flashback: a new control plane for wireless networks asaf cidon (stanford), kanthi nagaraj (ucla),...

Flashback: A New Control Plane for Wireless Networks

Asaf Cidon (Stanford), Kanthi Nagaraj (UCLA), Pramod Viswanath (UIUC), Sachin Katti (Stanford)

Stanford University

Agenda

1. Motivation and Overview2. Wi-Fi PHY Primer3. Design of Flashback4. Experiment Results5. Higher Layer Applications

December 21, 2011 Slide 2

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.


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)


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


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


Wi-Fi PHY Primer: Bit Rates and Channel Codes


1 2 3 4 5 6 7 80

10

20

30

40

50

60

BPSK 1/2

BPSK 3/4QPSK 1/2

QPSK 3/4

16-QAM 1/2

16-QAM 3/4

64-QAM 1/2

64-QAM 3/4

SNR

Mb/

s

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


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


Flashback Receiver Design

ADC Sync 64 FFT Equalizer

Demodul-ator

Flash Demodulator

Flash Eraser

Flash Detector

Viterbi Decoder

Data Packet

Control Message

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


5 5.5

6 6.5

7 7.5

8 8.5

9 9.5

10 10.5

11 11.5

12 12.5

13 13.5

14 14.5

15 15.5

16 16.5

17 17.5

18 18.5

19 19.5

201000

10000

100000

Maximum Flash Rates Using Optimal Bitrates

SNR (dB)

Max

imum

Num

ber o

f Fla

shes

per

Sec

ond

QPSK 1/2

QPSK 3/4

16-QAM 1/2 16-QAM 3/4

Minimum = 5,0005000

1000 10000 1000000

0.5

1

1.5

2

2.5

Overall Packet Loss Rate of Data Plane

Flashes per Second

Pack

et L

oss

Rate

[Per

cent

age]

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

5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19 19.5 200

1

2

3

4

5

6

7

8

9

10

Error Rates of Flashes

False Negative Rate of Flashes, R=5000 False Positive Rate of Flashes, R=5000

SNR [dB]

Perc

enta

ge

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


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


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200

50

100

150

200

250

300

350

400

450

500

Flashback-MAC's Throughput Improvement vs. Wi-Fi

CSMA/CA 20-80% Uplink-Downlink Data Traffic CSMA/CA 100-0% Uplink-Downlink Data TrafficRTS/CTS, 20-80% Uplink-Downlink Data Traffic RTS/CTS 100-0% Uplink-Downlink Data Traffic

Number of Nodes

Perc

enta

ge

CSMA/CA

RTS/CTS

Example 2: QoS


4 6 8 10 12 14 16 18 200.1

1

10

100

1000

10000

Queue Latency of Delay Sensitive Packets

Flashback, 100% Uplink Traffic, 2 Latency Sensitive NodesCSMA/CA, 100% Uplink Traffic, 2 Latency Sensitive NodesRTS/CTS, 100% Uplink Traffic, 2 Latency Sensitive Nodes

Number of Nodes

Late

ncy

[ms]

Example 3: Estimate SNIRs

• Clients flash at constant power receivers can estimate link SNR estimate SNIR ()– APs can know SNIR of all the links in the network– Use flashes to communicate between APs– Maximize spatial reuse


Thank You!

Stanford University

flashback: a new control plane for wireless networks asaf cidon (stanford), kanthi nagaraj (ucla),...

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