## 2 Related Works  
In a TDMA-based VANET protocol, each vehicle can occupy more than one time-slot per frame; as such, it provides the following advantages over the competition-based IEEE 802.11p standard [3]. * Equal access to the channel for all vehicle nodes. * Efficient channel utilization without collisions. * High reliability of communications. * Deterministic access time even for high traffic loads. * QoS for real-time applications. In VANET environments, the network topology changes frequently because vehicles are moving at various speeds. Therefore, a variety of distributed TDMA-based MAC protocols have been proposed considering the advantages of TDMA and the characteristics of VANET. AD Hoc Medium Access Control (ADHOC MAC) is a basic distributed TDMA-based MAC protocol designed to provide efficient and reliable data transmission in VANET environments [7]. As shown in Figure 3, each vehicle periodically generates and broadcasts an FI (frame information) message including information about its own 1-hop neighbors and its own time-slot allocation. Because a vehicle receiving the FI message recognizes the neighbors of the vehicle broadcasting the FI message, all vehicles can obtain information about the existence and resource allocation status of 2-hop neighbor vehicles. A new vehicle entering the network listens to the channels of each frame to obtain the unused time-slot information and selects an available time-slot for transmitting data in the next frame based on this information. If the vehicle successfully secures the time-slot, it will continue to use the same time-slot until a collision occurs or the connection is released. However, ADHOC MAC has disadvantages in that it does not take into consideration the moving direction and speed of the vehicle. Figure 2: Merging collision. Vehicular Ad Hoc Network MAC (VeMAC) is a distributed TDMA-based VANET protocol that improves on the disadvantages of ADHOC MAC by considering the moving direction of a vehicle [8]. Each vehicle, like the ADHOC MAC, broadcasts an FI message to its 1-hop neighbor vehicles. All vehicles can obtain information about the presence and resource allocation status of 2-hop neighbor vehicles. In VeMAC, each frame is evenly divided considering the direction of the vehicles. As shown in Figure 4, a frame is divided into two sub-frames. A vehicle traveling to the left occupies the left direction time-slot, and a vehicle traveling to the right occupies the right time-slot. Each vehicle may occupy a time-slot in other directions if there is no remaining time-slot. Because used time-slots are classified according to the moving direction of the vehicle, this results in lowering the merging collision rate. On the other hand, VeMAC did not take into account the relative speed difference between the vehicles; therefore, merging and access collisions may occur when there are many vehicles moving at different speeds. The collision-free reservation-based MAC (CFR MAC) is a distributed TDMA protocol that improves on VeMAC by considering the relative speed and traffic flow of each vehicle [9]. Like VeMAC, the resource allocation of each vehicle is divided into two sub-frames and resources are allocated to the vehicles according to their moving directions (left and right). As we just discussed, if vehicles moving in the same direction have different speeds, a merging collision problem may occur. To solve this problem in CFR MAC, the left/right sub-frame of each frame is subdivided into three levels (high, middle, and low), and resources are allocated accordingly, as shown in Figure 5. Each vehicle calculates its relative speed from the difference between its own speed and the average speed of neighbor vehicles. If the relative speed is lower than the standard deviation speed, it is set to have a \"low\" phase. If it is higher than the standard deviation, it is set to have a \"high\" phase. The rest of the vehicles are determined to have a \"middle\" phase.