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Istituto Superiore Mario Boella

Istituto Superiore Mario Boella. Slotted VANET – One Year After. R. Scopigno, BWA Lab – scopigno@ismb.it. Agenda. Survey on solutions by international literature The need for a comparative analysis The state-of-art of MS- Aloha Upcoming activities by ISMB. Agenda.

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Istituto Superiore Mario Boella

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  1. Istituto Superiore Mario Boella Slotted VANET – OneYearAfter R. Scopigno, BWA Lab– scopigno@ismb.it

  2. Agenda • Survey on solutions by internationalliterature • The need for a comparative analysis • The state-of-art of MS-Aloha • Upcomingactivities by ISMB

  3. Agenda • Survey on solutions by internationalliterature • The need for a comparative analysis • The state-of-art of MS-Aloha • Upcomingactivities by ISMB

  4. Slotted MAC VANETs • Synchronousprotocols are supposed to improve VANET performances… in theory • Coordination over time-space: Higher PDR • FixedLatency and determinism • Thesefeaturescannot be taken for granted • Slottedprotocols can worsen performances • E.g.Hidden Terminal (HT): A,B can select the same slot x and continuouslytransmit on the same, mutuallyinterfering to C • In CSMA/CA the effect in not so disruptive: evenwithout RTS/CTS interferenceisnotdeterministic and PDR keeps high • Studiesneed to considerallpossiblecauses of HT from real world (urbanobstructions by corners, fading, mobility), otherwisepurelyidealresults • E.g.BlockingProtocol by scarse slot reuse

  5. Some Examples • Severalcases of HT • A, C in LOS simultaneouslyselect the same (withoutack) • B can heareitherone (A?) or none • No onewilleverdiscoverit HT only by sensing • ..OR.. A↔B; C↔B but NOT A↔C • A, C cannotheareachother; C mayselect the slot alreadyused by A and collide • B can heareitherone (A?) or none • No onewilleverdiscoverit HT only by sensing • ..OR.. Itmayhappenalsoif an ack-ed setup isinvolved • A, C maygetclose due to mobility • Allthesecaseshave a huge impact • A continuous ACK isrequired, by all the nodes(a priori unknown set), two hop-far • Possiblethreat from Overheads • Effects becomes more relevant with lower number of slots (increased probability of collision) • What is worse: notable reducion in PDR at low distances A B C

  6. International Solutions • Solutions must be carefullyevaluated • Some of themcouldworsenratherthanimprove performance • Literatureincludes 3 mainapproacheswhichhaveneverbeenquantitavelycompared • S-TDMA (Self-Configuring TDMA) from HalmstadtUniv • Solution developed for shipsurvelliance in open spaces (originally no HT solution) • Fullyspecified for ships; missingdetailsabout VANET adaptation(exactmessage formats - especially for signalling, management of HT,…) • MS-Aloha (Mobile SlottedAloha) from ISMB • developedspecifically for VANETsagainst HT and to solve scalability • DTDMA (Decentralized TDMA) from Toyota • MS-Aloha and DTDMA share a common rationaleagainst HT

  7. Agenda • Survey on solutions by internationalliterature • The need for a comparative analysis • The state-of-art of MS-Aloha • Upcomingactivities by ISMB

  8. Comparative Analysis • Solutions must be carefullyevaluated • A possible standard shouldinherit the points of strenght of eachtechnology • S-TDMA, MS-Aloha, DTDMA haveneverbeencompared • Common initialrationale (slots, absolutesynchronization) • Differentapproaches • Whatrequirements? What target figures? Whattheirweights? • A proposal of requirements - based on literature - follows • Metrics, additionalrequirements and scenarios can be suggested • Suggestionsplease: scopigno@ismb.it

  9. VANET MAC Requirements (i) • Decentralized • In order to work without any fixed infrastructure • Immune from the problem of HT • Meant to prevent disruptive undected collisions • Fading and obstructions (shadowing) by buildings should be considered • Reactive to network topology changes • Aimed at coping with mutual mobility and sudden appearance of nodes • Scalable both with traffic and with the number of stations • To properly manage resources (involves effective slot reuse) • Involving a low protocol overhead, for sake of efficiency • For sake of efficiency and against issues raised by acks preventing HT • Priority(and/or pre-emption) • To improve dynamic service multiplexing • Prevention of blocking states • Virtually infinite slot re-use 9

