Efficient Control Plane Design in Wireless Networks

Presented by Xiaoyu JI23, June, 2013

How to Schedule a Wireless Network?

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VoIPFile Sync

Cellular: Decoupled Control Plane

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Data PlaneControl Plane Contro

l Plan

eData

Plane

You go first

Data data data data

You go second

Data data data dataI want to

transmit sync

I want to transmit VoIP

Cellular networks pay high price for centralized control

VoIPFile Sync

Wi-Fi: Implicit Control

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Is anyone transmitting?No. Let’s go!

Is anyone transmitting?Yes. I must back off.

1 Mississippi,2 Mississippi…

Data Plane Data

Plane

Data data data data

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

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The Challenge: the Best of Both Worlds

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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– 802.11ec[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

• 802.11ec stands for 802.11 with Encoded Control

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August 15, 2012 Slide 9

August 15, 2012 Slide 10

August 15, 2012 Slide 11

August 15, 2012 Slide 12

August 15, 2012 Slide 13

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.)

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How can we send control messages without interfering with data packets?

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Data data data dataFlash

OFDM is a Grid

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Symbol Symbol Symbol Symbol

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Symbol Symbol Symbol Symbol0

1

2

3

60

61

62

63S

ub

carr

ier

Po

siti

on

… ……

0 4 8 12 16Time []

20

MH

z

*Refer to IEEE 802.11 specification

Redundancy Protects from Errors

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Symbol Symbol

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Symbol Symbol

Symbol Symbol

Wi-Fi Packet

Symbol Error Symbol Symbol

Error Error Symbol Symbol

Symbol Symbol Symbol Symbol

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Symbol Symbol Symbol Error

Symbol Symbol Symbol Error

Symbol Symbol Symbol Error

Symbol Symbol Symbol Error

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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

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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

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Symbol Symbol Symbol Symbol

Symbol Symbol Flash Symbol

Symbol Symbol Symbol Symbol

Symbol Symbol Symbol Symbol

Flash Symbol Symbol Symbol

Symbol Symbol Symbol Symbol

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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?

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Symbol Symbol Symbol Symbol

Symbol Symbol Flash Symbol

Symbol Symbol Symbol Symbol

Symbol Flash Symbol Symbol

Flash Symbol Symbol Symbol

Symbol Symbol Symbol Symbol

Symbol Symbol Symbol Symbol

Symbol Symbol Symbol Symbol0

1

2

3

60

61

62

63

… ……

0 4 8 12 16Time []

Su

bca

rrie

r

• Messages encoded by relative distance between consecutive flashes– Each digit is relative

distance– Digit 1: 60 – 3 = 57– Digit 2: 62 - 60 = 2

Demand Map

Node 5 150 KB Bulk Transfer

Node 1 20 KB Low Latency

Demand Map

Node 5 150 KB Bulk Transfer

Flashback-MAC

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FlashFlash

Node 1Node 5

ACK + piggyback

Data Data Data DataData Data Data 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– …

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Other In-band Control Plane

• Side-channel– In 802.15.4 networks– Exploit data payload redundancy in DSSS– Space encoding

• Hitchhike– Exploit preamble redundancy– Support multiple controls – Low SNR requirement

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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

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Future directions

• Preamble based control in 802.11 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.

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THANK YOU

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