Augmenting Mobile 3G Using WiFi

Aruna Balasubramanian, Ratul Mahajan, Arun Venkataramani 2011-04-04

Jimin Lee [email protected]

Multimedia and Mobile communications Laboratory

Outline

 Introduction  Measurement  Wiffler  Prediction-based offloading  Fast switching  Evaluation  Conclusion

2/23

Introduction

 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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Measurement

 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

4/23

Measurement

 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

5/23

Measurement

 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

6/23

Measurement

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

7/23

Measurement

 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

8/23

Measurement

 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

9/23

Wiffler

 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

10/23

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

11/23

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

Parallel Operating if(WiFi is available) Send data on WiFi If(W < S and 3G is available) Send data on 3G

12/23

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)

13/23

Fast 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

14/23

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

15/23

Evaluation

 Deployment on 20 vehicular nodes  Simulations

16/23

Evaluation

 Deployment on 20 vehicular nodes  Prediction-based offloading Prediction-based offloading WiFi when available

Data offloaded to WiFi

30% 11%

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

17/23

Evaluation

 Deployment on 20 vehicular nodes  Fast switching to 3G Fast switching WiFi when available

Time w/ good voice quality

68% 42%

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

18/23

Evaluation

 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

19/23

Evaluation

 Wiffler increases data offloading to WiFi

20/23

Evaluation

 Prediction reduces completion time

21/23

Evaluation

 Fast switching improves performance of demanding apps

22/23

Conclusion

 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

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