  10. VANET MAC Requirements (ii) • Deterministic in delay • In order to guarantee a fixed delivery-time for safety messages; • Reliable • Providing a high packet-delivery rate (almost ideal) and preventing collisions; • Fair among the nodes • All nodes have at least one opportunity to access the channel within each time period; • At some degree compatible with 802.11p • This would shorten the time-to-market and simplify the migration path as well as coexistence • It should include a back-up solution for missing synch • In case (urban canyons) GPS were not available, the protocol should work – even if in a slightly degraded way • It should preferably include measures against Denial-of-Service (DoS) Attacks • If a connection-oriented protocol is not immune from DoS it can be blocked by fakenodes • As if in CSMA/CA a node did not repsect CSMA/CA waiting times

  11. Agenda • Survey on solutions by internationalliterature • The need for a comparative analysis • The state-of-art of MS-Aloha • Upcomingactivities by ISMB

  12. MS-Aloha: State of the Art (i) • MS-AlohaFeatures • A trailer (FI) appended to each slot with the node’sview of the channel • Against HT and to managemobility(one-periodrefresh) • A slot is free ifitisannounced free by everyone in the radio range • The FI isaggregated (and forwarded) no more than 2 hops(slot re-use) • Astinyaspossible to avoidheavyoverheads (short-identification) • Preemption (high priority connection can reject a lowerpriorityone) • Slot Re-Use and infinite re-use by dynamicthresholds • A cross-layerthreshold can be set-up • If a FI isreceived in a frame with [Power < Thr] the information isprocessed (butnotforwarded) • Thrisdynimicallyraised and lowered by eachnodebased on the number of free slotswhichitconsiders free • Infinite slot reuse

  13. MS-Aloha: State of the Art (ii) • MS-AlohaResults • Tested by simulations (NS-2) involving • Nakagami fading • Cumulative Signal-to-Noise and Interference (SNIR) Model (by Mercedes) • Obstruction by buildings(by ISMB) • Large number of nodes • Settings: 224 slots (446 us), 0.1s frame, guardtime 1µs (*), 12Mbps linerate • In eachslot: 200B + 802.11p frame + 802.11p PLCP (PLCP used to detect frame start) • Java visualization-tool to study NS-2 outputs(of MS-Aloha, CSMA/CA and adptable to anyprotocol) – by ISMB • Encouraging comparative results to CSMA/CA • Higher PDR, fixedlatency, effective slot reuse • Lower overhead (η=0.3) than CSMA/CA(η=0.13-0.03)no statisticalwaiting time • Both in case of unicast and broadcast, despite 336Byte overhead in FI • (*) … seenext slide … • Simulationswithoutbuildings: • In addition to determinism, MS-Alohahas an almostideal PDR at small distances • The differencebetween 90% and 100% ishuge ( 0 distance) • the differencebetween 20 and 17% isalmostnegligible (200m) • In CSMA/CA interferenceisevenlydistributed; in MS-Alohaiscoordinated by slot re-use and startsonlywhenslotsgetexhasuted • thresholdsmake slot re-use stronger and thiscausescloserinterferences

  14. MS-Aloha: Possible Settings(iii) This slide contains additional details on the oral answers to the main objections arisen during the presentation • So far the protocolsimulations and settingshavebeenaimed to validate the algorithms (HT, slot re-use, mobility management… ) • Onlyidealsettings on guard-time • HoweverSynchronization under mobility (up to 1440km/h) is <250nsasdemonstrated by two commercial products • Lessthan 250 ns with the Datum ET6000 (with 6 satellites in view) asexplainedat p.4 of the link • Between 25 and 100ns with the following GPS timing receiver (from Instrumentation Technology Systems) • Reducing the number of slots – from 224 – and increasing the guardtime (Tg) isnot an issue • Moving to 220 slots (from 224) the Tg becomes 13,1 µs • Moving to 200 slots the Tg becomes 78,5 µs (alsorelevanteffects on FI trailer) • Thanks to dynamicthresholds the lowernumber of slotsiscounteracted by a stronger re-use • Ithasalreadybeendemonstrated to scarcely and gracefullyaffect PDR • Wehavealreadystartedsimulations with new settings. Resultsforwardedsoon • Suggestions on the expectedprecisions of GPS clocks (also in hold-on mode) aswellas on other time-constraints are welcome • Details on scenarios to be investigated are welcome too • Requests from meeting Jan2010 (Sophiantipolis) on obstruction, alreadystudied (#7 of next slide)

