## 3 Communication Strategies in Multicore Systems
### LET in Multicore Systems
* Completion of Execution: The task must complete its execution within the assigned LET period. If the task finishes before the LET period expires, it remains suspended until its next activation period. At the end of the LET, the output data are written to shared memory, ensuring data consistency and predictability throughout the system. Once the current LET ends, the next LET period begins, either for the same task or for a group of tasks in the system. Figure 3 shows a visual representation to understand the operational dynamics of the LET model. While the LET model enhances predictability and data consistency in multicore real-time systems, it has some limitations, as it was initially designed for classical single-core real-time systems. The model does not account for the parallel execution of multiple tasks across different cores. Each core operates with an independent scheduler, resulting in read/write operations with different timings across cores, even when operations are fixed at the core task level. Figure 4 illustrates an example of various executions of different tasks allocated to different cores using Simulink's SoC Blockset. This scenario was modeled for the producer task to start periodic execution every 10 ms at \(t=0\). The consumer periodic task was set to have a rate of 10 ms with a variable start time to emulate variable initialization times. Although the cycle time was fixed, the offset between the tasks on each core varied on each simulation run. These tasks included producer-consumer pairs, a simulation of real-time data acquisition tasks, and synchronized transmission tasks. TDMA in Multicore Systems  
Time-Division Multiple Access (TDMA) is a time-slot method widely used in communications and embedded systems. According to [30], TDMA is a time-triggered communication technology where the time axis is statically divided into slots, with each slot statically assigned to a component. A component is permitted to send a message only during its designated slot. Figure 5 illustrates a simple example of a single channel shared among four transmitter entities and four receiver entities to exchange data within a given time period \(T\). The channel can be any transmission medium, such as a memory cell or a carrier signal. Figure 4: Simulation of eight executions with varying latencies between read and write operations. This figure demonstrates the impact of different execution cycle starts for the receiver task on the second core. Despite having a fixed cycle time, the offset between tasks on each core varies, highlighting the challenges in achieving consistent timing across multiple cores. Figure 3: Visual representation of the operational dynamics of each task under the LET framework. The figure illustrates the predefined time frames within which tasks are executed. Each task follows a cycle of running, suspension/preemption, and termination, with specific periods for waiting. This structure ensures consistent and predictable execution, enhancing system predictability and reliability. This concept allows data-producing entities to utilize the full channel capacity to transmit information during a defined time interval, while consumer entities can access this information within the same timeframe. In a channel, information is only available during a specific period, necessitating time synchronization to ensure the desired data are transmitted to the intended recipients. This method is also resource-efficient, as multiple producers and consumers can share a single channel. In embedded systems, resources are limited and time constraints exist; therefore, TDMA has been widely adopted to address scheduling and resource management challenges. With this setup, emitters can transmit data at a fixed rate, while receivers can read data within the same time slots, facilitating communication synchronization and optimizing channel capacity by allowing multiple transmitters to use the same channel through time-slot multiplexing. TDMA has been proposed as a solution in several studies. For instance, Ref.