## 4 Performance Evaluation
### Simulation Results
This is primarily due to intensified competition among ship stations for access to RAC slots used in allocating slots for VDE-TER transmission. The increased contention for RAC slots delays the transmission of request messages for slot allocation. This delay, in turn, increases the transmission waiting time for VDE-TER data packets,  
Figure 5: Aggregate throughput: (**a**) static ship scenario and (**b**) dynamic ship scenario. significantly impacting the average latency. The average latency is shorter when the RI is set to 6 compared to when it is set to 2. This is because fewer AIS message transmissions occur across the network with a larger RI, reducing delays in VDE-TER data transmission. In the figure, SOCA consistently demonstrates shorter average latency regardless of the RI. This is because SOCA reduces overlapping slots by selecting TDMA channels with sufficient non-interference slots to minimize collisions, thereby reducing the retransmission of AIS messages. However, the existing VDES randomly selects TDMA channels, which often results in insufficient available slots or the allocation of slots that are more likely to experience interference during VDE-TER data transmission. Accordingly, it leads to longer latency compared to SOCA. Quantitatively, when RI is 2 and 6, SOCA achieves 12.63% and 10.36% lower average latency, respectively, compared to the existing VDES. In Figure 6b, the average latency in the dynamic ship scenario is affected by the number of ship stations, similar to the static ship scenario. However, in the dynamic ship scenario, the average latency increases gradually and then rises sharply when the number of ship stations exceeds 20, compared to the static scenario. When the number of ship stations is  
Figure 6: Average latency: (**a**) static ship scenario and (**b**) dynamic ship scenario. 20 or fewer, the proportion of the slowest ship stations (i.e., those moving at 10 knots) is small, resulting in a negligible impact on VDE-TER data transmission delay. Specifically, a larger RI increases the transmission interval of AIS messages, thereby reducing the transmission opportunities available to ship stations. That is, the number of NTSs available for transmitting both AIS messages and VDE-TER data decreases. Conversely, as the number of the slowest ship stations increases, the average number of NTSs per ship station decreases, which lengthens the VDE-TER data transmission interval and significantly increases the average latency. Accordingly, the average latency in the dynamic ship scenario is longer than in the static ship scenario. Quantitatively, the average latency in the dynamic ship scenario is 9.39% and 54.99% higher than in the static ship scenario when the RI is set to 2 and 6, respectively. In addition, in the dynamic ship scenario, SOCA achieves a 11.99% higher average latency than that of the existing VDES. Figure 7a,b illustrate the variations in the collision ratio in static and dynamic ship scenarios, respectively. The collision ratio refers to the proportion of the average number of collided slots within a single frame (\(n_{avgColSlot}\)) to the total number of slots in that frame, and can be expressed as  
\[\text{Collision Ratio}=n_{avgColSlot}/2250 \tag{13}\]  
In Figure 7a, the collision ratio is affected by the number of ship stations since frequent AIS message transmissions can increase collisions with messages transmitted over TDMA channels. Accordingly, as the RI decreases, collisions increase due to the higher number of AIS messages transmitted in the network. As a result, the collision ratio is higher when the RI is 2 compared to when the RI is 6. In the figure, the collision ratio of SOCA is lower than that of the existing VDES because SOCA identifies non-interference slots based on the NIT and allocates slots for VDE-TER data transmission accordingly.