CSMA/CN: Carrier Sense Multiple Access with Collision Notification Souvik Sen, Romit Roy Choudhury, Srihari Nelakuditi

Collision in Wireless Networks T1 R T2 time 2

Collision in Wireless Networks T1 R T2 time 2

Collision in Wireless Networks T1 R T2 time 2

Collision in Wireless Networks T1 R T2 Collision time 2

Collision in Wireless Networks T1 R T2 Collision ACK Timeout time 2

Collision in Wireless Networks T1 R T2 Collision ACK Timeout time Retransmit 2

Collision in Wireless Networks T1 R T2 time Not Efficient! Collision ACK Timeout Retransmit 3

Collision in Wireless Networks T1 R T2 Collision ACK Timeout time Not Efficient! Retransmit Better if T1 stops right after collision 3

Collision in Wired Networks R T2 Collision Ethernet 4

Collision in Wired Networks R T2 Collision Ethernet ✦ Transmitter aborts transmission on collision 4

Collision in Wired Networks R T2 Collision Ethernet ✦ Transmitter aborts transmission on collision ✦ Transmitter senses the signal while transmitting 4

Collision in Wired Networks R T2 Collision Ethernet ✦ Transmitter aborts transmission on collision ✦ Transmitter senses the signal while transmitting If (sensed != transmitted), abort 4

Collision in Wired Networks R T2 Collision Ethernet ✦ Transmitter aborts transmission on collision ✦ Transmitter senses the signal while transmitting If (sensed != transmitted), abort Collision Detection (CSMA/CD) 4

Why not do CSMA/CD in Wireless?

Why not do CSMA/CD in Wireless? Has been hard because.....

Wireless Signal Propagation Collision Signal power Distance 6

Wireless Signal Propagation T1 does not send and listen in parallel T1 R T2 Collision Signal power Distance 6

Wireless Signal Propagation T1 does not send and listen in parallel T1 R T2 Collision Signal not same at different locations Signal power Distance 6

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

Is CSMA/CD Beneficial in Wireless? T1 R1 8

Is CSMA/CD Beneficial in Wireless? T2 T1 R2 R1 8

Is CSMA/CD Beneficial in Wireless? T2 T1 R2 R1 8

Is CSMA/CD Beneficial in Wireless? Dont Transmit! T2 T1 R2 R1 8

Is CSMA/CD Beneficial in Wireless? Dont Transmit! T2 T1 T3 R2 R1 R3 8

Is CSMA/CD Beneficial in Wireless? Dont Transmit! T2 T1 T3 R2 R1 R3 Collision 8

Is CSMA/CD Beneficial in Wireless? Dont Transmit! Collision Detected T2 T1 T3 R2 R1 R3 Collision 8

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

Is CSMA/CD in Wireless Beneficial? Channel free now Abort Tx! T2 T1 T3 R2 R1 R3 9

Is CSMA/CD in Wireless Beneficial? Lets Transmit! T2 T1 T3 R2 R1 R3 10

Is CSMA/CD in Wireless Beneficial? Lets Transmit! T2 T1 T3 R2 R1 R3 CSMA/CD frees the channel for other transmissions 10

Can we imitate CSMA/CD on Wireless?

Practical Requirements? 1. Transmitter cannot detect collision ✦ Receiver needs to detect it Rx Tx 12

Practical Requirements? 1. Transmitter cannot detect collision ✦ Receiver needs to detect it Collision! Rx Tx 12

Practical Requirements? 1. Transmitter cannot detect collision ✦ Receiver needs to detect it 2. Receiver needs to convey collision notification to the transmitter Collision! Rx Tx 13

Practical Requirements? 1. Transmitter cannot detect collision ✦ Receiver needs to detect it 2. Receiver needs to convey collision notification to the transmitter Collision! Rx Tx 13

Practical Requirements? 1. Transmitter cannot detect collision ✦ Receiver needs to detect it Receiver needs to convey collision notification to the transmitter Transmitter needs an additional antenna ✦ To receive notification Collision! Rx Tx 14

Practical Requirements? 1. Transmitter cannot detect collision ✦ Receiver needs to detect it Receiver needs to convey collision notification to the transmitter Transmitter needs an additional antenna ✦ To receive notification Collision! Rx Tx 14

