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