QoS-aware AFDX: benefits of an efficient priority assignment for avionics flows

AFDX (Avionics Full Duplex Switched Ethernet) standardised as ARINC 664 is a major upgrade for avionics systems. The certification imposes to guarantee that the end-toend delay of any frame transmitted on the network is upperbounded and that no frame is lost due to buffer overflow. This guarantee is obtained thanks to a worst-case analysis assuming a FIFO scheduling policy of flows on each output port. For future aircraft, it is envisioned to modify AFDX switch and to use a Fixed Priority scheduling policy of flows (QoS AFDX using IEEE 802.p mechanisms). A worst-case analysis of such a network has been proposed, based on the Trajectory approach. But the remaining issue is to efficiently assign available priorities to the avionics flows inside the network without modifying the application knowledge. The objective is then to minimise overall the worst case end to end delay of flows and consequently to minimise needed buffer size at switch level. The main contribution of this paper deals with the assignment of priorities to the flows using the well-know Optimal Priority Assignment algorithm (OPA) which was first defined for monoprocessor preemptive systems. The schedulability test is then based on the worst case delay analysis of each flow allocated on the AFDX QoS network computed by the trajectory approach. The proposed mechanisms have been applied on an industrial AFDX case study using two priority levels and the overall worstcase delay could be reduced by 20 %.