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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