Alexander Afanasyev

1
Content Distribution in VANETs using Network
Coding Evaluation of the Generation Selection
Algorithms

• Alexander Afanasyev
• Tutors Seung-Hoon Lee, Uichin Lee

March 2, 2009
2
Content Distribution in Vehicular Ad-Hoc
Networks (VANETs)

• Applications
• Software updates and patches (e.g., navigation
  map, games)
• Multimedia data downloads (e.g., videos, news,
  etc.)

3
Content Distribution Challenges

• High mobility (i.e., highly dynamic networks)
• Error-prone channel (due to obstacles, multi-path
  fading, etc.)

4
CarTorrent BitTorrent-like Cooperative Content
Distribution in VANETs
A file is divided into pieces
Web Server
Exchange pieces via Vehicle-to-Vehicle
Communications
Download a file (piece by piece)
Not useful!

• Problem Peer Piece selection ? coupon
  collection problem

Cannot complete download!
5
Using Network Coding CodeTorrent
A file is divided into pieces
Web Server
Any linearly independent coded packet is helpful

• Network coding effectively mitigates the
  peer/piece selection problem and improves the
  performance!

6
Network Coding ProblemProcessing Overhead
Single Generation
Overhead
5/10/50 Generations

• Delay without O/H
• Small of generations is a better choice
• Larger of generations ? more severe coupon
  collection problem

7
Mitigating Coding Overheads

• Solution divide a file into small generations
• Problem too many generation causes a coupon
  collection problem
• Conflicting goals maximizing benefits of NC vs.
  minimizing coding O/H

50MB
8
Mitigating Coding Overheads

• Solution divide a file into small generations
• Problem too many generation causes a coupon
  collection problem
• Conflicting goals maximizing benefits of NC vs.
  minimizing coding O/H

10MB x 5
9
What is optimal strategy for generation
downloading?
Gen1
Gen2
Gen3
Global (neighbor status)
Request to ??
Gen1
Gen2
Gen3
Local (my status)

• Checking neighbor rank improve chances of
  linearly independent block, but
• Low-rank cars can also have valuable blocks
• Back to the BitTorrent problem of
  piece/generation selection
• Local status based decision (i.e., the least/the
  most downloaded generation, sequential order)?
• Neighbor status based decision?
• Random?

10
Generation Selection Strategies
Virtual Global Completeness Vector
Global Min Gen 4 Global Max Gen 3 Random
Random Sequential Gen 1
11
Simulation Setup

• Communications
• 802.11b 11Mbps Two-ray ground propagation
• Mobility
• Random Waypoint model w/ speed range of 0,20
  m/s
• Westwood area map 2400m2400m
• Nodes
• 3 APs file sources
• 200 nodes/40 interest level
• 80 nodes are downloading a file
• Download parameters
• 50 megabyte file
• 10 generations

Westwood area map
12
Downloading all generations in parallelGeneration
Progress
Global Min
Local Min
Random
Neighbor Min
13
Downloading all generations in parallelOverall
Progress
Neighbor-aware strategy improves at the beginning
of downloading
Local-aware and random strategies has smaller tail
confidence interval is calculated with
probability 95 using 8 simulations
14
Downloading all generations in parallelFinishing
times histogram

• Conclusions
• Network-aware strategy has long tail of finishing
  times
• Local and random strategies behave almost as good
  as global status-aware

15
Downloading generations (semi-)sequentiallyGenera
tion Progress
Global Max
Local Max
Neighbor Max
Sequential
16
Downloading generations (semi-)sequentiallyOveral
l Progress
!!! Neighbor-aware strategy outperforms local and
global one
confidence interval is calculated with
probability 95 using 8 simulations
17
Downloading generations (semi-)sequentially Finish
ing times histogram

• Conclusions
• Network-aware strategy outperforms other
  strategies
• Average finishing time for global/local max
  strategies 1.5 times worse than neighborhood
  status aware policy

18
Parallel vs Sequential DownloadingOverall
progress of the best strategies

• Conclusions
• Neighbor-aware generation choosing considerably
  improves chances for helpful block (linearly
  independent) at the beginning
• Local or random strategy improves download
  finishing time

19
Parallel vs Sequential DownloadingFinishing
times histogram

• Conclusions
• Neighbor-aware strategies have on average 20
  worse finishing times than local max strategy

20
Interesting Facts
Rank checking

• Checking generation rank of the available
  generation greatly improves performance for
  neighbor status aware strategies
• Integer vector gossiping decrease overall
  download performance

No rank check
Bool vector
Int vector
21
Conclusion

• Generation selection strategy in multi-generation
  CodeTorrent downloads have big impact on the
  overall download performance
• Local status aware strategies (local-min, random)
  have the best finishing performance
• Neighbor status aware strategies have the best
  start-up performance
• It is important to check rank for neighbor status
  aware strategies
• Future work
• Investigate performance of combined strategies
• Check performance using different node mobility
  models