Shyamnath Gollakota Dina Katabi The Hidden Terminals Problem Collision Alice Bob The Hidden Terminals Problem Alice Bob More Collisions Retransmissions Cant get any useful connections ID: 776256
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Slide1
Decoding 802.11 Collisions
Shyamnath GollakotaDina Katabi
Slide2The Hidden Terminals Problem
Collision!
Alice
Bob
Slide3The Hidden Terminals Problem
Alice
Bob
More Collisions!
Retransmissions
Can’t get any useful
connections
Slide4Can we take two collisions and produce the two packets?
Pa
Pb
Pa
Pb
Yes, we can!
Slide5ZigZag
Exploits 802.11’s behaviorRetransmissions Same packets collide againSenders use random jitters Collisions start with interference-free bits
∆1
∆2
P
a
P
b
P
a
P
b
Interference-free Bits
Slide6How Does ZigZag Work?
∆1
∆2
Find a chunk that is
interference-free
in one collisions and has
interference
in the other
1
∆1 ≠∆2
Decode and subtract
from the other collision
1
Slide7∆2
1
2
1
∆1
How Does ZigZag Work?
Find a chunk that is
interference-free
in one collisions and has
interference
in the other
∆1 ≠∆2
Decode and subtract
from the other collision
Slide8∆2
1
2
2
∆1
How Does ZigZag Work?
3
Find a chunk that is
interference-free
in one collisions and has
interference
in the other
∆1 ≠∆2
Decode and subtract
from the other collision
Slide9∆2
1
2
4
∆1
How Does ZigZag Work?
3
3
Find a chunk that is
interference-free
in one collisions and has
interference
in the other
∆1 ≠∆2
Decode and subtract
from the other collision
Slide10∆2
1
2
4
4
∆1
How Does ZigZag Work?
3
5
Find a chunk that is
interference-free
in one collisions and has
interference
in the other
∆1 ≠∆2
Decode and subtract
from the other collision
Slide11∆2
1
6
∆1
How Does ZigZag Work?
3
5
5
2
4
Find a chunk that is
interference-free
in one collisions and has
interference
in the other
∆1 ≠∆2
Decode and subtract
from the other collision
Slide12∆2
1
6
6
∆1
How Does ZigZag Work?
2
4
3
5
7
Find a chunk that is
interference-free
in one collisions and has
interference
in the other
∆1 ≠∆2
Decode and subtract
from the other collision
Slide13∆2
1
6
8
∆1
How Does ZigZag Work?
2
4
3
5
7
7
Find a chunk that is
interference-free
in one collisions and has
interference
in the other
∆1 ≠∆2
Decode and subtract
from the other collision
Delivered 2 packets in 2 timeslots
As efficient as if the packets did not collide
Slide14ZigZag
A receiver design that decodes collisions
As efficient as if the colliding packets were sent in separate time slots
Experimental results shows that it reduces hidden terminal losses from 72% to 0.7%
Slide15How does the AP know it is a collision and where the second packet starts?
Time
AP received a collision signal
∆
Slide16Detecting Collisions and the Value of ∆
Time
AP received signal
Packets start with known preamble
AP
correlates
known preamble with signal
Correlation
Time
Correlate
∆
Preamble Correlation
Detect collision and the value of
∆
Works despite interference
because correlation with an independent signal is zero
Slide17How Does the AP Subtract the Signal?
Channel’s attenuation or phase may change between collisionsCan’t simply subtract a chunk across collisions
Alice’s signal in first collision
Alice’s signal in second collision
Slide18Subtracting a Chunk
Decode
chunk into bits
Removes effects of channel during first collision
Re-modulate
bits to get channel-free signal
Apply effect of channel
during second collision
Use correlation to estimate channel despite interference
Now, can
subtract!
Slide19What if AP Makes a Mistake?
Slide20∆1
∆2
1
1
2
2
Bad News: Errors can propagate
3
Can we deal with these errors?
What if AP Makes a Mistake?
Slide21∆1
∆2
What if AP Makes a Mistake?
