Name : Dinesh Bilimoria Date: Oct 23rd, 2013

CSLI 5350G - Pervasive and Mobile ComputingWeek 6 - Paper Presentation“Exploiting Beacons for Scalable Broadcast Data Dissemination in VANETs”

Name: Dinesh BilimoriaDate: Oct 23rd, 2013

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Research Paper

Bibliography:

Schwartz, R. S., Das, K., and Scholten, H. and Havinga, P.(2012) Exploiting beacons for scalable broadcast data dissemination in VANETs. In: 9th ACM International Workshop on Vehicular Inter-Networking, Systems, and Applications, VANET 2012.

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Exploiting beacons for scalable broadcast data dissemination in VANETs

Broadcast Suppression Simple flooding, probability based, area based, neighbor knowledge

methods do not take into account the highly dynamic environment present on roads and predictable mobility pattern of vehicles’ movements.

In Broadcast Storm Problem, flooding scheme results in excessive redundancy, contention, and collision rates.

Broadcasting with CSMA/CA in the 802.11p is unreliable due to the lack of acknowledgments.

1-hop or 2-hop neighborhood information become outdated due to high speed of vehicles.

Adding neighborhood information to periodic messages results in high network overhead.

Current delay-based schemes still suffer from high levels of contention and collision when the number of vehicles rebroadcasting nearly simultaneously is high in dense networks.

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Broadcast Suppression Techniques Common approach to reduce broadcast redundancy and end-to-end delay in

VANETs is to give highest priority to the most distant vehicles towards the message direction.

Three ways of assigning this priority; Weighted p-Persistence;

- the farthest vehicles rebroadcast with highest probability

Slotted 1-Persistence; - vehicles with highest priority are given the shortest delay

before rebroadcasting

Slotted p-Persistence; - vehicles with highest priority the shortest delay and highest probability to rebroadcast

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Broadcast Suppression Techniques

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Issues and Challenges With delay-based schemes, scalability issues are seen as there is inability to

dynamically choose the optimal value for the number and boundaries of the time slots used.

Time slots matched to geographical regions within the transmission range of the sender but this can lead to an uneven distribution of vehicles in each time slot.

est. total time taken

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Issues and Challenges In EMDV, the sender determines the next relay vehicle based on

neighborhood information received from beacons. In centralized approach selecting the next relay vehicle, problem arises

when vehicles transmit messages with different power levels. Sub-optimal vehicle selection, leading to higher end-to-end delays

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Objective Distributed Optimized Time (DOT) protocol aims at always selecting the

farthest vehicles, i.e., optimal relay vehicles, while controlling transmission redundancy used to increase robustness.

Proposal DOT has the following characteristics;

(1) Time slot density control: - positioning information of 1-hop neighbors to control with precision the time slots’ boundaries- prevents the uneven distribution of vehicles among time slots(2) Distributed:- each vehicle takes the decision regarding when to retransmit a message in a distributed fashion- prevents sub-optimal selections of a relay vehicle

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Contributions Propose the DOT protocol to tackle broadcast suppression.

Assumptions DOT runs on top of the MAC layer, thereby requiring no modification to IEEE 802.11p Relies on the existence of periodic beacons transmitted by each vehicle at a certain

rate Beacons are defined to be transmitted in the form of WAVE Short Messages (WSMs),

according to the IEEE 1609 WAVE The IEEE WAVE standard determines that these messages carry information such as

the data rate, channel number and the transmission power level employed Contextual information ie vehicle’s geographical position, speed and acceleration to

be included Each vehicle is equipped with a device capable of obtaining the current vehicle’s

geographical position, such as a GPS receiver Consider the following message header structure: <Vehicle ID, Message ID, Time

Stamp, Vehicle’s Geographical Coordinates, Power Level>


Distributed Optimized Time (DOT) Each vehicle keeps a table of one-hop neighbors Tn. Each entry in Tn contains: <Vehicle ID, Expiration Time, Vehicle’s

Geographical Coordinates>. Time tolerance before removing an entry defined as tt = 2.5( 1 / bf), where bf

is the beaconing rate, e.g., 10 Hz. Every vehicle schedules a rebroadcast for message m with a time delay TSij. If any vehicle receives an echo of message m before TSij expires, it cancels

its rebroadcast and ignores future duplicates of m. TSij tasks (i) estimate which vehicles are within the transmission range of the

sender and received m (ii) Sort the entries of every vehicle in table Tn based on its geographical position.

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Distributed Sorting Algorithmsorts vehicles as

The time for vehicles to wait before rebroadcasting is given by:

# of vehicles in time slot

position of vehicles in

est. total time taken

additional delay


How does DOT work ? - All vehicles in the sender’s range assigned individual time slots Rebroadcasts are separated in time by multiples of slot time st.

- Prevent nearly simultaneous rebroadcasts among the two vehicles in each time slot, higher waits for additional delay

OPTIMALRESULTS

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Estimating vehicles in the Sender’s Range DOT depends on accurately estimating which vehicles are within the

transmission range Equation for Transmission Range

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Simulation Parameters

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Results Results of performance of protocols with respect to delivery ratio

DOT and Optimized Slotted 1-persistence achieve near 100% in every transmission range settings

DOT presents lowest delay since fewer hops

DOT controls the time slot density regardless of the current density of the vehicles

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Results Analysis of the performance and when varying the main parameters.

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Open problems Although this paper focused in suppressing broadcast using 1-hop neighbor

information, what happens when using 2-hop neighbor is used. Incorporate DOT broadcast suppression using store-carry-forward scheme. Analytical method is required instead of using estimating vehicles in the

sender’s range. Priority to be embedded in message header

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Exploiting beacons for scalable broadcast data dissemination in VANETsDiscussion points1. What is the difference between simple flooding and probability based flooding ? Simple flooding – no form of suppression

Probability flooding – rely on network topology to assign a probability to each broadcast

2. What is the difference between Weighted p-Persistence and Slotted 1-Persistence? Weighted p-Persistence;

- the farthest vehicles rebroadcast with highest probability Slotted 1-Persistence;

- vehicles with highest priority are given the shortest delay before rebroadcasting

3. Using the DOT sorting algorithm,what is vehicle v1’s sorting list? <v3,v2,v1,v0>

v0

v1

v2 v3

Name : Dinesh Bilimoria Date: Oct 23rd, 2013

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