Peculiarities of the vehicular environment make the design of the Physical (PHY) and Medium Access Control (MAC) layers for Vehicular Ad-hoc Networks (VANETs) very challenging. Technical solutions should carefully cope with (i) quickly changing network topologies caused by vehicles mobility, (ii) short connection lifetimes, (iii) multi-hop vehicle-to-vehicle communications, (iv) hostile environments for radio signal propagation, and (v) heterogeneous nature and quality requirements of various types of applications. The main aim of this chapter is to serve as an introduction for readers interested in vehicular network design, with a special focus on the MAC layer. It includes a detailed description of the major features and operating principles provided by PHY and MAC layers of the IEEE 802.11p and IEEE 1609 standard suites to support Wireless Access in Vehicular Environments (WAVE). The last part of the chapter contains a brief survey of some relevant MAC proposals in the scientific literature that try to cope with the challenges of vehicular networks. Most of them follow the contention-based channel access idea of the standard and propose extensions to the 802.11p MAC layer in order to achieve higher throughput and fairness; others capitalize on a centralized access to achieve deterministic service quality.
An Empirical study on MAC layer in IEEE 802.11p/WAVE based Vehicular Ad hoc Networks