST specifies: Intended channel change Perceived channels of other nodes

3-way Handshake Protocol x ACCEPT

3-way Handshake Protocol x UPDATE When a node ACCEPTed a REQUEST, it “freezes” its channel until corresponding response (UPDATE or ABORT) is received.

3-way Handshake Protocol x

3-way Handshake Protocol x REQUEST

3-way Handshake Protocol x REJECT

3-way Handshake Protocol x ABORT

3-way Handshake Protocol REQUEST y x REQUEST

3-way Handshake Protocol REJECT y x ACCEPT Break ties by predefined order of nodes - if x < y, y will be accepted to change.

3-way Handshake Protocol UPDATE y x ABORT Show how to Resolve the livelock.