One of the challenges for Vehicular Ad-hoc Networks (VANETs) is the design of the Medium Access Control (MAC) protocol.When exchanging messages between vehicles, there are network issues that must be addressed, including the hidden terminal problem, high density, high node mobility, and data rate limitations.A cluster-based MAC scheme is needed in VANETs to overcome the lack of specialized hardware for infrastructure and the mobility to support network stability and channel utilization.
-In the US, VANETs use 75 MHz of spectrum in the 5.850 to 5.925 GHz band specially allocated by the U.S. Federal Communications Commission for Vehicle-to-Vehicle communication (V2V) and Vehicle-to-Infrastructure communication (V2I) using Dedicated Short Range Communication (DSRC) technology .
Since safety applications of vehicular communicationhave stringent reliability and delay requirements, giving eachvehicle the time to send safety messages without interferingwith other vehicles is required. Also, safety messages arebased on broadcast transmission, so, using the IEEE 802.11RTS/CTS mechanism for collision avoidance is not feasiblein VANETs.Safety messages need to be sent every 100 msec.In high density, CCH interval will be higher than the SCH interval. So, non-safety mesg will be almost stopped.Traffic density: Low = 40Medium = 80High = 160
It divides the signal into different time frames.Each frame is divided into several time slots, where each node is assigned to a time slot to transmit.The length of the time slot may vary, based on the needs of the node assigned to it.The main advantage of TDMA is reducing interference between nodes.
TDMA Cluster-based MAC forVANETs (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
2 June 25th 2012Introduction• 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
3 June 25th 2012IntroductionDedicated Short Range Communication (DSRC) channels:
4 June 25th 2012IntroductionIEEE 1609.4 standard:
5 June 25th 2012Motivation• 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
6 June 25th 2012ArchitectureA MAC algorithm for VANETs using a new method for TDMA slotreservation 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
7 June 25th 2012ArchitectureTime Division Multiple Access (TDMA) is a method used to enable multiple nodes to transmit on the same frequency channel.
8 June 25th 2012Architecture In our architecture “clustering”: ▫ Clusterhead(CH) ▫ Single-hop cluster ▫ Cluster member (CM) ▫ Relay node (RN)
9 June 25th 2012ArchitectureTDMA 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
10 June 25th 2012Architecture• 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”.
11 June 25th 2012Architecture-Safety messages/beacons• On the CCH, vehicle j owns: • Mini-slot • On slot
12 June 25th 2012Architecture-non-safety messages• On the SCHs, vehicle j owns: • SCH • Slot
13 June 25th 2012Architecture 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 39cch [36(mini-slot) -41]
14 June 25th 2012ArchitectureDisseminating intra-cluster safety /governancemessages: ▫ Emergency messages: Disseminated by CH. N-bit vector for subset ▫ Cluster control messages: Disseminated by CH
15 June 25th 2012ArchitectureDisseminating intra-cluster non-safety messages: ▫ Unicast: Set up without CH intervention. Using the local ID for channel and slot time.
16 June 25th 2012Simulation and EvaluationParameters Values for DSRC Values for TDMACluster length 300 m 300 mTX Range 300 m 300 mSafety Packet Size 200 bytes 200 bytesData Rate 6 Mbps 6 MbpsCCHI 50 msec N/ASCHI 50 msec N/AMini Slot Size N/A 0.254 msecSCH Slot Size N/A 1.525 msecSpeed Limit 29 m/sec 29 m/sec
17 June 25th 2012Result – without CH re-broadcast
18 June 25th 2012Result – with CH re-broadcast
19 June 25th 2012Conclusion• 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.
20 June 25th 2012Questions? TDMA Cluster-based MAC for VANETs (TC-MAC) Mohammad S. Almalag, Stephan Olariu and Michele C. Weigle email@example.com Old Dominion University Department of Computer Science Norfolk, Virginia, USAPlease visit the Intelligent Networking and Systems (iNetS) Research Group in the Department ofComputer Science at Old Dominion University: http://www.cs.odu.edu/~inets/