Intelligent Transport Systems (ITS); Performance Evaluation of Self-Organizing TDMA as Medium Access Control Method Applied to ITS; Access Layer Part (V1.1.1)
The present document describes the use of time slotted MAC algorithms in VANETs. Two specific MAC methods@ self-organizing time division multiple access (STDMA) and mobile slotted Aloha (MS-Aloha)@ are described in detail@ not excluding other time slotted approaches. Time slotted approaches are suitable for road traffic safety applications as the maximum delay is predictable and channel access can be made fair among all participating nodes even during broadcast. However@ time slotted approaches do require synchronization between nodes to build a common framing structure for transmissions@ something that is not needed for non-time slotted approaches@ e.g. CSMA as used by ITS G5 [i.1]. In the literature of time slotted MAC protocols for VANETs@ synchronization is provided by a global navigation satellite system (GNSS) such as the global positioning system (GPS) or Galileo. The present document also describes the GNSS synchronization issue as well as proposals for dealing with synchronization when the GNSS signal is absent or weak@ which can occur in urban environments and tunnels. Further@ time slotted approaches use fixed-length time slots for transmissions@ implying that packet lengths are fixed. However@ as the physical (PHY) layer suggested for VANETs offers several transfer rates@ this means that different packet sizes can be obtained in the fixed time slots. The analysis of the most preferable configuration in this context constitutes the second technical topic covered by the present document. Finally the present document also deals with the coexistence between CSMA and time slotted MAC approaches nodes. The backward compatibility and coexistence are of crucial importance since the first generation of VANETs will use CSMA technology. This represents the third and final topic of the present document.