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Istituto Superiore Mario Boella

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Slotted VANET One Year After R. Scopigno, BWA Lab scopigno_at_ismb.it * – PowerPoint PPT presentation

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Title: Istituto Superiore Mario Boella


1
Istituto Superiore Mario Boella
Slotted VANET One Year After
R. Scopigno, BWA Lab scopigno_at_ismb.it
2
Agenda
  • Survey on solutions by international literature
  • The need for a comparative analysis
  • The state-of-art of MS-Aloha
  • Upcoming activities by ISMB

3
Agenda
  • Survey on solutions by international literature
  • The need for a comparative analysis
  • The state-of-art of MS-Aloha
  • Upcoming activities by ISMB

4
Slotted MAC VANETs
  • Synchronous protocols are supposed to improve
    VANET performances in theory
  • Coordination over time-space Higher PDR
  • Fixed Latency and determinism
  • These features cannot be taken for granted
  • Slotted protocols can worsen performances
  • E.g. Hidden Terminal (HT) A,B can select the
    same slot x and continuously transmit on the
    same, mutually interfering to C
  • In CSMA/CA the effect in not so disruptive even
    without RTS/CTS interference is not deterministic
    and PDR keeps high
  • Studies need to consider all possible causes of
    HT from real world (urban obstructions by
    corners, fading, mobility), otherwise purely
    ideal results
  • E.g. Blocking Protocol by scarse slot reuse

5
Some Examples
  • Several cases of HT
  • A, C in LOS simultaneously select the same
    (without ack)
  • B can hear either one (A?) or none
  • No one will ever discover it HT only by sensing
  • ..OR.. A?B C?B but NOT A?C
  • A, C cannot hear each other C may select the
    slot already used by A and collide
  • B can hear either one (A?) or none
  • No one will ever discover it HT only by sensing
  • ..OR.. It may happen also if an ack-ed setup is
    involved
  • A, C may get close due to mobility
  • All these cases have a huge impact
  • A continuous ACK is required, by all the nodes(a
    priori unknown set), two hop-far
  • Possible threat 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

6
International Solutions
  • Solutions must be carefully evaluated
  • Some of them could worsen rather than improve
    performance
  • Literature includes 3 main approaches which have
    never been quantitavely compared
  • S-TDMA (Self-Configuring TDMA) from Halmstadt
    Univ
  • Solution developed for ship survelliance in open
    spaces (originally no HT solution)
  • Fully specified for ships missing details about
    VANET adaptation(exact message formats -
    especially for signalling, management of HT,)
  • MS-Aloha (Mobile Slotted Aloha) from ISMB
  • developed specifically for VANETs against HT and
    to solve scalability
  • DTDMA (Decentralized TDMA) from Toyota
  • MS-Aloha and DTDMA share a common rationale
    against HT

7
Agenda
  • Survey on solutions by international literature
  • The need for a comparative analysis
  • The state-of-art of MS-Aloha
  • Upcoming activities by ISMB

8
Comparative Analysis
  • Solutions must be carefully evaluated
  • A possible standard should inherit the points of
    strenght of each technology
  • S-TDMA, MS-Aloha, DTDMA have never been compared
  • Common initial rationale (slots, absolute
    synchronization)
  • Different approaches
  • What requirements? What target figures? What
    their weights?
  • A proposal of requirements - based on literature
    - follows
  • Metrics, additional requirements and scenarios
    can be suggested
  • Suggestions please scopigno_at_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 fake nodes
  • As if in CSMA/CA a node did not repsect CSMA/CA
    waiting times

11
Agenda
  • Survey on solutions by international literature
  • The need for a comparative analysis
  • The state-of-art of MS-Aloha
  • Upcoming activities by ISMB

12
MS-Aloha State of the Art (i)
  • MS-Aloha Features
  • A trailer (FI) appended to each slot with the
    nodes view of the channel
  • Against HT and to manage mobility (one-period
    refresh)
  • A slot is free if it is announced free by
    everyone in the radio range
  • The FI is aggregated (and forwarded) no more than
    2 hops (slot re-use)
  • As tiny as possible to avoid heavy overheads
    (short-identification)
  • Preemption (high priority connection can reject a
    lower priority one)
  • Slot Re-Use and infinite re-use by dynamic
    thresholds
  • A cross-layer threshold can be set-up
  • If a FI is received in a frame with Power lt Thr
    the information is processed (but not forwarded)
  • Thr is dynimically raised and lowered by each
    node based on the number of free slots which it
    considers free
  • Infinite slot reuse

