wifi-nano: reclaiming wifi efficiency through 800 ns slotsem9/publications/wifi-nanoslides.pdf ·...
TRANSCRIPT
Eugenio Magistretti † Krishna Kant Chintalapudi *
Božidar Radunović # Ramachandran Ramjee *
WiFi-Nano: ReclaimingWiFi Efficiency through
800 ns Slots
† Rice University * Microsoft Research India
# Microsoft Research Cambridge
Problem Overview
1997802.11
[Mbp
s]
§ 802.11 data-rates have increased from 1 Mbpsto 1 Gbps
1999802.11a/b
2003802.11g
2009802.11n
2012802.11ac/ad
Problem Overview[M
bps]
~55 Mbps
§ 802.11 data-rates have increased from 1 Mbpsto 1 Gbps
§ Throughput performance has not seen acommensurate increase
1997802.11
1999802.11a/b
2003802.11g
2009802.11n
2012802.11ac/ad
Contribution
WiFi-Nano increases 802.11throughput up to 100%
Why Throughput << Data-rate?
DIFS
MediumAccess101.5µs
Preamble20µs 224µs
Data
SIFS
ACK
7 6 5 4 3 2 1 0
405.5 µs
44µs54 Mbps
Overhead ~45%
Why Throughput << Data-rate?
DIFS
MediumAccess101.5µs
Preamble20µs 224µs
Data
SIFS
ACK
7 6 5 4 3 2 1 0
405.5 µs
44µs54 Mbps
7 6 5 4 3 2 1 0225.5 µs
300 Mbps
MediumAccess101.5µs
Preamble32µs 40µs
Data
SIFS
ACK36µs
DIFS
Overhead ~45%
Overhead ~82%
Why Throughput << Data-rate?
DIFS
MediumAccess101.5µs
Preamble20µs 224µs
Data
SIFS
ACK
7 6 5 4 3 2 1 0
405.5 µs
44µs54 Mbps
7 6 5 4 3 2 1 0225.5 µs
300 Mbps
MediumAccess101.5µs
Preamble32µs 40µs
Data
SIFS
ACK36µs
7 6 5 4 3 2 1 0221.5 µs
600 Mbps
MediumAccess101.5µs
Preamble40µs 20µs
Data
SIFS
ACK44µs
DIFS
DIFS
Overhead ~45%
Overhead ~82%
Overhead ~91%
Why Throughput << Data-rate?
DIFS
MediumAccess101.5µs
Preamble20µs 224µs
Data
SIFS
ACK
7 6 5 4 3 2 1 0
405.5 µs
60µs54 Mbps
7 6 5 4 3 2 1 0225.5 µs
300 Mbps
MediumAccess101.5µs
Preamble32µs 40µs
Data
SIFS
ACK52µs
7 6 5 4 3 2 1 0221.5 µs
600 Mbps
MediumAccess101.5µs
Preamble40µs 20µs
Data
SIFS
ACK60µs
Overhead ~45%
Overhead ~82%
Overhead ~91%
802.11 overhead dominates at high data-rates
Motivation
Preambles 40%SIFS 16%
Slot Overhead 42%
ACK 2%
Overhead Components
7 6 5 4 3 2 1 0
MediumAccess101.5µs
Preamble40µs 20µs
Data
SIFSDIFS
Preamble+ACK44µs
Motivation
Preambles 40%
ACK 2%
Overhead ComponentsSIFS 16%
Slot Overhead 42%
7 6 5 4 3 2 1 0
MediumAccess101.5µs
Preamble40µs 20µs
Data
SIFS
Preamble+ACK60µs
Motivation
Preambles 40%
ACK 2%
Overhead ComponentsSIFS 16%
Slot Overhead 42%
Communications OverheadCannot be removed!
