1 Vehicular Networks - Content distribution and data retrieval Slides are integrated from researchers at UCLA

Outline Urban Grid Car Torrent

Urban Grid “opportunistic” ad hoc networking From Wireless to Wired network Via Multihop

Opportunistic piggy rides in the urban mesh Pedestrian transmits a large file in blocks to passing cars, busses The carriers deliver the blocks to the hot spot

Car to Car communications for Safe Driving Vehicle type: Cadillac XLR Curb weight: 3,547 lbs Speed: 65 mph Acceleration: - 5m/sec^2 Coefficient of friction:.65 Driver Attention: Yes Etc. Vehicle type: Cadillac XLR Curb weight: 3,547 lbs Speed: 45 mph Acceleration: - 20m/sec^2 Coefficient of friction:.65 Driver Attention: No Etc. Vehicle type: Cadillac XLR Curb weight: 3,547 lbs Speed: 75 mph Acceleration: + 20m/sec^2 Coefficient of friction:.65 Driver Attention: Yes Etc. Vehicle type: Cadillac XLR Curb weight: 3,547 lbs Speed: 75 mph Acceleration: + 10m/sec^2 Coefficient of friction:.65 Driver Attention: Yes Etc. Alert Status: None Alert Status: Passing Vehicle on left Alert Status: Inattentive Driver on Right Alert Status: None Alert Status: Slowing vehicle ahead Alert Status: Passing vehicle on left

DSRC*/IEEE p : Enabler of Novel Applications Car-Car communications at 5.9Ghz Derived from a three types of channels: Vehicle-Vehicle service, a Vehicle-Gateway service and a control broadcast channel. Ad hoc mode; and infrastructure mode p: IEEE Task Group that intends to standardize DSRC for Car-Car communications * DSRC: Dedicated Short Range Communications

DSRC Channel Characteristics

Hot Spot Vehicular Grid as Opportunistic Ad Hoc Net

Hot Spot Power Blackout Power Blackout Vehicular Grid as Emergency Net

Power Blackout Power Blackout Vehicular Grid as Emergency Net

CarTorrent : Opportunistic Ad Hoc networking to download large multimedia files Published 2005

You are driving to Vegas You hear of this new show on the radio Video preview on the web (10MB)

Highway Infostation download Internet file

Incentive for “ad hoc networking” Problems: Stopping at gas station to download is a nuisance Downloading from GPRS/3G too slow and quite expensive Observation: many other drivers are interested in download sharing (like in the Internet) Solution: Co-operative P2P Downloading via Car-Torrent

CarTorrent: Basic Idea Download a piece Internet Transferring Piece of File from Gateway Outside Range of Gateway

Co-operative Download Vehicle-Vehicle Communication Internet Exchanging Pieces of File Later

Bit Torrent review Swarming: Parallel downloads among a mesh of cooperating peers –Scalable: System capacity increases with increase in number of peers Tracker –Handles peer discovery Centralized Tracker, single point of failure Observation: Might not work for Wireless scenarios, because of intermittent connectivity Issue: Mobility increases churn of nodes participating in a download

BitTorrent…. A picture.. Uploader/downloader Tracker Uploader/downloader

CarTorrent: Gossip protocol A Gossip message containing Torrent ID, Chunk list and Timestamp is “propagated” by each peer Problem: how to select the peer for downloading

Peer Selection Strategies Possible selections: 1) Rarest First: BitTorrent-like policy of searching for the rarest bitfield in your peerlist and downloading it 2) Closest Rarest: download closest missing piece (break ties on rarity) 3) Rarer vs Closer: weighs the rare pieces based on the distance to the closest peer who has that piece.

Experimental Evaluation

Impact of Selection Strategy

Analytical Model n √n log n

Why is the Car-Torrent solution attractive? Bandwidth at the infostation is limited and “not convenient” –It can become congested if all vehicles stop –It is a nuisance as I must stop and waste time GPRS and 3G bandwidth is also limited and expensive The car to car bandwidth on the freeway is huge and practically unlimited! Car to car radios already paid for by safe navigation requirement CarTorrent transmissions are reliable - they involve only few hops (proximity routing)