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A Cross-Layer Architecture to Exploit Multi-Channel Diversity

Jay A. Patel, Haiyun Luo, and Indranil Gupta

Department of Computer Science

University of Illinois at Urbana-Champaign

Distributed Protocols Research Group http://kepler.cs.uiuc.edu/

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Motivation: Mesh networks do not scale

• Wireless mesh networks: “Killer app”– MIT Roofnet

– Champaign-Urbana Wireless

• Contention: single channel– Intra-flow interference

– Inter-flow interference

– Worsens near gateway(s)

Gateway node

Can a single “commodity” transceiver exploit

multi-channel diversity?

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Challenges + Prior Work

• Neighbors must converge to exchange data– While exploiting multiple channels

• Locally opportunistic channel hopping– Multi-channel MAC [So:MobiHoc04]

– Seeded Slotted Channel Hopping [Bahl:MobiCom04]

• Limitations– Leads to node synchronization problem

– MAC Approach: Probable implementation issues

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Our Contributions

• Dominion: A cross-layer architecture– Simple MAC + Intelligent routing

– Key decisions shifted up, i.e., in to the software stack

• Deterministic channel hopping MAC protocol – Eliminate locally opportunistic behaviour

• Improves fairness

• Core logic resides at the routing layer– Graph-theoretic model: extensible and flexible

– Multi-path routing

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Split Topology: k subnetworks

• Frequency Division + CSMA Approach– Logical subnetworks: A subnetwork per channel

– Node ni homed at channel SHA1(ni) mod k

– Creates network and subnetwork partitions

• Route across network partitions?

f1

f2

f3

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Time is on our side...• Key: Periodically converge subnetworks

– Each pair of subnetworks switches to a common channel at a pre-determined time

• “Deterministic scheduling”– Based on modulo arithmetic– Can be generated simply with the parameter k– MAC uses this schedule

• Primary difference vs. IEEE 802.11

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A Sample Schedule

s2

s3

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s0

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s0

s1

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s3

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s1

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s0

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s0

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t0

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k = 3

f2

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f1

• Number of subnetworks: 2k• Schedule cycle: T= NextPrime(2k - 1)• Exactly 2 subnets converge on a channel• Every subnet converges every other subnet

s1

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Connectivity: A Visual Guide

DominionIEEE 802.11

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Routing

• Best route for A -> B?– Two routes: AB (direct) and AC -> CB (indirect)

• Which is the better route? It depends– Throughput-wise: AB

• Can we do better? YES! with multi-path routing – Latency-wise: is time-variant

• Addressed in a follow-up paper

A [s2]

B [s3]

C [s0]

t4

t2

t1

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Abstraction: Graph-Theoretic Model

• Convert link state to an abstract model• Edge weight assignment

– Connectivity edge = pf, temporal edge = 0• Locate shortest route using Dijkstra’s• Multi-path routing

– Prune all connectivity edges in route– Repeat: until no more routes found

A5

A0 A1 A2 A3 A4

C1 B4

Temporal EdgeConnectivity EdgeBase Edge

A [s2]B [s3]

C [s0]

t4

t2

t1

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Experiment Methodology

• Implementation– QualNet v3.9– 10 ms timeslots, 80 µs switching delay

• Only 11 channels used (out of 12 for 802.11a)• Topology

– 100 nodes, 1000m x 1000m– Uniform random placement– Random assignment of nodes to subnetworks

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Results

Distance-normalized aggregate throughput:Dominion vastly better than SSCH (86%) and 802.11 (1813%)

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1 3 5 10 20 30 40 50Number of flows

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80211-etx 80211-dsr ssch11 dominion11

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Results (continued)

• Jain’s fairness index shows that Dominion is fair– 1730% fairer than 802.11, and 315% fairer than SSCH

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1 3 5 10 20 30 40 50Number of flows

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's F

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ness In

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80211-etx 80211-dsr ssch11 dominion11

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Conclusion

• New cross-layer architecture– Dominion exploits k channels with only 1 radio– Eliminate locally opportunistic behavior

• Simple MAC: deterministic schedule

– Intelligence shifted upwards

• Suitable for static, wireless mesh networks– Excels in non-disjoint multi-flow scenarios

Distributed Protocols Research Group http://kepler.cs.uiuc.edu/

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Questions

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Future Work

• Dynamic subnetwork assignment– Based on two-hop “neighborhood”

• Extend the Graph-theoretic model– Optimize on end-to-end latency

• TCP improvement– Multiple routes leads to out-of-order packets

• Broadcast packets– Probabilistic approach

– Allow efficient dissemination of link-state at run-time

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Implementation

• QualNet v3.9• 10 ms timeslots, 80 µs switching delay• Source routing• Per-flow, per-timeslot queuing

– prevents head-of-line blocking

• Warnings reduce buffer overflow at intermediate nodes• Attempts only 1 DCF transmission per packet at a time

– Allows for on-time switching

• A packet is dropped after 14 DCF failures– akin to two 802.11 retries

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Experiment Methodology

• Implementation– QualNet v3.9– 10 ms timeslots, 80 µs switching delay

• 100 nodes, 1000m x 1000m– Uniform random placement– Random assignment of nodes to subnetworks

• Bootstrap process: measure quality of each link– 802.11 and SSCH: used to calculate static ETX routes– Dominion: network link-state

• Results are average of 5 independent trials– Only 11 channels used (out of 12 for 802.11a)

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Multi-Path Routing

• Using Dijkstra, locate shortest route

• Prune all connectivity edges in route– Reduces or eliminates inter-flow interference

• Repeat: until no more routes found

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Outline

• Motivation

• Related Work

• Dominion: Key Contributions

• Deterministic Scheduling

• Routing Intelligence

• Experimental Results

• Conclusion