Pushing the Limits of Wireless Networks Prof. Dina Katabi Jan 9, 2006.

High PerformanceSwitching and RoutingTelecom Center Workshop: Sept 4, 1997.

Pushing the Limits of Wireless Networks

Prof. Dina Katabi

Jan 9, 2006

But, wireless still struggle with bandwidth scarcity

Need a breakthrough!

The future is wireless

Current Approach

BobAlice

Router

Alice’s packetBob’s packet

Bob’s packetAlice’s packet

• Requires 4 transmissions

• Can we do it in 3 transmissions?

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 =

Network Coding

Routers mix bits in packets, potentially from different flows

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

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

?

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

Opportunism (1)

Opportunistic Listening: Every node listens to all packets It stores all heard packets for a limited time

Opportunism (1)

Opportunistic Listening: Every node listens to all packets It stores all heard packets for a limited time

D

A

B

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

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?

Efficient Coding

Arrows show next-hop

D

A

B

C

Efficient Coding

Arrows show next-hop

D

A

B

C

Bad Coding

C will get RED pkt but A can’t get BLUE pkt

Efficient Coding

Arrows show next-hop

D

A

B

C

Better Coding

Both A & C get a packet

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

Opportunism

• Flows arrive and leave at any time

• Follow any path

• No SchedulingO

GN

A

C

K

M

EF

B

DC

K

Performance

Simulation

• Emstar Simulator

• 100 nodes

• 800mx800m

• Senders and receivers are chosen randomly

• Metric: Total Throughput of the Network

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)

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

Now, we also have a prototype implementation

Rerun the Alice-and-Bob experiment using our Implementation on 3 Linux PCs

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

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

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!

Ongoing and Future Work

A practical approach to Network Coding in wireless networks, including:

• Reduce bandwidth consumption

• Reduce power consumption

• Security

• Mobility

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)