Vehicular Network Applications

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Vehicular Network Applications

• VoIP
• Web
• Email
• Cab scheduling
• Congestion detection
• Vehicle platooning
• Road hazard warning
• Collision alert
• Stoplight assistant

• Toll collection
• Deceleration warning
• Emergency vehicle warning
• Border clearance
• Traction updates
• Flat tire warning
• Merge assistance

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Congestion Detection

• Vehicles detect congestion when
• Vehicles gt Threshold 1
• Speed lt Threshold 2
• Relay congestion information
• Hop-by-hop message forwarding
• Other vehicles can choose alternate routes

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Deceleration Warning

• Prevent pile-ups when a vehicle decelerates
  rapidly

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Wireless Technologies for Vehicular Networks

• Cellular networks
• High coverage, low bandwidth, expensive
• WiFi networks
• Moderate coverage, high bandwidth, free
• Combine all of them to achieve low cost, high
  bandwidth, and high coverage

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Interactive WiFi Connectivity from Moving Vehicles
Aruna Balasubramanian, Ratul Mahajan Arun
Venkataramani, Brian N Levine, John Zahorjan
University of Massachusetts Amherst
Microsoft Research
University of Washington
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Target Scenarios

• A car is within the range of multiple APs
• How common?
• Low data rate but low delay
• Alternatives?

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Overview
Given enough coverage, can WiFi technology be
used to access mainstream applications from
vehicles?

• Existing work shows
• the feasibility of WiFi access at vehicular
  speeds
• focus on non-interactive applications. e.g.,
  road monitoring

Internet
8
Outline

• Can popular applications be supported using
  vehicular WiFi today?
• Performance is poor due to frequent disruptions
• How can we improve application performance?
• ViFi, a new handoff protocol that significantly
  reduces disruptions
• Does ViFi really improve application performance?
• VoIP, short TCP transfers

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VanLAN Vehicular Testbed
Uses MS campus vans Base stations(BSes) are
deployed on roadside buildings Currently 2 vans,
11 BSes
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Measurement study

• Study application performance in vehicular WiFi
  setting
• Focus on basic connectivity
• Study performance of different handoff policies
• Trace-driven analysis
• Nodes send periodic packets and log receptions

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Handoff policies studied

• Practical hard handoff
• Associate with one BS
• Current 802.11
• Ideal hard handoff
• Use future knowledge
• Impractical

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Handoff policies studied

• Practical hard handoff
• Associate with one BS
• Current 802.11
• Ideal hard handoff
• Use future knowledge
• Impractical
• Ideal soft handoff
• Use all BSes in range
• Performance upper bound

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Comparison of handoff policies
Disruption

• Summary
• Performance of interactive applications poor when
  using existing handoff policies
• Soft handoff policy can decrease disruptions and
  improve performance of interactive applications

Practical hard handoff
Ideal hard handoff
Ideal soft handoff
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Outline

• Can popular applications be accessed using
  vehicular WiFi?
• How can we improve application performance?
• ViFi, a practical diversity-based handoff
  protocol
• Does ViFi really improve application performance?
• VoIP, short TCP transfers

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Design a practical soft handoff policy

• Goal Leverage multiple BSes in range
• How often do we have multiple BSes?
• Not straightforward

Constraints in Vehicular WiFi 1. Inter-BS
backplane often bandwidth-constrained 2.
Interactive applications require timely
delivery 3. Fine-grained scheduling of packets
difficult
Internet
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Why are existing solutions inadequate?

• Opportunistic protocols for WiFi mesh (ExOR,
  MORE)
• Uses batching Not suitable for interactive
  applications
• Path diversity protocols for enterprise WLANs
  (Divert)
• Assumes BSes are connected through a high speed
  back plane
• Soft handoff protocols for cellular (CDMA-based)
• Packet scheduling at fine time scales
• Signals can be combined

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ViFi protocol set up

• Vehicle chooses anchor BS
• Anchor responsible for vehicles packets
• Vehicle chooses a set of BSes in range to be
  auxiliaries
• e.g., B, C and D can be chosen as auxiliaries
• ViFi leverages packets overheard by the auxiliary

Internet
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ViFi protocol

• Source transmits a packet
• If destination receives, it transmits an ack
• If auxiliary overhears packet but not ack, it
  probabilistically relays to destination
• If destination received relay, it transmits an
  ack
• If no ack within retransmission interval, source
  retransmits

Source
Dest
Downstream Anchor to vehicle
Dest
Source
Upstream Vehicle to anchor
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Why relaying is effective?

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Why relaying is effective?

• Losses are bursty
• Independence
• Losses from different senders independent
• Losses at different receivers independent

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Guidelines for probability computation
1. Make a collective relaying decision and limit
the total number of relays 2. Give preference to
auxiliary with good connectivity with destination
How to make a collective decision without
per-packet coordination overhead?
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Determine the relaying probability

• Goal Compute relaying probability RB of
  auxiliary B
• Step 1 The probability that auxiliary B is
  considering relaying
• CB P(B heard the packet) . P(B did not hear
  ack)
• Step 2 The expected number of relays by B is
• E(B) CB RB
• Step 3 Formulate ViFi probability equation, ?
  E(x) 1
• to solve uniquely, set RB proportional to
  P(destination hears B)
• Step 4 B estimates P(auxiliary considering
  relaying) and P(destination heard auxiliary)
  for each auxiliary

ViFi Practical soft handoff protocol uses
probabilistic relaying for coordination without
per-packet coordination cost
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ViFi Implementation

• Implemented ViFi in windows operating system
• Use broadcast transmission at the MAC layer
• No rate adaptation
• Deployed ViFi on VanLAN BSes and vehicles

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Outline

• Can popular applications be accessed using
  vehicular WiFi?
• Due to frequent disruptions, performance is poor
• How can we improve application performance?
• ViFi, a practical diversity-based soft handoff
  protocol
• Does ViFi really improve application performance?

