# Empirical Performance Models of MAC Protocols for Cooperative Platooning Applications
## 2 Surveying TDMA-Based V2V Communications
### TDMA-Based MAC Protocols  
**Direct TDMA Protocols:** This class of protocols implements TDMA directly over the physical layer with the TDMA layer controlling the exact transmission instants. Examples of this class include the following protocols. VeMAC [10] is a contention-free multi-channel protocol for vehicular networks that assigns disjoint sets of time slots to vehicles moving in opposite directions (Left and Right) and to a road-side unit (RSU). Each vehicle is assumed to have two transceivers: One tuned to the control channel (CCH), and another tuned to any service channel (SCH). If a moving vehicle detects that it cannot access a time slot of its set, it will attempt to access any available time slot reserved for vehicles moving in the opposite direction. Dedicated multi-channel MAC protocol (DMMAC) [11] supports  
Figure 1: Classification of time-division multiple access (TDMA)-based medium access control (MAC) protocols in vehicular ad-hoc networks (VANETs). an adaptive broadcasting mechanism to provide collision-free and delay-bounded transmissions. The DMMAC operation is similar to IEEE 802.11p, with the difference that the CCH interval is divided into two time periods: Slots for adaptive broadcast frames (ABF) and a contention-based reservation period (CRP). ABFs are sent at reserved time slots for collision-free delivery of safety messages, while during the CRP, vehicles reserve resources on service channels for non-safety applications. DMMAC implements a dynamic TDMA mechanism for basic channel (BCH) reservation based on the distributed access technique R-ALOHA. Vehicles Self-Organizing MAC protocol (VeSOMAC) [12] uses an in-band signaling scheme that carries information about allocated slots and allows fast slot reconfiguration following topology changes (e.g., platoon merging). It aims at fast TDMA slot reconfiguration without relying on roadside infrastructure or leader vehicles. VeSOMAC operates in both synchronous and asynchronous manner. In the former, all the vehicles are assumed to be time-synchronized by using GPS where they share the same frame and slot boundaries. In the latter, each vehicle maintains its own frame boundaries. STDMA [13], for self-organizing TDMA, is a decentralized TDMA scheme that uses periodic frames (super frames) further divided into time slots. When a vehicle joins the network, it first listens to the channel to get information from other vehicles positions and identifies the super frame. The joining vehicle then introduces itself to the network by determining the first transmission slot available; if all slots are occupied the vehicle will reuse the slot of the farthest away vehicle. After that, the vehicle transmits periodically in the slot it selected earlier. **Cluster-Based TDMA Protocols:** In this family of protocols, vehicles are clusters by physical proximity and relative position. A cluster head sets up the TDMA frame for that cluster and generally synchronizes the other vehicles in the group. In CBMAC [14], the medium is divided into multiple CCH and one data channel (DCH). Access to the CCH channels is based on CSMA/CA while the DCH channel uses TDMA to guarantee low transmission delay and collision-free transmissions within each cluster. To form clusters, all the vehicles tune to the CCH and elect one cluster head (CH). Each cluster member sends its position and speed to its CH periodically in its own TDMA time slot. To avoid inter-cluster interference, each CH selects a different orthogonal code from that of its neighbouring CHs. CBMMAC [15] combines contention-free and contention-based MAC protocols. CBMMAC deploys three main protocols: cluster configuration, intra-cluster and inter-cluster coordination communication, to avoid merging collisions and inter-cluster interference problems.