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. 2. 3. 4. 5. Motivation and Overview Wi-Fi.

Flashback: A New Control Plane for
Wireless Networks
Asaf Cidon (Stanford), Kanthi Nagaraj (UCLA), Pramod
Viswanath (UIUC), Sachin Katti (Stanford)
Stanford University
Agenda
1.
2.
3.
4.
5.
Motivation and Overview
Wi-Fi PHY Primer
Design of Flashback
Experiment Results
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
60
64-QAM 3/4
64-QAM 1/2
50
16-QAM 3/4
Mb/s
40
30
16-QAM 1/2
QPSK 3/4
20
10
0
QPSK 1/2
BPSK 3/4
BPSK 1/2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SNR
December 21, 2011
Slide 7
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
Detector
Flash
Eraser
Viterbi
Decoder
Flash
Demodulator
Control
Message
Data
Packet
Implementation
• Implementation using NI PXIe8130 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
Maximum Flash Rates Using Optimal Bitrates
Maximum Number of Flashes per Second
QPSK 1/2
16-QAM 1/2
16-QAM 3/4
QPSK 3/4
100000
10000
5000
Minimum = 5,000
1000
5
6
7
8
9
10
11
12
13
SNR (dB)
14
15
16
17
18
19
20
Overall Packet Loss Rate of Data Plane
2.5
Packet Loss Rate [Percentage]
2
1.5
1
0.5
0
1000
10000
Flashes per Second
100000
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
Error Rates of Flashes
10
9
8
Percentage
7
6
5
4
3
2
1
0
SNR [dB]
False Negative Rate of Flashes, R=5000
False Positive Rate of Flashes, R=5000
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
Flashback-MAC's Throughput Improvement vs. Wi-Fi
500
450
400
CSMA/CA
Percentage
350
300
250
200
150
RTS/CTS
100
50
0
1
3
5
7
9
11
13
15
17
Number of Nodes
CSMA/CA 20-80% Uplink-Downlink Data Traffic
CSMA/CA 100-0% Uplink-Downlink Data Traffic
RTS/CTS, 20-80% Uplink-Downlink Data Traffic
RTS/CTS 100-0% Uplink-Downlink Data Traffic
19
Example 2: QoS
Queue Latency of Delay Sensitive Packets
10000
Latency [ms]
1000
100
10
1
4
0.1
6
8
10
12
14
16
18
20
Number of Nodes
Flashback, 100% Uplink Traffic, 2 Latency Sensitive Nodes
CSMA/CA, 100% Uplink Traffic, 2 Latency Sensitive Nodes
December 21, 2011
RTS/CTS, 100% Uplink Traffic, 2 Latency Sensitive Nodes
Slide 19
Example 3: Estimate SNIRs
• Clients flash at constant power  receivers
can estimate link SNR  estimate SNIR
𝑆𝑖𝑔𝑛𝑎𝑙
(
)
𝑁𝑜𝑖𝑠𝑒+𝐼𝑛𝑡𝑒𝑟𝑓𝑒𝑟𝑒𝑛𝑐𝑒
– 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