25
Evaluation

• Evaluation based on VanLAN deployment
• ViFi reduces disruptions
• ViFi improves application performance
• ViFis probabilistic relaying is efficient
• Also in the paper Trace-driven evaluation on
  DieselNet testbed at UMass, Amherst
• Results qualitatively consistent

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ViFi reduces disruptions in our deployment
ViFi
Practical hard handoff
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ViFi improves VoIP performance

• Use G.729 codec

gt 100
ViFi
seconds
Practical hard handoff
Length of voice call before disruption
Disruption When mean opinion score (mos) is
lower than a threshold
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ViFi improves performance of short TCP transfers

• Workload repeatedly download/upload 10KB files

gt 50
gt 100
ViFi
Practical hard handoff
Number of transfers before disruption
Median transfer time (sec)
Disruption lack of progress for 10 seconds
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ViFi uses medium efficiently

• Efficiency
• Number of unique packets delivered/ Number of
  packets sent
• Its efficient for their testbed, but may not be
  the case in general. Why?

efficiency
ViFi
Practical hard handoff
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Conclusions

• Improves performance of interactive applications
  for vehicular WiFi networks
• Interactive applications perform poorly in
  vehicular settings due to frequent disruptions
• ViFi, a diversity-based handoff protocol
  significantly reduces disruptions
• Experiments on VanLAN shows that ViFi
  significantly improves performance of VoIP and
  short TCP transfers

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Comments

• Interesting problem domain
• Target low-bandwidth applications, for which
  cellular networks are sufficient
• Have multiple APs within range tuned into the
  same channel
• May not be common and lose spatial diversity
• Use the lowest data rate
• Common to have multiple or fewer than 1 relay(s)
  for each tx
• Relay is not compelling
• Uplink sufficient to relay data to one AP
• Downlink if best AP is selected, the need for
  relay is low
• If relay has to be used, MORE like opportunistic
  routing may be more efficient
• They dismissed opportunistic routing due to its
  potential large delay due to batch
• But their delay can be high since retx timeout is
  generally large in order to account for variable
  contention delay

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Motivation

• 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 cellular networks?
• Expensive 30-60/month
• 5GB/month -gt 2Kbps!
• 40 3G capable devices have no 3G plan
• iPod Touch sales iPhone sales
• Bandwidth and backhaul limitations
• Limited video quality (96-128kbps, lt 10min long)
• Carriers interested in WiFi offloading
• Arms race between
• Increase in cellular bandwidth
• Higher resolution screens and videos
• Goal Enable high bandwidth applications (e.g.,
  video) in vehicular networks via WiFi

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Opportunistic WiFi connectivity
Gas stations and local shops deploy APs
Internet
Devices in vehicles contact roadside APs
Passengers watch videos, download files

• Compelling usage scenario
• Taxi companies provide value-added services to
  passengers
• Previous work low-bandwidth applications
• We focus on delivering high-bandwidth content
• e.g. video streaming

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Synergy among connections
High b/w, short-lived
High b/w, low coverage
Wireless LAN
Mesh Network
VCD
High b/w, persistent
Internet Access
Vehicle Relay
Low b/w, persistent
High b/w, high delay
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Contributions

• New techniques for replication optimization
• Goal Fully utilize wireless bandwidth during
  contact
• Optimized wireline replication to
  Internet-connected APs
• Replication using vehicular relays to unconnected
  APs
• Use mesh 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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Evaluation Methodology

• Trace-driven simulation and emulation
• San Francisco cabs, Seattle buses, Shanghai cabs
• Two testbeds on UT campus
• 802.11b 14 APs deployed inside 8 campus
  buildings, 20-60ft from the road
• 802.11n 4 APs outdoor, 1-5ft from the road
• Smartphone and laptop clients
• HP iPAQ and HTC Tilt
• Stream H.264 videos at 64Kbps

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Summary Vehicular Content Distribution

• KNT A new mobility prediction algorithm
• Based on voting among K nearest trajectories
• 25-94 more accurate than 1st and 2nd order
  Markov models
• A series of novel replication schemes
• Optimized wireline replication and mesh
  replication
• Opportunistic vehicular relay based replication
• Extensive evaluation simulation testbed
  emulation
• Simulation using San Francisco taxi and Seattle
  bus traces
• 3-6x of no replication, 2-4x of wireline or
  vehicular alone
• Full-fledged prototype deployed on two real
  testbeds
• 14-node 802.11b testbed and 4-node 802.11n
  testbed
• 4.2-7.8x gain over no replication
• Emulab emulation with real AP/controller and
  emulated vehicles
• Show system works at scale and is efficient
• Validate our trace-driven simulator