Towards Collision Detection in Wireless Networks Souvik Sen, Naveen Santhapuri, Romit Roy Choudhury, Srihari Nelakuditi

2 Collision in Wireless Networks T1RT2 t0t0 t1t1 ACK Timeout Retransmit time Collision

3 Collision in Wireless Networks T1RT2 Collision t0t0 t1t1 Retransmit time Not Efficient! 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 T1RT2 Collision Ethernet BUS Collision Detection (CSMA/CD)

Can we do CSMA/CD in Wireless? Seems hard because.....

6 Wireless Signal Propagation T1 cannot send and listen in parallel Collision T1RT2 Distance Signal power 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 T2 T1 R1 T3 R3 Dont Transmit!

9 Is CSMA/CD Beneficial in Wireless? R2 Collision Detected T2 T1 R1 T3 R3 Dont Transmit! Abort Tx! Collision

10 Is CSMA/CD in Wireless Beneficial? R2 Collision Detected T2 T1 R1 T3 R3 Channel free now

11 Is CSMA/CD in Wireless Beneficial? R2 Collision Detected T2 T1 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?

13 Practical Requirements? 1. Transmitter cannot detect collision Receiver needs to detect it Tx Rx Collision!

14 Practical Requirements? 1. Transmitter cannot detect collision Receiver needs to detect it 2. Receiver needs to convey collision notification to the transmitter Tx Rx 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 Tx Rx 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 Tx Rx Collision! Practical Requirements?

17 Overview MAC PHY CrossLayer MAC PHY CrossLayer Data Transmission (S1) S=S1 TxRx If Collision, Notify Tx If Notification, Abort Tx Notify Collision (S2)

18 Overview S=S1+S2 MAC PHY CrossLayer MAC PHY CrossLayer Notify Collision (S2) Data Transmission (S1) S=S1 TxRx If Notification, Abort Tx If Collision, Notify Tx

19 Two Key Challenges S=S1+S2 2. Detect Collision in real time 2. Detect Collision in real time 1. Find Notification on Listening Antenna 1. Find Notification on Listening Antenna MAC PHY CrossLayer MAC PHY CrossLayer Notify Collision (S2) Data Transmission (S1) TxRx If Notification, Abort Tx If Collision, Notify Tx

We propose CSMA/CN We propose CSMA/CN Our key idea: Correlation 2. Detect Collision in real time 2. Detect Collision in real time 1. Find Notification on Listening Antenna 1. Find Notification on Listening Antenna

21 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 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 Correlation Sample Number Challenge 1: Detecting Notification

28 Challenge 2: Interference Detection Data T1 R Correlate for Preamble + SoftPHY Collision T2 R2 R1

What if transmitter starts second?

30 SOI starts after interference Data R Correlate (Sign(R1)) Sign(R1)Sign(R2) Collision T1 T2 R2 R1

31 Signal Correlation and Abort Data R Sign(R1) Corr (Sign(R1)) Notification! Stop Tx Collision T1 T2 R2 R1 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 like and PPR PPR detects interfered portion of received packet Transmitter sends only the interfered portion

33 Notification Detection at Tx Notification Signal << Self Signal How weak can the notification signal be? MAC PHY

34 How weak the notification signal be? Signal power Self Signal Notification Signal } 18 dB

35 How weak the notification signal be? Signal power } 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 bytes Bytes 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/CN

40 Traced Based Evaluation Upto 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 Group