  15. MS-Aloha: Published Papers (iv) • H.A. Cozzetti, Riccardo M. Scopigno, “RR-Aloha+: A Slotted and Distributed MAC Protocol for Vehicular Communications”, IEEE VNC 2009 • H.A. Cozzetti, Riccardo M. Scopigno, Luca Casone, Giuseppe Barba, “Comparative Analysis of IEEE 802.11p and MS-Aloha in Vanet Scenarios”, IEEE VON 2009 • R. Scopigno, A. Cozzetti, “Mobile Slotted Aloha for Vanets”, IEEE VTC-fall 2009 • R. Scopigno, A. Cozzetti, “GNSS Synchronization in Vanets”, IEEE-IFIP NTMS 2009 • A. Cozzetti, R. Scopigno, L. Lo Presti, “Architectures for the Integration of GNSS Receiver and Vanets Tranceriver”, ICINS 2010 • R. Scopigno, A. Cozzetti, “Comparative Analysis of Time-Space Efficiency in CSMA/CA and Slotted Vanets”, IEEE VTC-fall 2010 • R. Scopigno, A. Cozzetti, “Signal Shadowing in Simulation of Urban Vehicular Communications”, IEEE-IARIA ICWMC 2010 • L.Pilosu, A.Cozzetti, R. Scopigno, “Layered and Service-Dependent Security in CSMA/CA and Slotted Vanets”, DSRC workshop at ICST QShine 2010 • R. Scopigno, A. Cozzetti, L. LoPresti “Benefits of Tightly-Coupled Architectures for the Integration of GNSS Receiver and Vanet Tranceiver” (in Russian), Concern CSRI Elektropribor, JSC Journal Gyroscopy and Navigation, Issue 4, December 2010- ISSN 0869-7035

  16. MS-Aloha without & with Buildings(i) • Simulation Settings: • 5x5 grid-topology (the area is wide 750m) with double lane roads; • 600 nodes moving at 60 km/h in opposite directions; • Broacast Traffic - 10 HzApplication Rate; • 200 byte payload (safety application); • 7 dBm (5 mW) Transmission Power; • -96 dBm Wireless Interface Sensitivity. • MS-Aloha Settings: • 224 slots; • DynamicThreshold Disabled. • Video FI • Video 3-Hops • MS-Alohaworksalso with obstructions (itsolves HT) • Evenwithoutbuildings (limitingpropagation) and thresholds 224 slots can manage 600 nodes • Without building largernumber of collisions and less free slots 16

  17. MS-Aloha without BuildingsDynamic Threshold Disabled & Enabled (ii) • Simulation Settings: • 5x5 grid-topology (the area is wide 750m) with double lane roads; • 600 nodes moving at 60 km/h in opposite directions; • Broacast Traffic - 10 HzApplication Rate; • 200 byte payload (safety application); • 7 dBm (5 mW) Transmission Power; • -96 dBm Wireless Interface Sensitivity. • MS-Aloha Settings: • 224 slots; • DynamicThreshold Disabled & Enabled. • Video FI • Video 3-Hops • More free slots with Thresholds (forcedreuse) and lowernumber of collisions • worse PDR due to interferences by closernodesreusingslots • At 2 sec Thrunloads FI (1st sim) and mapschangescolourat the end • The two-hop spanisshrinked (2nd sim) 17 17

  18. MS-Aloha vs CSMA/CA without Buildings (iii) • Simulation Settings: • 5x5 grid-topology (the area is wide 750m) with double lane roads; • 600 nodes moving at 60 km/h in opposite directions; • Broacast Traffic - 10 HzApplication Rate; • 200 byte payload (safety application); • 7 dBm (5 mW) Transmission Power; • -96 dBm Wireless Interface Sensitivity. • MS-Aloha Settings: • 224 slots; • DynamicThreshold Disabled. • Video RX Pkt & PDR • Afterinitialtransition MS-Alohasucceeds in receiving a highernumberpfpackets (centre) • Bordereffectsmakereceivelesspackets • Thiseffectisemphasized by time-spacecoordination of MSA 18 18

  19. Agenda • Survey on solutions by internationalliterature • The need for a comparative analysis • The state-of-art of MS-Aloha • Upcomingactivities by ISMB

  20. ISMB: Upcoming Activities in the Field • New simulations • Improvedrealism in channel/mobilitymodelling • Comparative analysis to otherprotocols • Analysis of degradationdepending on clock delivery • Additional open issues • Strongercompatibility with CSMA/CA • Workaround for missingsynchronization • Pre-emption and service multiplexing • Nesting of upper-layerprotocols (geo-routing) • VisualizationTool and extended comparative analyses • Extension of V_SuperCar to supportoutputs of otherprotocols • Possibly with otherslottedprotocols (S-TDMA, DTDMA) • Possibleimplementation • Possibleframework • project (ItalianMinistry of Research), currentlysupporting the research • proposal (submitted to FP7 call on Transport) and otherpendingregionalinitiatives

  21. Thank you for your kind attention

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