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Enabling High-Bandwidth Vehicular Content Distribution

Upendra Shevade, Yi-Chao Chen, Lili Qiu, Yin Zhang, Vinoth Chandar, Mi Kyung Han, Han Hee Song and Yousuk

Seung

The University of Texas at Austin

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• People want to communicate while on the move– Average one way commute (2005):

• US: 24.3min, World: 40min

– Passengers want to watch videos, listen to songs, etc.

• Why not just use 3G?– 3G expensive: $30-$60/month

• 5GB/month -> 2Kbps!• 40% 3G capable devices have no 3G plan• iPod Touch sales ~ iPhone sales

– Bandwidth and backhaul limitations• Limited video quality (96-128kbps, < 10min long) • Carriers interested in WiFi offloading

– Arms race between • Increase in cellular bandwidth• Higher resolution screens and videos

Motivation

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Opportunistic WiFi connectivity

Internet

• Compelling usage scenario• Taxis and buses provide value-added services to

passengers

• Previous work: low-bandwidth applications• We focus on delivering high-bandwidth content

• e.g. video streaming

Devices in vehicles contact roadside APs

Gas stations and local shops deploy APs

Passengers watch videos, download files

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Challenges in Vehicular WiFi• Vehicles move at high speed

– WiFi contacts are short, intermittent and difficult to predict– 70% contacts less than 10sec [Cartel]

• Sparse AP coverage– Dense coverage over large area expensive

• Internet access links to APs are bottleneck– DSL throughput between 768Kbps to 6Mbps– WiFi capacity much higher

• 11b: 22.2Mbps, 11n (2.4GHz): 39.7Mbps, 11n (5GHz): 56.1Mbps– Naïve solution does not work well

• Download from Internet during contact• Insufficient b/w if data fetched during contact

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Key Ideas• Wireless b/w (e.g. 56Mbps) >> Internet access

b/w(~1Mbps)– Predict which APs a car will visit in the near future– Pre-fetch content at AP before vehicle arrives– Vehicle can download at wireless capacity

• Use vehicles to carry data between APs– Replicate content to those APs not connected to the Internet– APs as content caches for other vehicles

• Use mesh network as backhaul alternative– APs are often close together to form mesh networks– Content can be replicated over mesh in addition to Internet

links– Mesh network can act as content cache

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Synergy among connections

AP Wireless

Internet Access Vehicle Relay

High b/w, short-lived

High b/w, high delay

Low b/w, persistent

Mesh Network

High b/w, low coverage

VCD High b/w, persistent

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Contributions• New techniques for replication optimization– Goal: Fully utilize wireless bandwidth during contacts– Optimized wireline replication to Internet-connected

APs– Replication using vehicular relays to unconnected

APs– Use mesh n/w for replication and caching

• New algorithm for mobility prediction– Predict set of APs that will be visited by vehicle

• Critical for success of replication techniques

– Algorithm: voting among K nearest trajectories

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VCD ArchitectureController

Content Source

Internet

• Download and upload data • Upload GPS location updates, video demands, what videos a

device has

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Wireline Replication Controller collects

vehicle demands for interval (i+1) and what content is

present at vehicles and APs

Predicts set of APs visited by vehicle in interval (i+1)

Computes what content should be replicated to which APs

Content servers replicate content to APs

At start of interval i

Vehicle downloads content from APs

During interval (i+1)

During interval i

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Optimized Wireline Replication

Interval length, Content present at cars and APs, car demand, AP-to-visit

Content to transfer to APs and content to download to cars Total content downloaded to cars weighted by interest,

while minimizing the amount of content replicated to APs

Total download from AP to car bound by wireless capacity

Per-file download to car bound by file size, what car already hasPer-file download to car cannot exceed what AP already has and what is replicated to it from the Internet

Per-file replication to AP bound by file size and what AP already hasTotal replication to AP does not exceed Internet access link capacity

For each interval i, compute replication strategy maximizing user satisfaction for interval (i+1)

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Vehicular Replication

• Vehicles act as data relays between APs

• Simple strategy: Epidemic Dissemination– Vehicle uploads content to AP

• Based on expected future demand at AP• AP computes future demand, car notifies what it has• AP requests content from the car

– Vehicle downloads content from AP• First of it’s own interest• In remaining time, download content randomly