13
MS-Aloha State of the Art (ii)
  • MS-Aloha Results
  • 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 each slot 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 any protocol)
    by ISMB
  • Encouraging comparative results to CSMA/CA
  • Higher PDR, fixed latency, effective slot reuse
  • Lower overhead (?0.3) than CSMA/CA
    (?0.13-0.03)no statistical waiting time
  • Both in case of unicast and broadcast, despite
    336Byte overhead in FI
  • () see next slide
  • Simulations without buildings
  • In addition to determinism, MS-Aloha has an
    almost ideal PDR at small distances
  • The difference between 90 and 100 is huge ( 0
    distance)
  • the difference between 20 and 17 is almost
    negligible (200m)
  • In CSMA/CA interference is evenly distributed in
    MS-Aloha is coordinated by slot re-use and starts
    only when slots get exhasuted
  • thresholds make slot re-use stronger and this
    causes closer interferences

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 protocol simulations and settings have
    been aimed to validate the algorithms (HT, slot
    re-use, mobility management )
  • Only ideal settings on guard-time
  • However Synchronization under mobility (up to
    1440km/h) is lt250ns as demonstrated by two
    commercial products
  • Less than 250 ns with the Datum ET6000 (with 6
    satellites in view) as explained at 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) is not an issue
  • Moving to 220 slots (from 224) the Tg becomes
    13,1 µs
  • Moving to 200 slots the Tg becomes 78,5 µs
    (also relevant effects on FI trailer)
  • Thanks to dynamic thresholds the lower number of
    slots is counteracted by a stronger re-use
  • It has already been demonstrated to scarcely and
    gracefully affect PDR
  • We have already started simulations with new
    settings. Results forwarded soon
  • Suggestions on the expected precisions of GPS
    clocks (also in hold-on mode) as well as on other
    time-constraints are welcome
  • Details on scenarios to be investigated are
    welcome too
  • Requests from meeting Jan2010 (Sophiantipolis) on
    obstruction, already studied (7 of next slide)

15
MS-Aloha Published Papers (iv)
  1. H.A. Cozzetti, Riccardo M. Scopigno, RR-Aloha
    A Slotted and Distributed MAC Protocol for
    Vehicular Communications, IEEE VNC 2009
  2. 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
  3. R. Scopigno, A. Cozzetti, Mobile Slotted Aloha
    for Vanets, IEEE VTC-fall 2009
  4. R. Scopigno, A. Cozzetti, GNSS Synchronization
    in Vanets, IEEE-IFIP NTMS 2009
  5. A. Cozzetti, R. Scopigno, L. Lo Presti,
    Architectures for the Integration of GNSS
    Receiver and Vanets Tranceriver, ICINS 2010
  6. R. Scopigno, A. Cozzetti, Comparative Analysis
    of Time-Space Efficiency in CSMA/CA and Slotted
    Vanets, IEEE VTC-fall 2010
  7. R. Scopigno, A. Cozzetti, Signal Shadowing in
    Simulation of Urban Vehicular Communications,
    IEEE-IARIA ICWMC 2010
  8. L.Pilosu, A.Cozzetti, R. Scopigno, Layered and
    Service-Dependent Security in CSMA/CA and Slotted
    Vanets, DSRC workshop at ICST QShine 2010
  9. 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 Hz Application 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-Aloha works also with obstructions (it solves
    HT)
  • Even without buildings (limiting propagation) and
    thresholds 224 slots can manage 600 nodes
  • Without building larger number of collisions and
    less free slots

16
17
MS-Aloha without Buildings Dynamic 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 Hz Application 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 (forced reuse)
    and lower number of collisions
  • worse PDR due to interferences by closer nodes
    reusing slots
  • At 2 sec Thr unloads FI (1st sim) and maps
    changes colour at the end
  • The two-hop span is shrinked (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 Hz Application 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
  • After initial transition MS-Aloha succeeds in
    receiving a higher number pf packets (centre)
  • Border effects make receive less packets
  • This effect is emphasized by time-space
    coordination of MSA

18
18
19
Agenda
  • Survey on solutions by international literature
  • The need for a comparative analysis
  • The state-of-art of MS-Aloha
  • Upcoming activities by ISMB

20
ISMB Upcoming Activities in the Field
  • New simulations
  • Improved realism in channel/mobility modelling
  • Comparative analysis to other protocols
  • Analysis of degradation depending on clock
    delivery
  • Additional open issues
  • Stronger compatibility with CSMA/CA
  • Workaround for missing synchronization
  • Pre-emption and service multiplexing
  • Nesting of upper-layer protocols (geo-routing)
  • Visualization Tool and extended comparative
    analyses
  • Extension of V_SuperCar to support outputs of
    other protocols
  • Possibly with other slotted protocols (S-TDMA,
    DTDMA)
  • Possible implementation
  • Possible framework
  • project (Italian Ministry of Research),
    currently supporting the research
  • proposal (submitted to FP7 call
    on Transport) and other pending regional
    initiatives

21
Thank you for your kind attention
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