7 6 5 4 3 2 1 0
MediumAccess101.5µs
Preamble40µs 20µs
Data
SIFS
Preamble+ACK60µs
Motivation
Preambles 40%
ACK 2%
Overhead ComponentsSIFS 16%
Slot Overhead 42%
7 6 5 4 3 2 1 0
MediumAccess101.5µs
Preamble40µs 20µs
Data
SIFS
Preamble+ACK60µs
Motivation
Preambles 40%
ACK 2%
Overhead ComponentsSIFS 16%
Slot Overhead 42%
Single Link Case
Motivation
Preambles 34%
ACK 2%
Overhead ComponentsSIFS 14%
Slot Overhead 23%
Multiple Links Case (30 Links)
Contention Collisions 27%Contention Collisions 27%
Contention Collisions 27%Contention Collisions 27%
Motivation
Preambles 34%
ACK 2%
Overhead ComponentsSIFS 14%
Slot Overhead 23%
û10x Reduction
û
Nano Slots
§ Reduce slot durationq by designing a solution to
parallelize historically serial operations
127.6 µs
7.6µs 40µs 20µs 44µsMediumAccess
7 6 5 4 3 2 1 0221.5 µs
MediumAccess101.5µs
Preamble40µs 20µs
Data
SIFS
ACK44µs
DIFS802.11
WiFi-Nano
Nano Slots
§ Reduce slot durationq by designing a solution to
parallelize historically serial operations
127.6 µs
7.6µs 40µs 20µs 44µsMediumAccess
7 6 5 4 3 2 1 0221.5 µs
MediumAccess101.5µs
Preamble40µs 20µs
Data
SIFS
ACK44µs
DIFS802.11
WiFi-Nano
Why is Slot 9 µs?
§ 802.11a/n slot duration of 9 µs is close to theminimum feasible
PropagaDon
û4 µs
A0
B1
802.11 Slot
Clear ChannelAssessment (CCA)
1 µs
Why is Slot 9 µs?
§ 802.11a/n slot duration of 9 µs is close to theminimum feasible
PropagaDon Clear ChannelAssessment (CCA)
Rx/Tx HardwareTurnaroundü
4 µs 4 µs
A3
B1
0
PropagaDon Clear ChannelAssessment (CCA) û4 µs
A0
B1
802.11 Slot
1 µs
Why is Slot 9 µs?
Key assumption: Preamble detection and transmission are serial
PropagaDon Clear ChannelAssessment (CCA)
PropagaDon Clear ChannelAssessment (CCA)
û
ü
4 µs
4 µs 4 µs
A0
B1
A3
B1
802.11 Slot
0Rx/Tx HardwareTurnaround
§ 802.11a/n slot duration of 9 µs is close to theminimum feasible
Speculative Preambles
Preamble detection and transmission occur in parallel
Preamble
Preamble
01
2 1 0
Preamble
3 2 1 0
Node C
Node B
Node A
û
û
Speculative Preambles
1. As soon as the backoffexpires, a node transmits itspreamble
Slot Time (800 ns)
Preamble
Preamble
01
2 1 0
Preamble
3 2 1 0
Node C
Node B
Node A
û
û
Speculative Preambles
1. As soon as the backoffexpires, a node transmits itspreamble
Slot Time (800 ns)
Preamble
Preamble
01
2 1 0
Preamble
3 2 1 0
Node C
Node B
Node A
û
û
Speculative Preambles
1. As soon as the backoffexpires, a node transmits itspreamble
2. CCA: A node transmitting apreamble continues toattempt to detect incomingpreambles
Slot Time (800 ns)
Preamble
Preamble
01
2 1 0
Preamble
3 2 1 0
Node C
Node B
Node A
û
û
Speculative Preambles
1. As soon as the backoffexpires, a node transmits itspreamble
2. CCA: A node transmitting apreamble continues toattempt to detect incomingpreambles
Slot Time (800 ns)
Preamble
Preamble
01
2 1 0
Preamble
3 2 1 0
Node C
Node B
Node A
û
û
Speculative Preambles
1. As soon as the backoffexpires, a node transmits itspreamble
2. CCA: A node transmitting apreamble continues toattempt to detect incomingpreambles
Slot Time (800 ns)
Preamble
Preamble
01
2 1 0
Preamble
3 2 1 0
Node C
Node B
Node A
û
û
Speculative Preambles
1. As soon as the backoffexpires, a node transmits itspreamble
2. CCA: A node transmitting apreamble continues toattempt to detect incomingpreambles
3. A node aborts itstransmission if it detects apreceding preamble
Slot Time (800 ns)
Preamble
Preamble
01
2 1 0
Preamble
3 2 1 0
Node C
Node B
Node A
û
û
Speculative Preambles
1. As soon as the backoffexpires, a node transmits itspreamble
