Multimedia and Mobile communications Laboratory
Augmenting Mobile 3G Using WiFiAugmenting Mobile 3G Using WiFi
Aruna Balasubramanian, Ratul Mahajan, Arun Venkataramani
2011-04-04
Jimin [email protected]
2/23
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