Multimedia and Mobile communications Laboratory

Augmenting Mobile 3G Using WiFiAugmenting Mobile 3G Using WiFi

Aruna Balasubramanian, Ratul Mahajan, Arun Venkataramani

2011-04-04

Jimin Leejmlee@mmlab.snu.ac.kr

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OutlineOutline

Introduction Measurement Wiffler

Prediction-based offloading Fast switching

Evaluation Conclusion

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IntroductionIntroduction

Mobile Internet access is suffering today The ubiquitous deployment of cellular data networks has

drawn millions of users Mobile data is growing exponentially This is creating immense pressure on the limited spectrum of

networks Is more spectrum the answer?

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MeasurementMeasurement

To study 3G and WiFi network characteristics What is the availability of 3G and WiFi networks as seen by a vehicle user? What are the performance characteristics of the two networks?

Testbeds Outdoor testbeds that include effects present in real vehicular

settings such as noise, interference, traffic patterns Conducted across three cities

Amherst, Seattle, Sfo

Vehicular nodes with 3G and WiFi radios Amherst: 20 buses Seattle: 1car Sfo: 1car

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MeasurementMeasurement

Methodology The vehicles visit many locations multiple times each day

Amherst : 12days, Seattle: 6days, Sfo: 3days

The software on the vehicle includes the two programs First program scans the 3G and WiFi channels simultaneously Second program sends and receives data to a server

Both server and vehicle log the characteristics of the data transfer

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MeasurementMeasurement

Availability The server and the vehicle periodically send data to each other

over UDP An interface(3G or WiFi) is considered available if at least one

packet was received in the interval Availability is defined as the number of available 1-second intervals

divided by the total number of intervals

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MeasurementMeasurement

Availability (cont’d) WiFi availability is lower than 3G

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MeasurementMeasurement

Performance To measure the upstream and downstream UDP throughput The server and the vehicle send 1500-byte packets every 20ms. WiFi throughput is lower than 3G

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MeasurementMeasurement

Summary The availability of WiFi is poorer than 3G WiFi throughput is also much lower than 3G throughput

Augmenting 3G using WiFi How can we reduce 3G usage by using WiFi? The simplest policy

To send data on WiFi when available and switch 3G when WiFi is unavailable

First, Availability of WiFi can be low : 11% Second, WiFi throughput is lower than 3G

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WifflerWiffler

Key techniques Leveraging delay tolerance

Exploit the delay tolerance of apps to increase data offloaded to WiFi

Fast switching For apps with strict quality of service requirements Such as VoIP and video stream

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Leveraging delay tolerance Leveraging delay tolerance

The simplest solution To wait until the delay tolerance threshold to transfer

data on WiFi when available It may significantly increase the completion time

So, Wiffler uses the predictor to estimate offload capability of WiFi network

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Leveraging delay tolerance (cont’d) Leveraging delay tolerance (cont’d)

Prediction-based offloading Transfer required: S bytes by D seconds D: earliest delay tolerance threshold among queued transfers W: predicted WiFi capacity over future D seconds

if(WiFi is available)Send data on WiFi

If(W < S and 3G is available)Send data on 3G

ParallelOperating

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Leveraging delay tolerance (cont’d) Leveraging delay tolerance (cont’d)

WiFi throughput prediction We predict WiFi offload capacity

Based on an estimate of the average throughput offered by an AP and a prediction of the number of APs that will be encountered

AP meetings occur in bursts So, we can predict the number of AP encounters using a history-

based predictor Future AP encounters depend on recent past The mobile node keeps track of the last N Aps By using this information, we can compute the

• (# of APs) * (capacity per AP)

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Fast switching to 3GFast switching to 3G

Poor WiFi connectivity will hurt demanding apps Such as VoIP, video streaming

If WiFi is losing or delaying packets, we should send them on 3G as soon as possible Link-layer retransmissions take much time Variable medium access delays

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Fast switching to 3G (cont’d)Fast switching to 3G (cont’d)

Motivation Waiting for WiFi link-layer retransmissions incurs delay Losses are bursty in the vehicular environment

The simple mechanism It sends the packet on 3G if the WiFi link-layer fails to deliver

the packet within a delay threshold It’s better to send time-sensitive packets on 3G rather than

waiting for likely more failures on WiFi

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EvaluationEvaluation

Deployment on 20 vehicular nodes Simulations

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EvaluationEvaluation

Deployment on 20 vehicular nodes Prediction-based offloading

Data offloaded to WiFi

Prediction-based offloading 30%

WiFi when available 11%

Transfer size: 5MB, Delay tolerance: 60 secs,Inter-transfer gap: random with mean 100 secs

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EvaluationEvaluation

Deployment on 20 vehicular nodes Fast switching to 3G

Time w/ good voice quality

Fast switching 68%

WiFi when available 42%

VoIP-like traffic: 20-byte packet every 20 msWith standard MOS metric

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EvaluationEvaluation

Simulations To evaluate Wiffler’s prediction-based offloading and

fast switching from others Alternative strategies

Impatient : use WiFi when available Patient : waits until the threshold Oracle : perfect future knowledge

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EvaluationEvaluation

Wiffler increases data offloading to WiFi

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EvaluationEvaluation

Prediction reduces completion time

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EvaluationEvaluation

Fast switching improves performance of demanding apps

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ConclusionConclusion

Paper develops techniques to combine mutiple interfacees with different costs and ubiquitousness 3G is costly but more ubiquitous WiFi is cheaper but intermittently available

It overcomes WiFi’s poor availability by leveraging delay tolerance of applications and a fast switching mechanism