Good News: Temporal Diversity
A bit is unlikely to be affected by noise in both collisions
Get two independent
decodings
Slide22Errors propagate differently in the two
decodings
For each bit, AP picks the decoding that has a higher PHY confidence [JB07, WKSK07]
Which decoded value should the AP pick?
∆1
∆2
1
1
2
2
3
AP Decodes Backwards as well as Forwards
Slide23ZigZag Generalizes
Slide24ZigZag Generalizes
∆1
∆2
1
2
1
2
Flipped order
Slide25Flipped orderDifferent packet sizes
ZigZag Generalizes
∆1
∆2
1
2
1
2
Slide26ZigZag Generalizes
1
2
3
1
2
3
1
2
3
Flipped order
Different packet sizes
Multiple colliding packets
1
2
1
2
2
1
3
3
3
Slide27ZigZag Generalizes
Flipped orderDifferent packet sizesMultiple colliding packets Capture effectSubtract Alice and combine Bob’s packet across collisions to correct errors
∆1
∆2
P
a1
P
b
P
a2
P
b
3 packets in 2 time slots
better than no collisions
Slide28Performance
Slide29Implementation
USRP Hardware
GNURadio software
Carrier Freq: 2.4-2.48GHz
BPSK modulation
Slide30USRPs
Testbed
10% HT, 10% partial HT, 80% perfectly sense each other
Each run randomly picks an AP and two clients
Co-located 802.11a nodes to find out about HTs and created the same collision patterns by the USRPs
802.11a
Slide31Throughput Comparison
Throughput
CDF of concurrent flow pairs
Slide32Throughput Comparison
802.11
Throughput
CDF of concurrent flow pairs
Hidden Terminals
Partial Hidden Terminals
Perfectly Sense
Slide33Throughput Comparison
ZigZag
Throughput
CDF of concurrent flow pairs
802.11
Hidden Terminals get high throughput
Slide34Throughput Comparison
ZigZag
Throughput
CDF of concurrent flow pairs
802.11
ZigZag Exploits Capture Effect
ZigZag improved average Throughput by 25%
Slide35Throughput Comparison
ZigZag
Throughput
CDF of concurrent flow pairs
802.11
Improved hidden terminals loss rate from 72% to 0.7%
Hidden Terminals
Slide36Is ZigZag as efficient as if the colliding packets were sent in separate slots?
For every SNR,
Check that ZigZag can match the BER of collision-free receptions
Slide37Is
ZigZag as efficient as if packets were collision-free Receptions?
SNR in dB
Bit Error Rate (BER)
Slide38Collision-Free
Receptions
Is ZigZag as efficient as if packets were collision-free Receptions?
SNR in dB
Bit Error Rate (BER)
Slide39Collision-Free
Receptions
Is ZigZag as efficient as if packets were collision-free Receptions?
ZigZag-Decoded Collisions
SNR in dB
Bit Error Rate (BER)
ZigZag is as efficient as if the colliding packets were sent separately
Slide40Three Colliding Senders
Collision!
Alice
Bob
Chris
Nodes picked randomly from testbed
Slide41Three Colliding Senders
ZigZag extends beyond two colliding senders
CDF of runs
Per-Sender Throughput
Alice
Bob
Chris
Slide42Related Work
RTS-CTSExcessive Overhead; Administrators turn it offInterference CancellationUnsuitable for 802.11 because of bit rate adaptation
Interference cancelation operates on one collision
Undecodable
Alice’s
Info
Rate
Bob’s
Info
Rate
Rmax
Rmax
Slide43Related Work
RTS-CTS
Excessive Overhead; Administrators turn it off
Interference Cancellation
Unsuitable for 802.11 because of
bit rate adaptation
ZigZag operates on two collisions
Can
decode
Alice’s
Info
Rate
Bob’s
Info
Rate
Rmax
Rmax
Slide44Conclusion
ZigZag is a receiver design that resolves collisions
It is as efficient as if the colliding packets were sent in separate time slots
It reduces hidden terminal losses from 72% to 0.7%
It enables aggressive MAC
More concurrency