## 3 MAC Classification in VANETs Based on Contention Mechanism  
Traditional classification of the MAC protocol in the literature [38; 39] includes contention-based, contention-free, and hybrid protocols. Similarly, in this section, we studied some traditional MAC protocols in VANETs and classified them as contention-based, contention-free, and hybrid. This section ended by stating some of the recent advancement in vehicular communication, giving future directions. **Contention-based protocols**: Commonly referred to as \"listen before talk\". The nodes perceive the channel first; if it is free, it transmits. The system contends for channel access; the node that wins transmits through the channel. It is primarily used in a sparse network scenario so that the bandwidth can be utilized efficiently. These types of protocols cannot be used in real-time applications due to the time-bound and reduced throughput caused by collision. IEEE 802.11p and CSMA-based protocols are examples of this type of protocol. **Contention-free protocols**: The nodes need not compete to gain channel access, and the time period of transmission in the channel is pre-allocated. Collision is avoided during data transmission in the channel. The transmission frames are divided into slots, and the nodes are synchronized for channel access. Although this type of protocol provides quality of service (QoS), it suffers from improper bandwidth utilization due to explicit time slot allocation. The main challenge is the proper allocation of channel resources among nodes. TDMA, FDMA, and SDMA are examples of this type of protocol. **Hybrid protocols**: It merges the advantages of the contention-free and contention-based protocols to achieve QoS and to enhance network performance. It is primarily applicable for safety applications as there are minor delays and improved throughput. CSMA with TDMA [40], Token-Ring passing, and clustering-based protocols are some examples of this type of protocol. ALOHA is one of the introductory MAC protocols [41] for radio packet networks. In ALOHA, there is throughput reduction for which S-ALOHA slotted ALOHA [42] was proposed, which splits the medium into various time slots and strives to transmit at the beginning of the time slot. Multiple access with collision avoidance (MACA) [43] was proposed to overcome the hidden terminal problem using the handshaking method via RTS/CTS communication. The MACA for wireless (MACAW) [44] adds functionality to MACA to make it more robust when it comes to detecting collisions in WLAN data transmission. It requires nodes to send acknowledgements after every successful frame transmission. Similarly, BTMA [45] was proposed to overcome the hidden node problem by splitting the channel into data channels and a control channel. MC-MAC [46] uses two codes, one for control packet transmission and the other for data packet transmission. ADHOC-MAC [47] uses a slotted frame and dynamic TDMA mechanism; R-ALOHA [48] was designed for dynamic TDMA in a distributed way. The reliable R-Aloha (RR-Aloha) [49] architecture was designed similar to ADHOC-MAC, which uses distributed MAC and UTRA-DD (UMIS terrestrial radio access time division duplex) for physical channel access with the single-hop broadcast. Directional antenna-based MAC [50] is yet another development in past years that uses the GPS of the terminals and restricts transmissions to a geographical area. WAVE (802.11p) [51] mechanisms have no predetermined schedule, and channel access is random for vehicles, resulting in transmission collisions in a dense network scenario. 802.11p uses CSMA/CA; safety-critical applications cannot be guaranteed QoS due to its contention-based nature. Contention-based MAC protocols are designed to increase scalability under heavy loads by considering parameters such as the physical carrier sense threshold, contention window, and the transmission power control. WAVE employs the use of GPS to synchronize DSRC radios installed on all vehicles.