Practical Requirements? 1. Transmitter cannot detect collision ✦ Receiver needs to detect it Receiver needs to convey collision notification to the transmitter Transmitter needs an additional antenna ✦ To receive notification Collision! Rx Tx 15

Overview MAC MAC CrossLayer CrossLayer PHY PHY Tx Rx 16

Overview Tx Rx 16 MAC S=S1 MAC PHY PHY Data Transmission (S1) CrossLayer CrossLayer PHY PHY Tx Data Transmission (S1) Rx 16

Overview Tx Rx Detect Collision 16 MAC S=S1 MAC PHY PHY CrossLayer CrossLayer PHY PHY Tx Data Transmission (S1) Rx 16

Overview Tx Rx Detect Collision 16 Look for Notification MAC S=S1 MAC CrossLayer CrossLayer PHY PHY Tx Data Transmission (S1) Rx 16

Overview Tx Rx 17 If Collision, Notify Tx Look for Notification MAC S=S1 MAC CrossLayer CrossLayer PHY PHY Tx Data Transmission (S1) Rx 17

Overview Tx Rx 17 If Collision, Notify Tx Look for Notification MAC S=S1 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 17

Overview Tx Rx 18 If Collision, Notify Tx Look for Notification MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 18

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

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

Two Key Challenges Tx Rx 20 If Notification, Abort Tx If Collision, Notify Tx MAC S=S1+S2 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 20

Two Key Challenges Detect Collision in Real Time Tx Rx 20 If Notification, Abort Tx If Collision, Notify Tx MAC S=S1+S2 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 20

Two Key Challenges Find Notification on Listening Antenna Detect Collision in Real Time If Notification, Abort Tx If Collision, Notify Tx MAC S=S1+S2 MAC CrossLayer CrossLayer Notify Collision (S2) PHY PHY Tx Data Transmission (S1) Rx 20

Find Notification on Listening Antenna Detect Collision in Real Time We propose CSMA/CN

Find Notification on Listening Antenna Detect Collision in Real Time We propose CSMA/CN Our key idea: Signal Correlation

Signal Correlation Background Signal Known Signal

Signal Correlation Correlation Background Signal Known Signal

Signal Correlation Correlation Sample Number 27

Signal Correlation Whenever there is a known signal, Sample Number Whenever there is a known signal, there is a jump in correlation 27

Find Notification on Listening Antenna Detect Collision in Real Time We propose CSMA/CN Our key idea: Signal Correlation

Detect Collision in Real Time Find Notification on Listening Antenna We propose CSMA/CN Our key idea: Signal Correlation

Challenge 1: Collision Detection Data R2 R R1 30

Challenge 1: Collision Detection Data Data R2 R R1 30

Challenge 1: Collision Detection Data Data R2 R R1 Correlate for Preamble + SoftPHY hints 30

Challenge 1: Collision Detection Data Data R2 R R1 Collision Correlate for Preamble + SoftPHY hints 30

What if transmitter starts second?

My signal starts after interferer Data R2 R R1 32

My signal starts after interferer Data Data R2 R R1 32

My signal starts after interferer Data Data Sign(R1) Sign(R2) R2 R R1 32

My signal starts after interferer Data Data Sign(R1) Sign(R2) R2 R R1 Correlate (Sign(R1)) 32

My signal starts after interferer Data Data Sign(R1) Sign(R2) R2 R R1 Collision Correlate (Sign(R1)) 32

Signal Correlation and Abort Sign(R1) Data Data Sign(R1) Sign(R2) R2 R R1 Collision Correlate (Sign(R1)) 33

Signal Correlation and Abort Notification! Stop Tx Corr (Sign(R1)) T1 T2 Sign(R1) Data Data Sign(R1) Sign(R2) R2 R R1 Collision Correlate (Sign(R1)) 33

Signal Correlation and Abort ACK Sign(R1) ACK Sign(R2) T2 T1 Data Data R2 R R1 34

Find Notification on Listening Antenna Detect Collision in Real Time We propose CSMA/CN Our key idea: Signal Correlation

Challenge 2: Detecting Notification Collision notification is on same channel Hard to decode notification MAC PHY ✦ Self-signal too strong ✦ Our approach: use correlation ✦ Let Tx and Rx share a unique signature Tx correlates with shared signature Detects collision notification, aborts 36

