towards collision detection in wireless networks souvik sen, naveen santhapuri, romit roy choudhury,...

Towards Collision Detection in Towards Collision Detection in Wireless NetworksWireless Networks

Souvik Sen, Naveen Santhapuri, Romit Roy Choudhury, Srihari Nelakuditi

2

Collision in Wireless Networks

T1 R T2

t0t1

ACK Timeout

Retransmit

timeCollision

3

Collision in Wireless Networks

T1 R T2

Collision

t0t1

Retransmit

time

Not Efficient!

T1 should have stopped right after collision

T1 should have stopped right after collision

ACK Timeout

4

Collision in Wired Networks

Transmitter aborts transmission on collision✦ Transmitter senses the signal while transmitting✦ If (sensed != transmitted), abort

T1 R T2

Collision

Ethernet BUS

Collision Detection (CSMA/CD)Collision Detection (CSMA/CD)

Can we do CSMA/CD in Wireless?Can we do CSMA/CD in Wireless?

Seems hard because.....

6

Wireless Signal Propagation

T1 cannot send and listen in parallel

Collision

T1 R T2

Distance

Signalpower

Signal not same at different locations

But what if we could do CSMA/CD in wireless?But what if we could do CSMA/CD in wireless?

8

Is CSMA/CD Beneficial in Wireless?

R2

Collision Detected

Collision

T2T1

R1

T3

R3

Dont Transmit!

9

Is CSMA/CD Beneficial in Wireless?

R2

Collision Detected

T2T1

R1

T3

R3

Dont Transmit!

Abort Tx!

Collision

10

Is CSMA/CD in Wireless Beneficial?

R2

Collision Detected

T2T1

R1

T3

R3

Channel free now

11

Is CSMA/CD in Wireless Beneficial?

R2

Collision Detected

T2T1

R1

T3

R3

CSMA/CD frees the channel for other transmissions

CSMA/CD frees the channel for other transmissions

Lets Transmit!

Can we imitate CSMA/CD on Wireless?Can we imitate CSMA/CD on Wireless?

13

Practical Requirements?

1. Transmitter cannot detect collision✦ Receiver needs to detect it

TxRx

Collision!

14

Practical Requirements?

1. Transmitter cannot detect collision✦ Receiver needs to detect it

2. Receiver needs to convey collision notification to the transmitter

TxRx

Collision!

15

1. Transmitter cannot detect collision✦ Receiver needs to detect it

2. Receiver needs to convey collision notification to the transmitter

3. Transmitter needs an additional antenna✦ To receive notification

TxRx

Collision!

Practical Requirements?

16

1. Transmitter cannot detect collision✦ Receiver needs to detect it

2. Receiver needs to convey collision notification to the transmitter

3. Transmitter needs an additional antenna✦ To receive notification

TxRx

Collision!

Practical Requirements?

17

Overview

MAC

PHYCro

ssLa

yer MAC

PHY Cro

ssLa

yer

Data Transmission (S1)

S=S1

Tx

Rx

If Collision,Notify Tx

If Notification,Abort Tx

Notify Collision (S2)

18

Overview

S=S1+S2MAC

PHYCro

ssLa

yer MAC

PHY Cro

ssLa

yer

Notify Collision (S2)

Data Transmission (S1)

S=S1

Tx

Rx

If Notification,Abort Tx

If Collision,Notify Tx

19

Two Key Challenges

S=S1+S2

2. Detect Collisionin real time

2. Detect Collisionin real time

1. Find Notification onListening Antenna

1. Find Notification onListening Antenna

MAC

PHYCro

ssLa

yer MAC

PHY Cro

ssLa

yer

Notify Collision (S2)

Data Transmission (S1)Tx

Rx

If Notification,Abort Tx

If Collision,Notify Tx

We proposeCSMA/CN

We proposeCSMA/CN

Our key idea: CorrelationOur key idea: Correlation

2. Detect Collisionin real time

2. Detect Collisionin real time

1. Find Notification onListening Antenna

1. Find Notification onListening Antenna

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Challenge 1: Detecting Notification

Hard to decode notification on same channel✦ Self-signal too strong

Let Tx and Rx share a unique signature

Tx correlates with shared signature✦ Detects collision notification, aborts

Observe: No decoding, just correlateObserve: No decoding, just correlate

MAC

PHY

22

Self Signal

Notification Signature

Challenge 1: Detecting Notification

23

Self Signal

Notification Signature

Correlation

Challenge 1: Detecting Notification

24

Notification Signature

Correlation

Self Signal

Challenge 1: Detecting Notification

25

Notification Signature

Correlation

Self Signal

Challenge 1: Detecting Notification

26

Notification Signature

Correlation

Self Signal

Challenge 1: Detecting Notification

27

Whenever there is a notification, there is a jump in correlation

Whenever there is a notification, there is a jump in correlation

Cor

rela

tion

Sample Number

Challenge 1: Detecting Notification

28

Challenge 2: Interference Detection

Data Data

T1

R

Correlate for Preamble+

SoftPHY

Collision

T2

R2R1

What if transmitter starts second?What if transmitter starts second?

30

SOI starts after interference

Data Data

R

Correlate (Sign(R1))

Sign(R1)

Sign(R2)

Collision

T1T2

R2R1

31

Signal Correlation and Abort

Data Data

R

Sign(R1)

Corr (Sign(R1))

Notification!Stop Tx

Collision

T1T2

R2R1

Correlate (Sign(R1))

Sign(R1)

Sign(R2)

32

Performance Evaluation

7 node USRP testbed

Zigbee CC2420 PHY

Max data rate: 250Kbps

Signature size: 5 bytes

Compare with 802.11-like and PPR✦ PPR detects interfered portion of received packet

• Transmitter sends only the interfered portion

33

Notification Detection at Tx

Notification Signal << Self SignalNotification Signal << Self Signal

How weak can the notification signal be?

MAC

PHY

34

How weak the notification signal be?

Signalpower

Self Signal

Notification Signal

}18 dB

✔

35

How weak the notification signal be?

Signalpower

✘}18 dB

Self Signal

Notification Signal

36

Interference Detection at Rx

Interference detection accuracy of 93%

Receiver should detect interference quickly

Quicker detection Faster Tx abortion

MAC

PHY

37

Interference Detection: Speed

CSMA/CN predicts collision within 7 bytesCSMA/CN predicts collision within 7 bytesBytes after interferer started

38

Testbed Experimentation

One link doing CSMA/CN

CSMA/CN link has an exposed and hidden terminal

Whenever CSMA/CN link fails due to interference

✦ CSMA/CN link stops✦ Exposed terminal transmits reducing channel

wastage

39

Testbed Throughput

PPR continues to transmit under collision, worse than CSMA/CNPPR continues to transmit under collision, worse than CSMA/CN

40

Traced Based Evaluation

Upto 50% gain in per link throughputUpto 50% gain in per link throughput

50%

Throughput in Kbps

41

Summary

CSMA/CN imitates CSMA/CD in wireless

Rx uses correlation to detect interference

Tx uses correlation to detect notification

Others can utilize freed-up channel

42

Limitation and Future Work

Improve Correlation:✦ Suppress known self signal

Multiple Interferers:✦ Interference detection logic yet to be tested

Interference due to notification:✦ Collision notification can interfere with other links✦ Recover using FEC

Questions, comments?Thank you

Duke SyNRG Research Grouphttp://synrg.ee.duke.edu