1. Flashback: A New Control Plane for Wireless Networks Asaf Cidon (Stanford), Kanthi Nagaraj (UCLA), Pramod Viswanath (UIUC), Sachin Katti (Stanford) Stanford University

2. Agenda 1.Motivation and Overview 2.Wi-Fi PHY Primer 3.Design of Flashback 4.Experiment Results 5.Higher Layer Applications December 21, 2011Slide 2

3. Wireless Control Channels Wireless networks require control channels for synchronization and coordination across multiple clients Example: LTE – Dedicated frequencies for control and coordination – Used for resource allocation, QoS, scheduling, power level information, etc. December 21, 2011Slide 3

4. Unlicensed Networks Are Out of Control Unlicensed networks do not have an explicit control channel - they use implicit coordination – RTS/CTS – Collision prevention and backoff mechanisms (CSMA/CA) – 802.11e QoS queues Problems of implicit control mechanisms – Overhead on data channel – Do not scale with number of nodes, congested networks – Limited central control (lack of fairness, starvation) December 21, 2011Slide 4

5. The Holy Grail: Control Channel for Wi-Fi Our goal: a control channel for Wi-Fi – Centrally Managed: AP provides coordination and QoS through control channel – Independence: Data and control independent – Simplicity: Throw away RTS/CTS, CSMA/CA Constraint: low-overhead – Backwards compatibility – No big hardware changes December 21, 2011Slide 5

6. Wi-Fi PHY Primer: OFDM OFDM widely used in wireless networks Key idea: multiple narrowband sub-carriers at a low symbol rate – Main advantage: cope with severe channel conditions (frequency-selective fading) without complex equalization filters December 21, 2011Slide 6

7. Wi-Fi PHY Primer: Bit Rates and Channel Codes December 21, 2011Slide 7

8. Flashback Intuition Wi-Fi channel codes have robust SNR margins (~3db) – Insight: even if we lose a couple of bits here and there, channel codes will prevent data loss Key idea: erase subcarrier instead of treating it as an error – Gives us an even higher SNR margin December 21, 2011Slide 8

9. Flashback in a Nutshell Control signaling using ‘flashes’ – High power single sub-carrier flash sent on top of data transmission – Receiver can detect flashes independently of on-going data transmission – If flash detected, erase the sub-carrier from data packet – Flashes are not modulated (i.e. they are binary) Flashes provide a near-zero overhead separate PHY control channel – Backwards compatible – No synchronization required December 21, 2011Slide 9

10. Flashback Receiver Design ADCSync64 FFT Equalizer Demodul -ator Flash Demodulator Flash Eraser Flash Detector Viterbi Decoder Data Packet Control Message

11. Implementation Implementation using NI PXIe- 8130 RTOS Dual-Core Controller – NI PXIe-7965R FlexRIO, NI 5781 BB Transceiver Setup – 1 data transmitter, 1 flash transmitter, 1 receiver – ~300 runs for each data point – Flashes sent at 8-10 db relative to data transmission December 21, 2011Slide 11

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14. Improving Flash Detection Flash detection is not perfect: flashing node is not synchronized to transmitter node – Flashes can be ‘smeared’ over 2 symbols in time Solution: – Run additional FFT to detect if flash is smeared over 2 symbols

16. Applications Given control channel PHY, we can use Flashback to improve the MAC: – Get rid of overhead in RTS/CTS – Implement QoS scheduling – Use flashes for estimating SNIRs between networks and improving spatial reuse – Use flashes to indicate power/sleep modes December 21, 2011Slide 16

17. Example 1: Don’t RTS, Just Flash AP assigns flash subcarriers during association Clients maintain overall flash rate by estimating number of nodes Flash instead of RTS – Wait until AP is listening Benefits – No RTS = no contention period = no overhead! – AP can do smart scheduling by estimating SNRs of nodes using flashes December 21, 2011Slide 17

19. Example 2: QoS December 21, 2011Slide 19

20. Example 3: Estimate SNIRs December 21, 2011Slide 20

21. Thank You! Stanford University