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[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

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

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 does not scale with data-rate

Preambles 40%

ACK 2%

Motivation

Single Link Case

Overhead ComponentsSIFS 16%

Slot Duration 42%

Communications OverheadCannot be removed!

7 6 5 4 3 2 1 0

MediumAccess101.5µs

Preamble40µs 20µs

Data

SIFSDIFS

Preamble+ACK44µs

Motivation

Preambles 34%

ACK 2%

Overhead ComponentsSIFS 14%

Slot Duration 23%

Multiple Links Case (30)

Collisions 27%Collisions 27%

Objective

§ Reduce slot duration

q and reduce the occurrence of collisionsq … while preserving fairness

§ Remove SIFS

Collisions 27%Collisions 27%

Slot Duration 23%

SIFS 14%

Preambles 34%

ACK 2%

Challenge: Slot Duration

§ 802.11a/n slot duration of 9 µs is close to theminimum feasible

PropagaDon

û4  µs

A0

B1

802.11 Slot

Clear  ChannelAssessment  (CCA)

PropagaDon Clear  ChannelAssessment  (CCA)

Rx/Tx  HardwareTurnaroundü

4  µs 4  µs

A3

B1

0

Key assumption: Preamble detection and transmission are serial

Preamble detection and transmission occur in parallel

Speculative Preambles

Clear Channel Assessment may take multiple slots

The slot time can be reduced to a round-trip propagation delay

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

§ Collisions

§ Fairness

§ SIFS

§ Speculative Preambles

Objectives Techniques

ü

§ Speculative ACKs

§ Counter Roll-back

§ Minimum Slot Size

§ Probabilistic CollisionResolution

Implementation§ Detect preambles and their starting time under

interference

§ Simultaneously transmit and detect preambles

LatticeCorrelator

[PN1, PN2,PN3]PN1

PN2

PN3

Δ[PN1]

[PN2]

[PN3]

+[PN1,PN2]

++

[PN2,PN3]

Δ

Δ

Δ

Δ

QHNoise Canceller

Analog Radio Front-end

Rx TxAnalog Self-InterferenceCanceller

§ Interference may require longer preambles

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?

Slightly longer preambles permit to maintain reliability

Efficiency

Efficiency f(data rate, #nodes)

WiFi-Nano increases the throughput up to 100%

Efficiency

Efficiency f(data rate, slot time)

.11a/n.11b

WiFi-Nano

WiFi-Nano increases the throughput up to 100%

How to Achieve More?

q Small packets (TPC ACKs)q Short flows (HTTP)q Delay sensitive applications (TPC, VoIP)

Frame Aggregation

§ Works only for single-link bulk downloads§ Practically difficult to achieve

Frame Aggregation

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 nsq Reduce the occurrence of 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