With the rapid development of wireless technologies, the Vehicular Ad Hoc Networks (VANETs) have recently received much attention. VANETs technologies aim to ensure traffic safety for drivers, provide comfort for passengers and reduce transportation time and fuel consumption with many potential applications. The achievement of these aims highly relies on efficient MAC protocols which determine the performance of packet transmission in terms of success rate, delay, throughput and bandwidth utilization. This chapter reviews the existing MAC protocols developed for VANETs. Initially, the IEEE 802.11p and DSRC standard are reviewed. Three TDMA-based MAC protocols, called CVIA, VeSOMAC and D*S, are then introduced. In addition, three MAC protocols that cope with the emergency-message broadcasting problem are proposed. Finally, a reliable MAC protocol which is developed based on the cluster topology is reviewed.
Top12.2 Ieee 802.11P And Dsrc Standards
DSRC (Dedicated Short Range Communications) (ASTM International E2213-03, 2003) is a well known standard supports both Public Safety and Private operations in roadside to vehicle and vehicle to vehicle communication environments. DSRC standard at 5.9 GHz band is projected to support low-latency wireless data communications between vehicles and from vehicles to roadside units. The DSRC specification is meant to be an extension of the IEEE 802.11 technology into the outdoor high-speed vehicle environment. In fact, the Physical Layer (PHY) of DSRC is adapted from IEEE 802.11a PHY based on Orthogonal Frequency Division Multiplex (OFDM) technology. Moreover, the Multiple Access Control (MAC) layer of DSRC is very similar to the IEEE 802.11 MAC based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol with some minor modifications.
DSRC is meant to be a complement to cellular communications by providing very high data transfer rates in circumstances where minimizing latency in the communication link and isolating relatively small communication zones are important. DSRC protocol layer is developed based on physical, data link and applications layers of traditional OSI model. In the Physical layer, DSRC defines physical parameters for uplink and downlink communication and is mainly working in the 5.9 GHz band (U.S.) or 5.8 GHz band (Japan, Europe). In the data link layer, DSRC defines frame format, frame wrapper and the procedures of MAC and Logical Link Control (LLC). Application layer includes the fragmentation and defragmentation of data application service and service primitive for a variety of applications, including electronic toll collection, emergency warning system, vehicle safety service, commerce transactions via cars, electronic parking payments, probe data collection and so forth.
The standard of DSRC is comprised of IEEE 802.11p and IEEE 1609 family. Figure 1 shows the correspondence between DSRC and OSI models. IEEE 802.11p (Draft 7.0, 2009) is a draft amendment to the IEEE 802.11 standard to add Wireless Access in the Vehicular Environment (WAVE). As an extension of IEEE 802.11, IEEE 802.11p defines how data exchange between high-speed vehicles and between the vehicles and RSU in 5.9 GHz (5.85-5.925 GHz) band. IEEE 1609 is a higher layer standard based on IEEE 802.11p. The IEEE 1609 family is consisted of IEEE 1609.1 (2006), IEEE 1609.2 (2006), IEEE 1609.3 (2007) as well as IEEE 1609.4 (2006).
Figure 1. Protocol stack relating to OSI model
IEEE 1609.1 plays the role of WAVE Resource Manager which specifies a DSRC application overlying WAVE and allows remote site applications to communicate with OBUs or RSUs. As a standard of application layer, IEEE 1609.1 conducts application-level information interchanges.
IEEE 1609.2 supports WAVE security services for applications. It defines secure message formats, specifies methods for securing WAVE management messages and application messages and processes secure messages of DSRC/WAVE systems. In addition, it handles exception of vehicle-originating safety messages and provides administrative functions necessary for core security functions.