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/
1
Motivation: Mesh networks do not scale
• Wireless mesh
networks: “Killer app”
– MIT Roofnet
– Champaign-Urbana Wireless
• Contention: single
channel
Gateway node
– Intra-flow interference
– Inter-flow interference
– Worsens near gateway(s)
Can a single “commodity” transceiver exploit
multi-channel diversity?
2
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
3
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
f1
f2
f3
• 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?
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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
k=3
t0
t1
t2
t3
t4
s0
s1
s2
s3
s4
s5
s1
s0
s5
s2
s3
s4
s4
s0
s1
s5
s3
s5
s4
s0
s1
s2
s2
s3
s5
s0
s1
s3
s1
s4
s2
s0
s2
s3
s4
s5
f1
•
•
•
•
f2
Number of subnetworks: 2k
Schedule cycle: T= NextPrime(2k - 1)
Exactly 2 subnets converge on a channel
Every subnet converges every other subnet
f3
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Connectivity: A Visual Guide
IEEE 802.11
Dominion
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Routing
t1
C [s0]
t2
A [s2]
t4
B [s3]
• 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
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Abstraction: Graph-Theoretic Model
A5
A0
A1
A2
A3
A4
Temporal Edge
C1
Connectivity Edge
Base Edge
• Convert link state to an abstract model
• Edge weight assignment
– Connectivity edge = pf, temporal edge = 0
t1
• Locate shortest route using Dijkstra’s
• Multi-path routing
– Prune all connectivity edges in route
A [s2]
– Repeat: until no more routes found
B4
C [s0]
t2
t4
B [s3]
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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
Throughput (in 109 bit-meters/s)
16
14
12
10
8
6
4
2
0
1
3
80211-etx
5
10
20
30
Number of flows
80211-dsr
ssch11
40
50
dominion11
Distance-normalized aggregate throughput:
Dominion vastly better than SSCH (86%) and 802.11 (1813%)
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Results (continued)
1
Jain's Fairness Index
0.8
0.6
0.4
0.2
0
1
3
80211-etx
5
10
20
30
Number of flows
80211-dsr
ssch11
40
50
dominion11
• Jain’s fairness index shows that Dominion is fair
– 1730% fairer than 802.11, and 315% fairer than SSCH
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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/
14
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
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•
•
•
•
•
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Motivation
Related Work
Dominion: Key Contributions
Deterministic Scheduling
Routing Intelligence
Experimental Results
Conclusion
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