Transcript Towards Collision Detection in Wireless Networks

Towards Collision Detection in
Wireless Networks
Souvik Sen,
Naveen Santhapuri, Romit Roy Choudhury, Srihari Nelakuditi
Collision in Wireless Networks
T1
R
t0
T2
t1
Collision
time
ACK Timeout
Retransmit
2
Collision in Wireless Networks
T1
R
t0
T2
t1
Collision
time
ACK Timeout
Not Efficient!
Retransmit
T1 should have stopped
right after collision
3
Collision in Wired Networks
T1
R
T2
Collision
Ethernet BUS
Transmitter aborts transmission on collision
✦ Transmitter senses the signal while transmitting
✦ If (sensed != transmitted), abort
Collision Detection (CSMA/CD)
4
Can we do CSMA/CD in Wireless?
Seems hard because.....
Wireless Signal Propagation
T1 cannot send and listen in parallel
T1
R
T2
Collision
Signal
power
Signal not same at different locations
Distance
6
But what if we could do
CSMA/CD in wireless?
Is CSMA/CD Beneficial in Wireless?
Dont
Transmit!
Collision
Detected
T2
T3
T1
R2
R1
Collision
R3
8
Is CSMA/CD Beneficial in Wireless?
Dont
Transmit!
Collision
Abort Tx!
Detected
T2
T3
T1
R2
R1
Collision
R3
9
Is CSMA/CD in Wireless Beneficial?
Channel
free now
Collision
Detected
T2
T3
T1
R2
R1
R3
10
Is CSMA/CD in Wireless Beneficial?
Lets
Transmit!
Collision
Detected
T2
T3
T1
R2
R1
R3
CSMA/CD frees the channel for
other transmissions
11
Can we imitate CSMA/CD on Wireless?
Practical Requirements?
1. Transmitter cannot detect collision
✦ Receiver needs to detect it
Collision!
Tx
Rx
13
Practical Requirements?
1. Transmitter cannot detect collision
✦ Receiver needs to detect it
2. Receiver needs to convey
collision notification to the transmitter
Collision!
Tx
Rx
14
Practical Requirements?
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
Collision!
Tx
Rx
15
Practical Requirements?
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
Collision!
Tx
Rx
16
Overview
If Collision,
Notify Tx
If Notification,
Abort Tx
S=S1
MAC
Notify Collision (S2)
PHY
Tx
PHY
Data Transmission (S1)
CrossLayer
CrossLayer
MAC
Rx
17
Overview
If Collision,
Notify Tx
If Notification,
Abort Tx
S=S1+S2
S=S1
MAC
Notify Collision (S2)
PHY
Tx
PHY
Data Transmission (S1)
CrossLayer
CrossLayer
MAC
Rx
18
Two Key Challenges
1. Find Notification on
Listening Antenna
2. Detect Collision
in real time
If Collision,
Notify Tx
If Notification,
Abort Tx
S=S1+S2
MAC
Notify Collision (S2)
PHY
Tx
PHY
Data Transmission (S1)
CrossLayer
CrossLayer
MAC
Rx
19
1. Find Notification on
Listening Antenna
2. Detect Collision
in real time
We propose
CSMA/CN
Our key idea: Correlation
Challenge 1: Detecting Notification
Hard to decode notification on same channel
✦ Self-signal too strong
MAC
PHY
Let Tx and Rx share a unique signature
Tx correlates with shared signature
✦ Detects collision notification, aborts
Observe: No decoding, just correlate
21
Challenge 1: Detecting Notification
Self Signal
Notification Signature
22
Challenge 1: Detecting Notification
Correlation
Self Signal
Notification Signature
23
Challenge 1: Detecting Notification
Correlation
Self Signal
Notification Signature
24
Challenge 1: Detecting Notification
Correlation
Self Signal
Notification Signature
25
Challenge 1: Detecting Notification
Correlation
Self Signal
Notification Signature
26
Correlation
Challenge 1: Detecting Notification
Sample Number
Whenever there is a notification,
there is a jump in correlation
27
Challenge 2: Interference Detection
T2
T1
Data
Data
R1
R
R2
Collision
Correlate for Preamble
+
SoftPHY
28
What if transmitter starts second?
SOI starts after interference
T2
T1
Data
Data
Sign(R1)
R1
R
Correlate (Sign(R1))
Sign(R2)
R2
Collision
30
Signal Correlation and Abort
Notification!
Stop Tx
Corr (Sign(R1))
T1
T2
Sign(R1)
Data
Data
Sign(R1)
R1
R
Correlate (Sign(R1))
Sign(R2)
R2
Collision
31
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
32
Notification Detection at Tx
MAC
PHY
Notification Signal << Self Signal
How weak can the notification signal be?
33
How weak the notification signal be?
}✔
18 dB
Signal
power
Self
Signal
Notification
Signal
34
How weak the notification signal be?
}
Signal
power
18 dB
✘
Self
Signal
Notification
Signal
35
Interference Detection at Rx
MAC
PHY
Interference detection accuracy of 93%
Receiver should detect interference quickly
Quicker detection
Faster Tx abortion
36
Interference Detection: Speed
Bytes after interferer started
CSMA/CN predicts collision within 7 bytes
37
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
38
Testbed Throughput
PPR continues to transmit under
collision, worse than CSMA/CN
39
Traced Based Evaluation
50%
Throughput in Kbps
Upto 50% gain in per link throughput
40
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
41
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
42
Questions, comments?
Thank you
Duke SyNRG Research Group
http://synrg.ee.duke.edu