# A Preemptive-Resume Priority MAC Protocol for Efficient BSM Transmission in UAV-Assisted VANETs
## 3 System Model
### The MAC Layer Communication Protocol  
In densely populated traffic areas, such as congested urban roads or highways, vehicles often accumulate a substantial volume of safety information. As per the J2735 standard established by the Society of Automotive Engineers (SAE) in the United States [9], the increased number of vehicles leads to a significant rise in the frequency of BSMs received by UAVs per second. Consequently, important safety information may be discarded due to large transmission intervals, resulting in delayed or inadequate updates that compromise timeliness. This situation poses significant safety hazards. To mitigate such concerns, we have implemented a prioritization scheme for vehicles and their messages, enabling the prioritized validation and transmission of critical safety information. The subsequent section outlines the design of our MAC layer protocol. In this study, due to vehicles entering the UAV communication range as a Poisson process and each vehicle transmitting fixed-sized data packets of \(\beta\) bits at any given time, we adopt an M/G/1 queue model. Taking into account the priority categories, we assume there are r priority levels. Vehicles and their transmitted data packets of priority level i arrive according to a Poisson process with arrival rate \(\lambda_{i}\), with specific values obtained from Section 3.1. It is worth noting that the adopted priority mechanism in this study is Preemptive-Resume Priority [46], meaning that high-priority customers interrupt the service of low-priority customers. Upon interruption, a checkpoint is set, and the interrupted data enter the Shunts for storage. The service time of the lower-priority customer resumes at the checkpoint. For a customer of priority level i, due to the preemption rule, there are no lower-priority customers appearing ahead of them in their queue, i.e., \(\lambda_{i+1}=\ldots\ldots=\lambda_{\rm{r}}=0\). The following provides a detailed explanation of the priority setting. As shown in Figure 3, all devices are located inside the UAV for operation. The frame-header Checker (referred to as Checker in the diagram) is responsible for checking the header of the message packets received from the vehicles on the uplink, determining whether to accept them and dividing them into segments based on the vehicle's speed (included in the packet header). Buffer-A, Buffer-B, and Buffer-C represent three buffers that receive data packets from different priority levels and wait for instructions from the Arbiter. The Arbiter is responsible for scheduling the transmission order on the output link and accessing the outgoing packets. It has Shunts B and Shunts C buffers that store the remaining data packets of interrupted low and medium priority levels due to the Preemptive-Resume Priority rule. The information enters the frame-header Checker from vehicles on the Up Stream and the message priority is determined. The information transmitted by vehicles in the high-speed section enters Buffer-A as high-priority, while that transmitted by vehicles in the medium-speed section enters Buffer-B as medium-priority and that transmitted by vehicles in the low-speed section enters buffer Buffer-C as low-priority. The Arbiter is responsible for scheduling the transmission order on the output link and accessing the outgoing packets. If a medium-priority packet is interrupted by a high-priority packet, a breakpoint is set, and the remaining packets are transferred to Shunts B. If a low-priority packet is interrupted by a high- or medium-priority packet, a breakpoint is set and the remaining packets are transferred to Shunts C. We assume that the time it takes for packets to be transmitted to the buffer is negligible [47]. The protocol defines the mechanism for bandwidth allocation and packet scheduling. Assuming that the number of packets is negligible and that each buffer has an infinite capacity, the Arbiter can simply schedule the packets based on the following priority order (from highest to lowest).