Flashback: Decoupled Lightweight Wireless Control Asaf Cidon, Kanthi Nagaraj, Sachin Katti Stanford University Pramod Viswanath University of Illinois at Urbana-Champaign August 15, 2012Slide 1

How to Schedule a Wireless Network? August 15, 2012Slide 2 VoIPFile Sync

Cellular: Decoupled Control Plane August 15, 2012Slide 3 Data Plane Control Plane Data Plane You go first Data data data data You go second Data data data data I want to transmit sync I want to transmit VoIP Cellular networks pay high price for centralized control VoIPFile Sync

Wi-Fi: Implicit Control August 15, 2012Slide 4 Is anyone transmitting? No. Let’s go! Is anyone transmitting? Yes. I must back off. 1 Mississippi, 2 Mississippi… Data Plane Data data data data Hidden Node Data data data data Collision File SyncVoIP

The Challenge: the Best of Both Worlds August 15, 2012Slide 5 Data data data data I want to transmit Can we get the benefits of centralized control? While retaining Wi-Fi’s asynchronous and distributed properties Without designating spectrum

Flashback Flashback is a decoupled, lightweight control plane – Decoupled: send control messages concurrently with data messages on the same channel – Lightweight: barely impacts network performance (<1% packet loss) – Control Plane: enables rich set of applications (efficient scheduling, QoS enforcement, power savings, fast association, etc.) August 15, 2012Slide 6

How can we send control messages without interfering with data packets? August 15, 2012Slide 7 Data data data data Flash

OFDM is a Grid August 15, 2012Slide 8 Symbol Subcarrier Position … … … MHz

Redundancy Protects from Errors August 15, 2012Slide 9 Symbol Wi-Fi Packet SymbolErrorSymbol Error Symbol Error Symbol Error Symbol Error Symbol Error Symbol

Rate Adaptation Adds Redundancy Rate adaptation trades off redundancy and throughput – Redundancy is added in significant discrete chunks – Dropping a packet is very costly Rate adaptation errs on the conservative side August 15, 2012Slide 10

August 15, 2012Slide 11

Exploiting Margin Key insight: intentionally interfere Leverage OFDM grid structure Localize interference in the OFDM grid Flashes: high powered single subcarrier signal Single frequency sinusoid on particular time slot Slide 12 Symbol FlashSymbol FlashSymbol

Slide 13 Receiver Detects Flashes in Parallel 1.Flashes are easy to detect at receiver – Power spike on single subcarrier 2.Erase flashes from data packet 3.Decode flash and data packet in parallel August 15, 2012 Flash

How is the Control Message Encoded? August 15, 2012Slide 14 Symbol FlashSymbol FlashSymbol FlashSymbol … … … Subcarrier Messages encoded by relative distance between consecutive flashes – Each digit is relative distance – Digit 1: 60 – 3 = 57 – Digit 2: = 2

Practical Concerns Flash transmitter is not synchronized to receiver – CFO problem solved using relative frequencies – Time sync problem solved by detecting flashes at whole and half samples AGC – Commodity ADCs have sufficient dynamic range to accommodate both data and flashes August 15, 2012Slide 15

Implementation Implemented OFDM PHY supporting Flashback – Receiver + transmitter Implementation using NI Virtex-5 LX30 FPGA based software radios – Designed with LabVIEW August 15, 2012Slide 16

Transmitter Implementation August 15, 2012Slide 17 Encoder Modulator 64 IFFT Cyclic Prefix Flash Encoder DAC Data Packet Control Message MUX

Receiver Implementation August 15, 2012Slide 18 ADCSync64 FFTEqualizer Demodul- ator Flash Decoder Flash Eraser Flash Detector Viterbi Decoder Data Packet Control Message Modular, requires minimal changes

Maximum Flash and Data Rate August 15, 2012Slide 19 50,000 flashes per second 175 kb/sec 125,00 flashes per second 400 kb/sec

Demand Map Node 5150 KBBulk Transfer Node 120 KBLow Latency Demand Map Node 5150 KBBulk Transfer Flashback-MAC August 15, 2012Slide 20 Flash Node 1Node 5 ACK + piggyback Data DataData Data

August 15, 2012Slide 21 Centralized scheduling  4X throughput

August 15, 2012Slide 22 Decoupled Control = High Utilization

August 15, 2012Slide 23 Flashback enforces QoS in extreme scenarios

Broader Implications of Flashback Decoupling is key property of Flashback Enables hitherto impossible applications – Power duty cycling – Fast association – Coexistence across networks – Peer discovery – … August 15, 2012Slide 24

Summary What: Flashback is a decoupled lightweight control plane How: Cause localized interference to send control messages Why: Centralized scheduling facilitates many applications August 15, 2012Slide 25