pushing the limits of wireless networks prof. dina katabi jan 9, 2006
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High PerformanceSwitching and RoutingTelecom Center Workshop: Sept 4, 1997.
Pushing the Limits of Wireless Networks
Prof. Dina Katabi
Jan 9, 2006
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But, wireless still struggle with bandwidth scarcity
Need a breakthrough!
The future is wireless
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Current Approach
BobAlice
Router
Alice’s packetBob’s packet
Bob’s packetAlice’s packet
• Requires 4 transmissions
• Can we do it in 3 transmissions?
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A Network Coding Approach
BobAlice
Router
Alice’s packetBob’s packet
Bob’s packetAlice’s packet
3 transmissions instead of 4 Save bandwidth Increase Throughput
3 transmissions instead of 4 Save bandwidth Increase Throughput
XOR =
XOR =
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Network Coding
Routers mix bits in packets, potentially from different flows
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How to apply network coding?
• MulticastS R
R
R
• Given Sender & Receivers• Given Flow Rate & Capacities
State-of-the Art
Min-Cost Flow Optimization
Find the routing, which dictates the encoding
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How to apply network coding?
• Multicast
• Given Sender & Receivers• Given flow rate & capacities
State-of-the Art
Min-Cost Flow Optimization
Find the routing, which dictates the encoding
• Unicast
• Many Unknown Changing Sender & Receivers • Unknown and bursty traffic
In Practice
?
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How to apply network coding?
• Multicast
• Given Sender & Receivers• Given flow rate & capacities
State-of-the Art
Min-Cost Flow Optimization
Find the routing, which dictates the encoding
In Practice
Opportunism
• Unicast
• Many Unknown Changing Sender & Receivers • Unknown and bursty traffic
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Opportunism (1)
Opportunistic Listening: Every node listens to all packets It stores all heard packets for a limited time
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Opportunism (1)
Opportunistic Listening: Every node listens to all packets It stores all heard packets for a limited time
D
A
B
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Opportunism (1)
Opportunistic Listening: Every node listens to all packets It stores all heard packets for a limited time
Node sends Reception Reports to tell its neighbors what packets it heard • Reports are annotations to packets • If no packets to send, periodically send reports
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Opportunism (2)
Opportunistic Coding: To send packet p to neighbor A, XOR p
with packets already known to A• Thus, A can decode
But how to benefit multiple neighbors from a single transmission?
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Efficient Coding
Arrows show next-hop
D
A
B
C
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Efficient Coding
Arrows show next-hop
D
A
B
C
Bad Coding
C will get RED pkt but A can’t get BLUE pkt
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Efficient Coding
Arrows show next-hop
D
A
B
C
Better Coding
Both A & C get a packet
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Efficient Coding
Arrows show next-hop
D
A
B
C
Best Coding
A, B, and C, each gets a packet
To XOR n packets, each next-hop should have the n-1 packets encoded with the packet it wants
To XOR n packets, each next-hop should have the n-1 packets encoded with the packet it wants
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Opportunism
• Flows arrive and leave at any time
• Follow any path
• No SchedulingO
GN
A
C
K
M
EF
B
DC
K
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Performance
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Simulation
• Emstar Simulator
• 100 nodes
• 800mx800m
• Senders and receivers are chosen randomly
• Metric: Total Throughput of the Network
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No Coding
Our Scheme
Network Throughput (KB/s)
0
200
400
600
800
1000
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57
No. of Flows
No Coding
Opportunistic Coding vs. Current(Simulation Results)
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Opportunistic Coding vs. Current(Simulation Results)
No Coding
Our Scheme
Network Throughput (KB/s)
0
200
400
600
800
1000
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57
No. of Flows
Opportunistic Coding
No Coding
Orders of magnitude higher throughput in dense networks
Orders of magnitude higher throughput in dense networks
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Now, we also have a prototype implementation
Rerun the Alice-and-Bob experiment using our Implementation on 3 Linux PCs
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Preliminary Results of our Prototype Implementation
(Alice and Bob Experiment)
1
1.2
1.4
1.6
1.8
2
1 2 5.5 11
802.11 Bit Rates (Mb/s)
Ratio of Throughput with Coding to No-Coding
Opportunistic Coding Doubles the Throughput
Opportunistic Coding Doubles the Throughput
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Why More than 25%?
MAC is fair 1/3 BW for each node
• Without coding, relay needs twice as much bandwidth as Alice or Bob
• With coding, all nodes need equal bandwidth
Alice
Relay
Bob
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Conclusion
• Introduced an opportunistic approach to Network Coding
• Drastic throughput improvement in dense networks
• First implementation of network coding in a wireless network
• Simple, Practical, and It Works!
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Ongoing and Future Work
A practical approach to Network Coding in wireless networks, including:
• Reduce bandwidth consumption
• Reduce power consumption
• Security
• Mobility
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Questions?
No Coding
Our Scheme
0
200
400
600
800
1000
1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 No. of Flows
Opportunistic Coding
No Coding
Drastic throughput increase in dense networks
Drastic throughput increase in dense networks
Network Throughput (KB/s)