2. CCA: A node transmitting apreamble continues toattempt to detect incomingpreambles
3. A node aborts itstransmission if it detects apreceding preamble
Slot Time (800 ns)
Preamble
Preamble
01
2 1 0
Preamble
3 2 1 0
Node C
Node B
Node A
û
û
Speculative Preambles
1. As soon as the backoffexpires, a node transmits itspreamble
2. CCA: A node transmitting apreamble continues toattempt to detect incomingpreambles
3. A node aborts itstransmission if it detects apreceding preamble
Slot Time (800 ns)
Preamble
Preamble
01
2 1 0
Preamble
3 2 1 0
Node C
Node B
Node A
û
û
Speculative Preambles
1. As soon as the backoffexpires, a node transmits itspreamble
2. CCA: A node transmitting apreamble continues toattempt to detect incomingpreambles
3. A node aborts itstransmission if it detects apreceding preamble
Slot Time (800 ns)
Medium Access time decreases from 101.5 µs to 7.6 µs
WiFi-Nano Design
§ Slot Time Duration
§ Contention Collisions
§ Fairness
§ SIFS
§ Speculative Preambles
Objectives Techniques
ü
§ Speculative ACKs
§ Counter Roll-back
§ Minimum Slot Size
§ Probabilistic CollisionResolution
Key Design Points§ Detect preambles and their starting time under
interference
LatticeCorrelator
[PN1, PN2,PN3]PN1
PN2
PN3
Δ[PN1]
[PN2]
[PN3]
+[PN1,PN2]
++
[PN2,PN3]
Δ
Δ
Δ
Δ
Key Design Points§ Detect preambles and their starting time under
interference
§ Simultaneously transmit and detect preambles
LatticeCorrelator
PN1
PN2
PN3
Δ[PN1]
[PN2]
[PN3]
+[PN1,PN2]
++
[PN2,PN3]
Δ
Δ
Δ
Δ
[PN1, PN2,PN3]
Analog Self-InterferenceCanceller
QHNoise Canceller
Analog Radio Front-end
Rx Tx
Key Design Points§ Detect preambles and their starting time under
interference
§ Simultaneously transmit and detect preambles
LatticeCorrelator
PN1
PN2
PN3
Δ[PN1]
[PN2]
[PN3]
+[PN1,PN2]
++
[PN2,PN3]
Δ
Δ
Δ
Δ
§ Interference may require longer preambles
[PN1, PN2,PN3]
Analog Self-InterferenceCanceller
QHNoise Canceller
Analog Radio Front-end
Rx Tx
Results
§ Experimentsq Reliability of Preamble
Detection
q Efficiency Gain and Analysis
q Fairness
Lyrtech Qualnet
Preamble Detection
Can nodes Can nodes reliably detect preamblesreliably detect preambles despite self-interference? despite self-interference?
Preamble Detection
Can nodes Can nodes reliably detect preamblesreliably detect preambles despite self-interference? despite self-interference?
Preamble Detection
Slightly longer preambles permit to maintain reliability
Can nodes Can nodes reliably detect preamblesreliably detect preambles despite self-interference? despite self-interference?
Efficiency
Efficiency f(data rate, #nodes)
WiFi-Nano increases the throughput up to 100%
2XGains
How to Achieve More?
q Small packets (TCP ACKs)q Short flows (HTTP)q Delay sensitive applications (VoIP)
Frame Aggregation
§ Effective mainly for single-link bulk downloads§ Practically difficult to achieve
Frame Aggregation
§ Practically difficult to achieve
Frame Aggregation
§ Practically difficult to achieve
Frame Aggregation
Even at 18 kB, WiFi-Nano gains 25% over 802.11 at 600 Mbps
§ Practically difficult to achieve§ Related work reports 18 kB as average aggregation
Summary
§ WiFi-Nano permits toq Reduce the slot time to 800 ns via speculative
preamblesq Reduce the occurrence of contention collisions to
nearly 0q Remove SIFS
802.11 overhead can be > 90%
WiFi-Nano increases 802.11throughput up to 100%
Q&A
Eugenio Magistretti † Krishna Kant Chintalapudi *
Božidar Radunović # Ramachandran Ramjee *
† Rice University * Microsoft Research India
# Microsoft Research Cambridge
WiFi-Nano: Reclaiming WiFiEfficiency through 800 ns Slots