wi-fi goes to town · best ap sequence is not left-to-right order problem: rapid multi-path fading...
TRANSCRIPT
Speaker: Zhenyu Song
Zhenyu Song, Longfei Shangguan, Kyle Jamieson{zhenyus, longfeis, kylej}@cs.princeton.edu
Wi-Fi Goes to Town: Rapid Picocell Switching for Wireless Transit Networks
This work is supported by the NSF grant No. 1617161 and Google Research Award.
Motivation
Billions of commuters on trains, light rails and in cars surf the internet
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ConferenceOnline VideoVR gaming
How can we increase the wireless network 1000X in bits/(second*dollar)?
Motivation
“The majority of capacity gains over the past 45 years is due to the decreased size of each cell.” ——Cooper
APAP2
increasing capacity gain
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AP1 AP3AP4
Two recent observations
The ESP8266 Wi-Fi and system-on-chip module,available ca.2016 for $5.
Very low-cost AP (<= $5)
Commodity APs can extract fine-grained channel state information
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[Halperin et al.]
Wi-Fi Goes to Town:Picocell AP network for transit
APAPAPAPAPAPAP
APAP
APAP
APAP
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Problem: picocells + vehicular speed
How to support switching between APs?
APAPAPAPAPAPAP
APAP
APAP
APAP
APAP
APAPAP
AP
We need to switch fast!6
Best AP sequence is not left-to-right order
Problem: rapid multi-path fading
AP1
Rapid (ms-scale) channel fading due to the multi-pathAP2 AP3
Chan
nel C
apac
ityTime (ms)
AP1AP2AP3
We need to switch at a millisecond level!7
1 2 1 2
Design
Who maintains states and makes switching decision
When to switch (to which AP)
How to switch
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Design
Wi-Fi Goes to Town: system architecture
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Design::who
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A controller maintains states and makes decisions
Design::when (and which)AP selection algorithm
Controller maintains an Effective SNR value window (10 ms), and selects AP with largest median value.
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AP1 AP2AP3
AP1AP2
Time
191314
14 1713
13
15
121318
1019
windowAP3
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window window
Design::how
Association
Uplink (from client to AP)
Downlink (from AP to client)
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Design::how
Wi-Fi Goes to Town: AP-client association
AP1
AP3AP2
propagate info
client info
hostapd AP2
hostapd AP1
hostapd AP3
extract
send to
All APs associate with client
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Design::how
Wi-Fi Goes to Town: Uplink flow
tunneling
Stripe and de-duplicate
AP1 AP3AP2
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same bssid
Design::how
Wi-Fi Goes to Town: Downlink flow
1 12233
AP1 AP3AP2
321
start
ack
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Design::how
Wi-Fi Goes to Town: Downlink packet synchronization
AP1 AP2
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tunneling
set as 80211 seq
Introduction of aggregation in 802.11n
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Sender Receiver
block ack
MPDUaggregate frame 1,2,3
MPDUaggregate frame 2,3,4
partial ackframe 1
1 2 3 4
sender windowsender window
Problem: block ack lost causes mac layer inefficiency
AP1 AP2
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AP1 needlessly retransmits whole aggregate
AP2 (adjacent)
kernel
Solution: block ack forwarding
AP1 AP3AP2
321 block ack
AP1 (associated)
kernel
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Implementation
Wi-Fi Goes to Town: Two Deployment Schemes
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Implementation: hardwareAP: TP-Link N750 AP, Larid directional antenna, Atheros CSI Tool [Xie et al.]
Controller: Lenovo Thinkpad T430
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Evaluation: questions
How much does Wi-Fi goes to town improve uplink reception rate?
Does Wi-Fi goes to town increase jitter?
Does Wi-Fi goes to town achieve higher end-to-end throughput?
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Short demo
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AP
Uplink packet
Downlink packet
retrans packet
Strawman: 802.11r (enhanced)(Original 802.11r) Fast handover:
Fast BSS transition.Client maintains time-averaged RSSI, and switch when
below threshold
(Enhanced) Fast nearby AP discovery:Each AP tells client nearby AP information Client overhears beacons
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Wi-Fi goes to town achieve loweruplink loss rate by over-hearing
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Upl
ink
loss
rate
TCP Download
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RSSI ↓, switch! (Too late)
Wi-Fi goes to town achieves seamless switching at speed
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TCP Download
Wi-Fi goes to town achieves seamless switching at speed
Wi-Fi goes to town achieves seamless switching at speed
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ms-scale switching
TCP Download
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Wi-Fi goes to town achieves higherend-to-end throughput
Single-client, download
802.11r’s performance degrades with speed while Wi-Fi goes to town not
Multi-client
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Wi-Fi goes to town achieves higherend-to-end throughput
Wi-Fi goes to town always achieves higher performance
Aver
age
Thro
ughp
ut (M
bit/s
)
Conclusion
First roadside hotspot network at vehicular speeds with meter-sized picocells.
Execute switch decisions at millisecond-level granularities.
First step in a line of work that will scale out the wireless capacity of roadside hotspot networks using small cells.
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