Challenge 2: Detecting Notification Collision notification is on same channel Hard to decode notification MAC PHY ✦ Self-signal too strong ✦ Our approach: use correlation ✦ Let Tx and Rx share a unique signature Tx correlates with shared signature Detects collision notification, aborts Observe: No decoding, just correlate 36

Notification Detection at Tx MAC PHY Notification Detection at Tx Notification Signal << Self Signal 37

Notification Detection at Tx MAC PHY Notification Detection at Tx Notification Signal << Self Signal How weak can the notification signal be? 37

How weak the notification signal be? Signal power Self Signal Notification Signal 38

}16 dB ✔ How weak the notification signal be? Signal power Self Signal 38

}16 dB ✘ How weak the notification signal be? Signal power Self Signal 39

How can we do better for weaker clients?

How can we do better for weaker clients? Transmitted signal is a known signal - Can be effectively canceled Opportunity: Transmit and listening antenna are on the same device

Detecting Notification: Signal Cancelation Wireless ✦ Pass the Tx signal over wire Wired ✦ Listen antenna has two copies of the Tx signal ✦ One over wire, other over wireless Both the copies have same filter, freq. offset effects Align the two signals using sampling offset info Subtract the wired signal from the wireless

Correlation Performance with Cancelation + /- False of Fraction 0.35 0.3 0.25 0.2 0.15 0.1 0.05 Fraction of False +/- False positives False negatives False positivies False negatives 0 4 8 12 16 20 24 28 32 36 (Self Signal) - (Notification Signature)(dB) (Self Signal) - (Notification Signature)(dB) Reliable detection for upto 34dB below

Detecting Notification: Antenna Orientation ✦ Antenna orientation matters ✦ Due to polarization effects Listener Tx Self Signal @ 1ft. antenna separation Self Signal @ 2ft. antenna separation Client signal at listening antenna Self Signal Notification Signal #1 60 50 #2 SNR in dB 40 30 #3 20 #4 10 #1 #2 #3 #4 Configuration Number

Detecting Notification: Antenna Orientation ✦ Antenna orientation matters ✦ Due to polarization effects Listener Tx Self Signal @ 1ft. antenna separation Self Signal @ 2ft. antenna separation Client signal at listening antenna Self Signal Notification Signal #1 60 50 18dB #2 SNR in dB 40 30 #3 20 #4 Best Configuration 10 #1 #2 #3 #4 Configuration Number 44

Detecting Notification: Antenna Orientation ✦ Antenna orientation matters ✦ Due to polarization effects Listener Tx Self Signal @ 1ft. antenna separation Self Signal @ 2ft. antenna separation Client signal at listening antenna Self Signal Notification Signal #1 60 50 18dB #2 SNR in dB 40 30 #3 20 6dB #4 Best Configuration 10 #1 #2 #3 #4 Configuration Number 44

How dissimilar should signatures be? Each CSMA/CN receiver needs 2 signatures Different frequency offsets add diversity Results show 6 out of 20 bytes diff enough Hamming Distance (bytes) 2 4 6 8 10 False Positive 17% 9% 2.8% 2.2% 1.6%

Performance Evaluation 10 node USRP testbed BPSK, QPSK modulation Signature size: 20 bytes Topologies with three links doing CSMA/CN Compare with 802.11-like and PPR ✦ PPR retransmits only suspected bits of the packet

Collision Detection at Rx MAC Collision Detection at Rx PHY Collision detection accuracy of 92% Receiver detects collision within 20 bytes Total turnaround time for CN signature 18us ✦ Quicker turnaround Faster Tx abortion

Testbed Throughput Gain 1 802.11-like PPR CSMA/CN Fraction of links 0.8 links of Fr ac ti on 0.6 25% 0.4 0.2 1 1.2 1.4 1.6 1.8 2 Throughput in Mbps 2.2 2.4 Tput in Mbps 48

Testbed Throughput Gain 1 802.11-like PPR CSMA/CN Fraction of links 0.8 links of Fr ac ti on 0.6 25% 0.4 0.2 1 1.2 1.4 1.6 1.8 2 Throughput in Mbps 2.2 2.4 Tput in Mbps Upto 25% median throughput gain over 802.11 48

Why CSMA/CN has gain over PPR? 1 802.11-like PPR CSMA/CN Fraction of links 0.8 links of Fr ac ti on 0.6 0.4 0.2 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 (Correct Bits/Transmitted Bits) Equivalent success ratio 49

