Efficient Control Plane Design in Wireless Networks Presented by Xiaoyu JI 23, June, 2013

How to Schedule a Wireless Network? Slide 2 VoIPFile Sync

Cellular: Decoupled Control Plane Slide 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 Slide 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 RTSCTSACK… Data plane based control plane

The shortages Low efficiency – Preamble + control bits + … – E.g., ACK=14 bytes with only 1 bit useful Large overhead – RTS/CTS, long duration Slide 5

The Challenge: the Best of Both Worlds Slide 6 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

Control Plane Design Out of band – ec[Mobicom’12] – Gap-sese[Infocom’13] – E-sese[Mobicom’09] In band control – Flashback[Sigcomm’12] – Side-channel[Mobicom’10] – Hitchhkie[Infocom’14] Others – uACK[Mobicom’12] Slide 7

802.11ec Fundamentally re-think the way control information is conveyed in order to guarantee low overhead and robustness ec stands for with Encoded Control Slide 8

August 15, 2012Slide 9

August 15, 2012Slide 10

August 15, 2012Slide 11

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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.) Slide 14

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

OFDM is a Grid Slide 16 Symbol Subcarrier Position … … … MHz *Refer to IEEE specification

Redundancy Protects from Errors Slide 17 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 Slide 18

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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 20 Symbol FlashSymbol FlashSymbol

Slide 21 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 Flash

How is the Control Message Encoded? Slide 22 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

Demand Map Node 5150 KBBulk Transfer Node 120 KBLow Latency Demand Map Node 5150 KBBulk Transfer Flashback-MAC Slide 23 Flash Node 1Node 5 ACK + piggyback Data DataData Data

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

Other In-band Control Plane Side-channel – In networks – Exploit data payload redundancy in DSSS – Space encoding Hitchhike – Exploit preamble redundancy – Support multiple controls – Low SNR requirement Slide 25

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Comparison In band No extra air time Exploit data redundancy – OFDM, DSSS, Preamble Capacity limited by the redundancy Interference with data – Preamble based, eliminating the interference Out of band Extra air time Directly transmit controls – CSS Capacity limited by length of L Decoupled with data Slide 27

Future directions Preamble based control in networks Compromise between in-band and out-of- band control plane – E.g., uACK uses a portion of channel recourse to deliver control messages. In-band control plane based applications – CTP, neighbor discovery, etc. Slide 28

THANK YOU Slide 29