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WiFi -Nano : Reclaiming WiFi Efficiency Through 800 ns Slots

WiFi -Nano : Reclaiming WiFi Efficiency Through 800 ns Slots. Eugenio Magistretti ,Krishna Kant Chintalapudi , Bozidar Radunovic , Ramachandran Ramjee Rice University Microsoft Research India Microsoft Research Cambridge. Presenter : Min Seong Kim. Problem Overview.

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WiFi -Nano : Reclaiming WiFi Efficiency Through 800 ns Slots

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  1. WiFi-Nano : Reclaiming WiFi Efficiency Through 800 ns Slots Eugenio Magistretti,Krishna Kant Chintalapudi,BozidarRadunovic, RamachandranRamjee Rice University Microsoft Research India Microsoft Research Cambridge Presenter : Min Seong Kim

  2. Problem Overview WiFi physical layerdata rates increased from 1Mbps to 1Gbps. WiFi MAC overheads (channel access and acks) has not seen similar reductions. Reduce WiFi MAC overheads!

  3. Why Throughput << Data-rate? Can’t get information.

  4. Motivation

  5. Motivation

  6. Objective • Reduce slot duration • And reduce the occurrence of collisions. • Preserving fairness • Remove SIFS (detail -> later)

  7. What is WiFi MAC Overheads? • 802.11n • 1500 byte data packets using DCF with RTS/CTS turned off

  8. Channel Access Overhead 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

  9. PHY Layer Preamble Overhead The transmission of data in every packet is precede by a physical layer preamble. The preamble is crucial in preparing the receiver for a successful reception.

  10. Acknowledgement Overhead In order to allow enough time for the receiver to process incoming 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!!

  11. Collision Overhead 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 result, 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 increases, leading to increased collisions.

  12. The primary focus To reduce channel access and collision overheads and improve the efficiency of WiFi.

  13. WiFi-Nano Overview 800 ns Slots! Speculative Preamble Transmission Speculative ACK Transmission. (already mentioned)

  14. Speculative Preamble Two WiFi-nano devices A and B contend for the same channel. 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.

  15. Speculative Preamble 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 preamble.

  16. Speculative Preamble Medium Access Time decreases from 101.5 us to 7.6 us Identical circumstance with previous slide.

  17. WiFi-Nano Overview

  18. Experiments (Testbed /Simulation Results) • DSP/FPGA based software defined radio platform – the SFF SDR from Lyretech. • Simulation Qualnet network simulator. • Experiments • Reliability of PreambleDetection • Efficiency Gain and Analysis • Fairness

  19. Preamble Detection (Testbed) • Analog Self-Interference Canceller. • Interference may require longer preambles.

  20. Efficiency (Testbed) Excluding preamble, ack Efficiency (data rate, slot time) WiFi-Nano increases the throughput up to 100%

  21. Efficiency (Simulation) Efficiency (data rate, #nodes) WiFi-Nano increases the throughput up to 100%

  22. WiFi-nano Overheads Collisions accounts for less than 1% of the time.

  23. Summary • 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%

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