Wi-Fi Goes to Town · Best AP sequence is not left-to-right order Problem: rapid multi-path fading...

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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Questions?zhenyus@cs.princeton.edu

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