Performance analysis of the Disrupted Static Priority scheduling for AFDX

The AFDX technology is used as backbone in several aircraft. It offers a high bandwidth (commonly 100Mb/s), and guaranteed per data flow a bound on the network traversal delay, while being a shared resource. To do so, it uses a segregation mechanism, the Virtual Link (VL), designed as a bus abstraction. Nevertheless, there may exist applications with such stringent latency requirements that a given AFDX topology can not ensure it. To provide low latency, the AFDX technology provides priority levels: but even in the highest priority level, the latency bound can still be too high with regard to applicative requirements. One cause of latency is the non-preemptive aspect of priority policy: when a high priority frame reaches an output port, it may have to wait the emission of a low priority frame. Then, a high priority frame might have to face a delay equal to the maximum packet emission time of the low priority flows per crossed switch. The scheduling policy called Disrupted Static Priority (DSP) aims to improve the latency guarantees, by interrupting low priority frames, and re-emitting them from the beginning. This policy is a trade-off between simplicity and efficiency. A quantitative comparison between the NP-SP and the DSP scheduling systems´ performance on a realistic case-study is provided in this article.