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• Nearby APs can be organized into mesh networks– Replicate content to APs using mesh in addition to Internet link– Fetch missing content from other mesh nodes rather than

Internet

• Changes to linear program– Constraint C3:

– Two new constraints:

– Objective function:• Add

Mesh Networks of APs

Per-file download to car cannot exceed what AP already has and what is replicated to it from the Internet and from the mesh

AP cannot replicate more content over mesh than it has

Interference constraint: Total active time of all mesh nodes cannot exceed 100%, assuming all nodes interfere with each other

Prefer a replication which uses less mesh traffic among ones supporting equal traffic demands

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Mobility Prediction• Predict which APs a car will meet in next

interval• Challenges:– Vehicles move at high speeds– GPS location updates from vehicles

• Low frequency• Irregular updates

– Road and traffic conditions highly dynamic

• Previous work: 1st and 2nd order Markov models– Do not perform well on our dataset

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Voting among K Nearest Trajectories

• Exploit history to predict contact:

Vehicle’s near history

Past trajectories from other vehicles

• Find K trajectories that most closely match the vehicle’s recent history

• Obtain future path for K trajectories

• Report all APs visited by at least T of K trajectories

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• 802.11b testbed– 14 APs deployed inside 8 campus

buildings– APs are 20-60ft from the road– 802.11b radios with fixed rate of

11Mbps– 3 APs form a mesh network– Smartphone clients

• HP iPAQ and HTC Tilt • Stream H.264 videos at 64Kbps

• 802.11n testbed– 4 APs deployed outdoors– APs are 1-5ft from the road– All 4 APs form mesh– Laptops used as clients

Testbed Deployment

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3 4

567,8,9,1

0, 11,1213, 14

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Evaluation Methodology• Trace-driven simulation– San Francisco Yellow Cabs

• 30 day trace of 500 Cabs• Contacts with APs: 1120 gas stations, 1620 coffee shops

– Seattle city buses• Several week-long traces of 1200 buses• Contacts with APs: 618 gas stations, 738 coffee shops

• Two campus testbeds: Prototype implementation– Over 100 hours of road tests

• Emulab– Run unmodified Controller and AP code– Virtual cars emulating trace mobility

• Evaluate our system at scale

• Effect of modifying various parameters

Evaluate under real world wireless effects

• Validate the simulator• Show system is efficient and light-weight

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#Correctly predicted APs#Total predicted APs

• Setup: Gas stations as APs, radio range = 200m, prediction interval 3min

1200 Seattle city buses

Mobility Prediction Results

Voting among K nearest trajectories performs best for our dataset

#Correctly predicted APs#Total APs actually visited

( 2 )

(1/precision+1/recall)

Bus mobility is more

predictable

500 San Francisco Yellow Cabs

APs: Gas stations, 100m range

Results – Simulation (1/2)• Setup: 50 cars, Zipf-like demands, 50% APs not connected to

Internet

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APs: Coffee Shops, 100m range

Internet is the bottleneck

Benefit from wireline replication

Vehicular relay helps!

Wireline+relay5.2X baseline

6.3X better than baseline

VCD achieves higher throughput by combining wireline, relay and mesh replication

Mesh adds 3-13%

Results – Simulation (2/2)• Setup: 50 cars, Zipf-like demands, 50% APs not connected to

Internet

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APs: Coffee Shops, 100m range

Mesh benefits 14-20%

Benefits increase with higher range and dense AP deployment

APs: Coffee shops, 200m range

Low

Medium

High

Video quality over 3G

• 802.11b testbed: 8 APs, 3 connected by mesh

• 802.11n testbed: 4 APs, all connected by mesh

Results - Testbed

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Download (kB)

Play time (sec)

No replication 29297 3662

Wireline 71930 8991

Wireline + Mesh

79440 9930

Full replication 92493 11562

Download (kB)

Play time (sec)

No replication 16857 2107

Wireline 123175 15387

Wireline + Mesh

130827 16353

Full replication 136479 17060

2.7X

7.8X

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Summary

• VCD: high-bandwidth content distribution– Synergy: Internet, wireless, relay, mesh

connectivity– Predict AP-vehicle contacts– Wireline replication: LP formulation– Vehicular and mesh replication

• Deploy on two campus testbeds– Smartphone and laptop clients

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