WiFi-Nano : Reclaiming WiFi Effi-ciency Through 800 ns Slots

Presenter : Min Seong Kim

Eugenio Magistretti,Krishna Kant Chintalapudi,Bozidar Radunovic, Ramachandran Ramjee

Rice University Microsoft Research India

Microsoft Research Cambridge

• WiFi physical layer data rates increased from 1Mbps to 1Gbps.

• WiFi MAC overheads (channel access and acks) has not seen similar reductions.

• Reduce WiFi MAC overheads!

Problem Overview

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Why Throughput << Data-rate?

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Can’t get information.

Motivation

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Motivation

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• Reduce slot duration– And reduce the occurrence of collisions.– Preserving fairness

• Remove SIFS (detail -> later)

Objective

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• 802.11n– 1500 byte data packets using DCF with RTS/CTS turned off

What is WiFi MAC Overheads?

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• Prior to transmitting next packet, the device must first sense that the channel is idle for the duration of DIFS.

• DIFS, 34 us long, comprises SIFS(16us) and 2 slots(each 9us).

• After DIFS, random number of backoff slots.• 0 to CW-1.• In 802.11 CW=16.• Average backoff 7.5 slots.• 34us + 9*7.5 slots = 101.5 us

• 500%, Data=20us

Channel Access Overhead

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• The transmission of data in every packet is precede by a physical layer preamble.

• The preamble is crucial in preparing the receiver for a suc-cessful reception.

PHY Layer Preamble Overhead

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• In order to allow enough time for the receiver to process in-coming data and prepare its radio for transmission, nodes must wait for SIFS (16us) before transmitting an ACK.

• Actual ACK is 4us!• 40 us is ACK preamble!!

• So, not wait SIFS time, just transmit ACK!

• Remove SIFS!!

Acknowledgement Overhead

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• When multiple devices contend, their backoff counters are decremented independently and in parallel.

• The wait time for accessing the channel is thus determined by the device with the minimum backoff counter value, As a re-sult, overhead (idle) due to channel access reduces as the number of contending devices increase.

• However, with increasing contention, the probability the two or more devices may choose to transmit in the same slot in-creases, leading to increased collisions.

Collision Overhead

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• To reduce channel access and collision overheads and im-prove the efficiency of WiFi.

The primary focus

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• 800 ns Slots!• Speculative Preamble Transmission• Speculative ACK Transmission. (already mentioned)

WiFi-Nano Overview

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• Two WiFi-nano devices A and B contend for the same chan-nel.

• We assume that 4 us are required by the devices to detect each others transmissions.

• B detects A’s transmissionfour slots later and aborts.

• Because B receive A’s preamble at time 1 on B.

Speculative Preamble

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• If we consider the near far problem, the time that devices can detect each other’s transmissions is different.

• In this circumstance, C detects B’s preamble first, not A’s pre-amble.

Speculative Preamble

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• Identical circumstance with previous slide.

Speculative Preamble

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Medium Access Time decreases from 101.5 us to 7.6 us

WiFi-Nano Overview

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• DSP/FPGA based software defined radio platform – the SFF SDR from Lyretech.

• Simulation Qualnet network simulator.

• Experiments– Reliability of Preamble Detection– Efficiency Gain and Analysis– Fairness

Experiments (Testbed /Simulation Re-sults)

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• Analog Self-Interference Canceller.– Interference may require longer preambles.

Preamble Detection (Testbed)

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• Efficiency (data rate, slot time)

• WiFi-Nano increases the throughput up to 100%

Efficiency (Testbed)

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Excluding preamble, ack

• Efficiency (data rate, #nodes)

• WiFi-Nano increases the throughput up to 100%

Efficiency (Simulation)

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• Collisions accounts for less than 1% of the time.

WiFi-nano Overheads

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• WiFi-Nano permits to– Reduce the slot time to 800 ns– Reduce the occurrence of collisions to nearly 0– Remove SIFS

• WiFi-Nano increases 802.11 throughput up to 100%

Summary

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