# Empirical Performance Models of MAC Protocols for Cooperative Platooning Applications
## 2 Surveying TDMA-Based V2V Communications
### TDMA-Based MAC Protocols
To this end, it redefines the functions of the seven DSRC channels: CH178 to the inter-cluster control (ICC) channel, CH174 the inter-cluster data (ICD) channel, CH172 is the cluster range control (CRC) channel, and the remaining channels (Ch176, 180, 182 and 184) are the cluster range data (CRD) channels. The CBT protocol [16] aims to develop contention-free intra-cluster and inter-cluster communications while minimizing collisions when two or more clusters are approaching each other. The protocol uses a simple transmit-and-listen scheme to quickly elect a vehicular-network coordinator (VC). The access time is divided into frames and each frame consists of \(n\) time slots, under the assumption that all vehicles are equipped with a GPS positioning system to support synchronization. **Overlay TDMA Protocols:** This group of protocols implements TDMA on top of the CSMA/CA layer of IEEE 802.11p. Thus, they can be readily implemented on commercial off-the-shelf (COTS) IEEE 802.11p interfaces and are tolerant to uncoordinated (non-complying) traffic. However, note that the transmission instants defined by the TDMA layer can be modified by the layer below. The fully distributed and infrastructure-free TDMA-based MAC protocol [17] (DTMAC) is based on VeMAC protocol and aims to alleviate its scalability limitation by allowing spatial reuse, i.e., simultaneous transmission in different geographical areas. The channel time is partitioned into frames, and each frame is further partitioned into the Left and Right sets. The road is dissected into small fixed areas in which the time slots can be reused between them simultaneously and without interference. The vehicles use their location to access the channel simultaneously and without collisions. PLEXE-slotted [6] divides a TDMA round into equal intervals among the number of vehicles in the platoon. Transmissions in the platoon take place in a cycle starting by the leader, which transmits with high power to synchronize all platoon members, followed by all the followers from first to last and transmittingwith reduced power. The followers just reach the neighbouring followers and propagate information down the platoon in a multi-hop fashion. A shortcoming of this protocol is the lack of support to handle extra-platoon periodic communications: If nearby platoons or vehicles transmit with similar period and in phase, this can lead to frequent collisions until clock drifts separate their transmissions in time. RA-TDMAp [7] is similar to PLEXE-slotted, as it also divides the TDMA round period in equal intervals among the number of vehicles in the platoon. However, it addresses the limitation of the PLEXE-slotted method (of co-existence with extra-platoon periodic transmissions) by shifting the phase of the TDMA frame cycle (or round) of the current platoon with respect to the phase of other platoons or vehicles. In RA-TDMAp, the followers transmissions start from the last (farthest from leader) to the first, and each follower piggybacks the delays observed during the current round to this communication flow. This allows the leader to delay the start of the next round by a value that accommodates external traffic and thus escapes, with high probability, the interference from other platoons or vehicles. When used over a set of neighbouring platoons, RA-TDMAp provides an implicit synchronization mechanism, by which platoons automatically adjust their cycle phase to avoid or reduce mutual interference.