VANETs from Theory to Practice (VTP 2012) San Francisco, California, USA June 25th , 2012

1. TDMA Cluster-based MAC for
VANETs (TC-MAC)
Mohammad S. Almalag, Stephan Olariu and Michele C. Weigle
Old Dominion University
Department of Computer Science
Norfolk, Virginia, USA
VANETs from Theory to Practice (VTP 2012)
San Francisco, California, USA
June 25th , 2012

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Introduction
• Challenges for VANETs:
Design of the Medium Access Control (MAC) protocol
• Exchanging messages between vehicles:
Hidden terminal problem, high density, high node mobility, and
data rate limitations
• A cluster-based MAC scheme is needed in VANETs:
To support network stability and channel utilization

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Introduction
Dedicated Short Range Communication (DSRC) channels:

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Introduction
IEEE 1609.4 standard:

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Motivation
• Safety message requirements
Non-safety Application Share
▫ High reliability
▫ Every 100 msec
Non-Safety messages
almost shut down
[Wang, ACM VANET 2008]
Low Medium High
Traffic Density
• Allow vehicles to efficiently exchange both safety and non-
safety messages while meeting safety message requirements

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Architecture
A MAC algorithm for VANETs using a new method for TDMA slot
reservation based on clustering of vehicles.
▫ Decrease collisions and packet drops in the channel
▫ Provide fairness in sharing the wireless medium
▫ Minimize the effect of hidden terminals

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Architecture
Time Division Multiple Access (TDMA) is a method used to enable
multiple nodes to transmit on the same frequency channel.

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Architecture
In our architecture “clustering”:
▫ Clusterhead(CH)
▫ Single-hop cluster
▫ Cluster member (CM)
▫ Relay node (RN)

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Architecture
TDMA Slot Reservation in TC-MAC:
▫ Assumptions:
 One transceiver on-board
 vehicles are equipped with GPS
 1 CCH (cch)
 6 SCHs (k)
 N vehicles
 τ is the slot size in SCHs

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Architecture
• Each vehicle in the cluster will receive a local ID (from 0
to N).
• The CH will have always ID 1.
• ID 0 is reserved for a “virtual vehicle”.

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Architecture-Safety messages/beacons
• On the CCH, vehicle j owns:
• Mini-slot
• On slot

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Architecture-non-safety messages
• On the SCHs, vehicle j owns:
• SCH
• Slot

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Architecture SCH: (39 mod 6) = 3 During slot
And
• Example: CCH: 4th mini-slot in slot 6-1=5
▫ Number of vehicles N = 61, k = 6, vehicle j’s ID= 39
39
cch [36
(mini-slot) -41]

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Architecture
Disseminating intra-cluster safety /governance
messages:
▫ Emergency messages:
 Disseminated by CH.
 N-bit vector for subset
▫ Cluster control messages:
 Disseminated by CH

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Architecture
Disseminating intra-cluster non-safety messages:
▫ Unicast:
 Set up without CH intervention.
 Using the local ID for channel and slot time.

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Simulation and Evaluation
Parameters Values for DSRC Values for TDMA
Cluster length 300 m 300 m
TX Range 300 m 300 m
Safety Packet Size 200 bytes 200 bytes
Data Rate 6 Mbps 6 Mbps
CCHI 50 msec N/A
SCHI 50 msec N/A
Mini Slot Size N/A 0.254 msec
SCH Slot Size N/A 1.525 msec
Speed Limit 29 m/sec 29 m/sec

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Result – without CH re-broadcast

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Result – with CH re-broadcast

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Conclusion
• We presented TC-MAC as a cluster-based TDMA
scheduling protocol for VANETs.
• We also explained a light weight slot reservation
algorithm.
• The simulation results show that TC-MAC is able
to efficiently and reliably deliver safety
messages.
• TC-MAC gives vehicles a chance to send and
receive non-safety messages at any time.

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Questions?
TDMA Cluster-based MAC for VANETs (TC-MAC)
Mohammad S. Almalag, Stephan Olariu and Michele C. Weigle
malmalag@cs.odu.edu
Old Dominion University
Department of Computer Science
Norfolk, Virginia, USA
Please visit the Intelligent Networking and Systems (iNetS) Research Group in the Department of
Computer Science at Old Dominion University: http://www.cs.odu.edu/~inets/