flashback: a new control plane for wireless networks asaf cidon (stanford), kanthi nagaraj (ucla),...
TRANSCRIPT
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.
December 21, 2011 Slide 3
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, 2011 Slide 4
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, 2011 Slide 5
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, 2011 Slide 6
Wi-Fi PHY Primer: Bit Rates and Channel Codes
December 21, 2011 Slide 7
1 2 3 4 5 6 7 80
10
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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
December 21, 2011 Slide 8
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, 2011 Slide 9
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
December 21, 2011 Slide 11
5 5.5
6 6.5
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8 8.5
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10 10.5
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12 12.5
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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
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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
December 21, 2011 Slide 16
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, 2011 Slide 17
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200
50
100
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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
December 21, 2011 Slide 19
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
December 21, 2011 Slide 20
Thank You!
Stanford University