Why CSMA/CN has gain over PPR? 1 802.11-like PPR CSMA/CN Fraction of links 0.8 links of Fr ac ti on 0.6 0.4 0.2 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 (Correct Bits/Transmitted Bits) Equivalent success ratio PPR continues to transmit under collision CSMA/CN aborts colliding transmission 49

Does CSMA/CN gain at high rates? ( % ) P P R ov er ga in Throughput Throughput gain over PPR (%) 24 22 20 18 16 6Mbps 18Mbps 36Mbps 54Mbps 14 12 10 0 5 10 15 20 Collisions due to hidden terminal (%) Collisions due to hidden terminal (%) 50

Does CSMA/CN gain at high rates? ( % ) P P R ov er ga in Throughput Throughput gain over PPR (%) 24 22 20 18 16 6Mbps 18Mbps 36Mbps 54Mbps 14 12 10 0 5 10 15 20 Collisions due to hidden terminal (%) Collisions due to hidden terminal (%) CSMA/CN gains also at higher rates 50

Does CSMA/CN gain at high rates? ( % ) P P R ov er ga in Throughput Throughput gain over PPR (%) 24 22 20 18 16 6Mbps 18Mbps 36Mbps 54Mbps 14 12 10 0 5 10 15 20 Collisions due to hidden terminal (%) Collisions due to hidden terminal (%) Alleviates collision losses due to hidden terminals and same backoff

Limitation and Future Work ✦ Improve Correlation ✦ Analog interference cancelers ✦ Interference due to notification ✦ Recover using FEC ✦ Coexistence with MIMO ✦ One listen antenna can be shared by MIMO antennas CSMA/CN allows aggressive network design Low CS threshold, backoff etc.

Taking a Step Back Signals Decoded Bits Control Data 53

Taking a Step Back Signals Decoded Bits Control Data Full Duplex 53

Taking a Step Back 53 Correlation Control Full Duplex Signals Decoded Bits Control Control Data Full Duplex 53

Taking a Step Back CSMA/CN 53 Correlation Control Full Duplex Signals Decoded Bits Control Control Data CSMA/CN Full Duplex 53

Summary CSMA/CN imitates CSMA/CD in wireless Uses correlation, enough for 1 bit feedback Traditional protocols: recovers collision CSMA/CN aborts collision to summarize imitates CD in wireless .. uses corr to convey one bit info about collision .. while traditional collisions recover much later .. imeediately aborts collisions .. 54

Summary Prevention is Better than Cure CSMA/CN imitates CSMA/CD in wireless Uses correlation, enough for 1 bit feedback Traditional protocols: recovers collision CSMA/CN aborts collision Prevention is Better than Cure 54

Duke SyNRG Research Group Questions, comments? Thank you Duke SyNRG Research Group http://synrg.ee.duke.edu

Collision Detection at Rx MAC Collision Detection at Rx PHY 1.6 Correctly decoded Preamble decoded but packet lost Preamble lost Detection Accuracy 1.4 Fraction of packets 1.2 1 0.8 0.6 0.4 0.2 2 4 6 8 10 12 SIR in dB (SNR S - SNR I) 14 56

Collision Detection at Rx MAC Collision Detection at Rx PHY 1.6 Correctly decoded Preamble decoded but packet lost Preamble lost Detection Accuracy 1.4 Fraction of packets 1.2 1 0.8 0.6 0.4 0.2 2 4 6 8 10 12 SIR in dB (SNR S - SNR I) 14 Collision Detection Accuracy of 92% 56

Throughput Evaluation using Simulation Throughput gain over PPR (%) 24 22 20 18 10% Hidden Terminal 16 6Mbps 18Mbps 36Mbps 54Mbps 14 12 10 5 10 15 20 25 30 35 40 45 Number of clients 57

Throughput Evaluation using Simulation Throughput gain over PPR (%) 24 22 20 18 10% Hidden Terminal 16 6Mbps 18Mbps 36Mbps 54Mbps 14 12 10 5 10 15 20 25 30 35 40 45 Number of clients More Collisions due to same backoff with more clients: More Throughput Gain 57

Detecting Notification: Correlation Performance 1.2 Correlation when Signature absent Correlation when Signature present Normalized Correlation Value 1 0.8 0.6 0.4 0.2 4 8 12 16 (Self Signal) - (Notification Signature)(dB) Reliable detection for until below 16dB 58