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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
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 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