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Shyamnath Gollakota Dina Katabi The Hidden Terminals Problem Collision Alice Bob The Hidden Terminals Problem Alice Bob More Collisions Retransmissions Cant get any useful connections ID: 776256

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Slide1

Decoding 802.11 Collisions

Shyamnath GollakotaDina Katabi

Slide2The Hidden Terminals Problem

Collision!

Alice

Bob

Slide3The Hidden Terminals Problem

Alice

Bob

More Collisions!

Retransmissions

Can’t get any useful

connections

Slide4Can we take two collisions and produce the two packets?

Pa

Pb

Pa

Pb

Yes, we can!

Slide5ZigZag

Exploits 802.11’s behaviorRetransmissions Same packets collide againSenders use random jitters Collisions start with interference-free bits

∆1

∆2

P

a

P

b

P

a

P

b

Interference-free Bits

Slide6How Does ZigZag Work?

∆1

∆2

Find a chunk that is

interference-free

in one collisions and has

interference

in the other

1

∆1 ≠∆2

Decode and subtract

from the other collision

1

Slide7∆2

1

2

1

∆1

How Does ZigZag Work?

Find a chunk that is

interference-free

in one collisions and has

interference

in the other

∆1 ≠∆2

Decode and subtract

from the other collision

Slide8∆2

1

2

2

∆1

How Does ZigZag Work?

3

Find a chunk that is

interference-free

in one collisions and has

interference

in the other

∆1 ≠∆2

Decode and subtract

from the other collision

Slide9∆2

1

2

4

∆1

How Does ZigZag Work?

3

3

Find a chunk that is

interference-free

in one collisions and has

interference

in the other

∆1 ≠∆2

Decode and subtract

from the other collision

Slide10∆2

1

2

4

4

∆1

How Does ZigZag Work?

3

5

Find a chunk that is

interference-free

in one collisions and has

interference

in the other

∆1 ≠∆2

Decode and subtract

from the other collision

Slide11∆2

1

6

∆1

How Does ZigZag Work?

3

5

5

2

4

Find a chunk that is

interference-free

in one collisions and has

interference

in the other

∆1 ≠∆2

Decode and subtract

from the other collision

Slide12∆2

1

6

6

∆1

How Does ZigZag Work?

2

4

3

5

7

Find a chunk that is

interference-free

in one collisions and has

interference

in the other

∆1 ≠∆2

Decode and subtract

from the other collision

Slide13∆2

1

6

8

∆1

How Does ZigZag Work?

2

4

3

5

7

7

Find a chunk that is

interference-free

in one collisions and has

interference

in the other

∆1 ≠∆2

Decode and subtract

from the other collision

Delivered 2 packets in 2 timeslots

As efficient as if the packets did not collide

Slide14ZigZag

A receiver design that decodes collisions

As efficient as if the colliding packets were sent in separate time slots

Experimental results shows that it reduces hidden terminal losses from 72% to 0.7%

Slide15How does the AP know it is a collision and where the second packet starts?

Time

AP received a collision signal

∆

Slide16Detecting Collisions and the Value of ∆

Time

AP received signal

Packets start with known preamble

AP

correlates

known preamble with signal

Correlation

Time

Correlate

∆

Preamble Correlation

Detect collision and the value of

∆

Works despite interference

because correlation with an independent signal is zero

Slide17How Does the AP Subtract the Signal?

Channel’s attenuation or phase may change between collisionsCan’t simply subtract a chunk across collisions

Alice’s signal in first collision

Alice’s signal in second collision

Slide18Subtracting a Chunk

Decode

chunk into bits

Removes effects of channel during first collision

Re-modulate

bits to get channel-free signal

Apply effect of channel

during second collision

Use correlation to estimate channel despite interference

Now, can

subtract!

Slide19What if AP Makes a Mistake?

Slide20∆1

∆2

1

1

2

2

Bad News: Errors can propagate

3

Can we deal with these errors?

What if AP Makes a Mistake?

Slide21∆1

∆2

What if AP Makes a Mistake?

