1 / 25

Reliable Broadcasting without Collision Detection… … in an Automotive Scenario

International Graduate School of Dynamic Intelligent Systems. Reliable Broadcasting without Collision Detection… … in an Automotive Scenario. Jaroslaw Kutylowski (UPB) Filip Zagorski (WUT) SOFSEM 2006. Motivation & Problem. traffic jam.

reed-christian
Download Presentation

Reliable Broadcasting without Collision Detection… … in an Automotive Scenario

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. International Graduate School of Dynamic Intelligent Systems Reliable Broadcasting without Collision Detection… …in an Automotive Scenario Jaroslaw Kutylowski (UPB) Filip Zagorski (WUT) SOFSEM 2006

  2. Motivation & Problem traffic jam How to notify other cars on the road about traffic jam?

  3. Motivation & Problem • Design limitations • cheap hardware • no collision detection • low transmission speed • low transmission distance • interference range = 2 x transmission distance • hard environmental conditions • curves, big trucks – transmission failures • reliability is necessary

  4. Agenda • Model & System Architecture • Size approximation • Broadcast • Experimental evaluation

  5. Model & System Architecture • Hardware requirements • Synchronized clocks • Location information (GPS) • Successful / unsuccessful transmission • collision information to sender • no information about collision to other stations • a collision is not undistinguishable from no transmission at all

  6. Model & System Architecture • Division in geographic sectors • For evaluations • we assume sector length of 250 meters

  7. Model & System Architecture • Message about traffic jam produced in start sector • Message broadcasted from one sector to the next one • Unique leader in each sector • Leader changed frequently • Only leaders retransmit information • In each sector we need • Size approximation algorithm • Leader election algorithm • Broadcast algorithm

  8. Model & System Architecture • Time division between size approximation, leader election and broadcast Size approximation Leader election Broadcast one time unit

  9. Model & System Architecture • Transmission distance • for size approximation and leader election • transmission distance = one sector size • for broadcast • transmission distance = two sector sizes • Interference distance • for size approximation and leader election • transmission distance = two sector size • for broadcast • transmission distance = four sector sizes

  10. Model & System Architecture • Time division between neighbor sectors • sectors numbered with consecutive IDs • a sector executes its algorithms only if • current time unit % 5 == sector ID % 5 Sector with ID%5==0 Sector with ID%5==1 Sector with ID%5==2 Sector with ID%5==3 Sector with ID%5==4 Sector with ID%5==0 one time unit five time units

  11. Model & System Architecture • Time division between neighbor sectors • Fact: when a sector is transmitting, the next sector can hear it without interference from other sectors • (the next sector transmitting is in distance 5 and its interference range is at most 4 sectors)

  12. Model & System Architecture • Every node hears a successful transmission with probabilitypr Leader transmits broadcast message … sector with traffic jam

  13. Size approximation • Leader election needs approximate number of stations in sector • Upper bound on number of stations in sector known – Nmax • Multi-round algorithm • each round has duration of Mj steps • every active station chooses randomly step to transmit in a round • each station counts number of successful transmissions • if station has successful transmission – station gets inactive

  14. Size approximation • Defining parameters Mj and number of rounds • First possibility • Compute Mjso that the expected number of successful transmission per time step in each round is maximized • For Mjwe compute the expected number of stations which get inactive Sj– computation of Mj+1relies on it • Disadvantage: deviations from expectation can cause low approximation ratio

  15. Size approximation • Second possibility • Taking deviations into account • using more rounds • giving better reliability • Why? • we can compute the deviation of Sj from its expectation -- number of stations which get inactive with high probability • computation of Mj+1 gets more reliable

  16. Size approximation • Deviation from expectation given by • Increase M so that necessary probability is reached Deviation from expectation Probability dependent on M Number of stations which get inactive

  17. Size approximation • Take n=Nmax=100 (worst case) • First possibility: 264 steps in 8 rounds, perfect estimation of number of stations in 45% of experiments • Second possibility: 592 steps in 7 rounds, perfect estimation in 100% of experiments

  18. Broadcast • Algorithm • Leader transmits message if it has not been resent from its sector often enough • No acknowledgements • Key problem • unreliability of wireless link (parameter pr) • transmission from sector S to S+1 • only a fraction of nodes from sector S+1 received message

  19. Broadcast • No acknowledgements • with link unreliability an acknowledgment is not reliable • leader may have received messages – others do not • leader acknowledges message although intuitively it should not! leader

  20. Broadcast • Investigate probability that message will be retransmitted in sector S after i rounds from hearing it from S-1 • these values are hard to compute! • We develop an approximation (lower bound) for the value of • P(i,x) describes the probability that message will reach sector S+i from S for the first time in exactly x rounds

  21. Broadcast • Final approximation • c computed numerically, dependent on smallest number of stations in sector expected

  22. Broadcast • Some computed values • link reliability set to • 2.5 km length road, with 10 sectors • probability to travel this distance in 20 time slots (2 seconds) is about 0.9

  23. Experimental evaluation • Helbing-type cellular automaton • link reliability set to • 6.75 km length road, with 27 sectors • traffic jam in last sector, counting arrival time in first sector • all broadcasts reachedthe first sector • average time 31 unit timeslots (3.1 second)

  24. Conclusion • We have shown • complete system for traffic jam notification • size approximation algorithm • not optimized for asymptotical performance • works well for small inputs • analysis for broadcast on a line under assumption of link unreliability

  25. International Graduate School of Dynamic Intelligent Systems Thank you for your attention!

More Related