Title: Istituto Superiore Mario Boella
1Istituto Superiore Mario Boella
Slotted VANET One Year After
R. Scopigno, BWA Lab scopigno_at_ismb.it
2Agenda
- Survey on solutions by international literature
- The need for a comparative analysis
- The state-of-art of MS-Aloha
- Upcoming activities by ISMB
3Agenda
- Survey on solutions by international literature
- The need for a comparative analysis
- The state-of-art of MS-Aloha
- Upcoming activities by ISMB
4Slotted 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
5Some 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
6International 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
7Agenda
- Survey on solutions by international literature
- The need for a comparative analysis
- The state-of-art of MS-Aloha
- Upcoming activities by ISMB
8Comparative 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
9VANET 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
10VANET 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
11Agenda
- Survey on solutions by international literature
- The need for a comparative analysis
- The state-of-art of MS-Aloha
- Upcoming activities by ISMB
12MS-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
13MS-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
14MS-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)
15MS-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
16MS-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
17MS-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
18MS-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
19Agenda
- Survey on solutions by international literature
- The need for a comparative analysis
- The state-of-art of MS-Aloha
- Upcoming activities by ISMB
20ISMB 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
21Thank you for your kind attention