Augmenting Mobile 3G Using WiFi

1. Augmenting Mobile 3G Using WiFi Aruna Balasubramanian, Ratul Mahajan, Arun Venkataramani 2011-04-04 Jimin Lee jmlee@mmlab.snu.ac.kr

2. Outline • Introduction • Measurement • Wiffler • Prediction-based offloading • Fast switching • Evaluation • Conclusion

3. 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?

4. 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

5. 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

6. 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

7. Measurement • Availability (cont’d) • WiFi availability is lower than 3G

8. 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

9. 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

10. 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

11. 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

12. 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 Parallel Operating

13. 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)

14. 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

15. 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

16. Evaluation • Deployment on 20 vehicular nodes • Simulations

17. Evaluation • Deployment on 20 vehicular nodes • Prediction-based offloading Transfer size: 5MB, Delay tolerance: 60 secs,Inter-transfer gap: random with mean 100 secs

18. Evaluation • Deployment on 20 vehicular nodes • Fast switching to 3G VoIP-like traffic: 20-byte packet every 20 ms With standard MOS metric

19. 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

20. Evaluation • Wiffler increases data offloading to WiFi

21. Evaluation • Prediction reduces completion time

22. Evaluation • Fast switching improves performance of demanding apps

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