Good News: Temporal Diversity

A bit is unlikely to be affected by noise in both collisions

Get two independent

decodings

Slide22Errors propagate differently in the two

decodings

For each bit, AP picks the decoding that has a higher PHY confidence [JB07, WKSK07]

Which decoded value should the AP pick?

∆1

∆2

1

1

2

2

3

AP Decodes Backwards as well as Forwards

Slide23ZigZag Generalizes

Slide24ZigZag Generalizes

∆1

∆2

1

2

1

2

Flipped order

Slide25Flipped orderDifferent packet sizes

ZigZag Generalizes

∆1

∆2

1

2

1

2

Slide26ZigZag Generalizes

1

2

3

1

2

3

1

2

3

Flipped order

Different packet sizes

Multiple colliding packets

1

2

1

2

2

1

3

3

3

Slide27ZigZag Generalizes

Flipped orderDifferent packet sizesMultiple colliding packets Capture effectSubtract Alice and combine Bob’s packet across collisions to correct errors

∆1

∆2

P

a1

P

b

P

a2

P

b

3 packets in 2 time slots

better than no collisions

Slide28Performance

Slide29Implementation

USRP Hardware

GNURadio software

Carrier Freq: 2.4-2.48GHz

BPSK modulation

Slide30USRPs

Testbed

10% HT, 10% partial HT, 80% perfectly sense each other

Each run randomly picks an AP and two clients

Co-located 802.11a nodes to find out about HTs and created the same collision patterns by the USRPs

802.11a

Slide31Throughput Comparison

Throughput

CDF of concurrent flow pairs

Slide32Throughput Comparison

802.11

Throughput

CDF of concurrent flow pairs

Hidden Terminals

Partial Hidden Terminals

Perfectly Sense

Slide33Throughput Comparison

ZigZag

Throughput

CDF of concurrent flow pairs

802.11

Hidden Terminals get high throughput

Slide34Throughput Comparison

ZigZag

Throughput

CDF of concurrent flow pairs

802.11

ZigZag Exploits Capture Effect

ZigZag improved average Throughput by 25%

Slide35Throughput Comparison

ZigZag

Throughput

CDF of concurrent flow pairs

802.11

Improved hidden terminals loss rate from 72% to 0.7%

Hidden Terminals

Slide36Is ZigZag as efficient as if the colliding packets were sent in separate slots?

For every SNR,

Check that ZigZag can match the BER of collision-free receptions

Slide37Is

ZigZag as efficient as if packets were collision-free Receptions?

SNR in dB

Bit Error Rate (BER)

Slide38Collision-Free

Receptions

Is ZigZag as efficient as if packets were collision-free Receptions?

SNR in dB

Bit Error Rate (BER)

Slide39Collision-Free

Receptions

Is ZigZag as efficient as if packets were collision-free Receptions?

ZigZag-Decoded Collisions

SNR in dB

Bit Error Rate (BER)

ZigZag is as efficient as if the colliding packets were sent separately

Slide40Three Colliding Senders

Collision!

Alice

Bob

Chris

Nodes picked randomly from testbed

Slide41Three Colliding Senders

ZigZag extends beyond two colliding senders

CDF of runs

Per-Sender Throughput

Alice

Bob

Chris

Slide42Related Work

RTS-CTSExcessive Overhead; Administrators turn it offInterference CancellationUnsuitable for 802.11 because of bit rate adaptation

Interference cancelation operates on one collision

Undecodable

Alice’s

Info

Rate

Bob’s

Info

Rate

Rmax

Rmax

Slide43Related Work

RTS-CTS

Excessive Overhead; Administrators turn it off

Interference Cancellation

Unsuitable for 802.11 because of

bit rate adaptation

ZigZag operates on two collisions

Can

decode

Alice’s

Info

Rate

Bob’s

Info

Rate

Rmax

Rmax

Slide44Conclusion

ZigZag is a receiver design that resolves collisions

It is as efficient as if the colliding packets were sent in separate time slots

It reduces hidden terminal losses from 72% to 0.7%

It enables aggressive MAC

More concurrency