Methods and Tools for the Temporal Analysis of Avionic Networks

Abstract

Thanks to the Integrated Modular Avionics concept [ARI (1991; 1997)], functions developed for civilian aircraft share computation resources. However, the continual growing number of these functions implies a huge increase in the quantity of data exchanged and thus in the number of connections between functions. Consequently, traditional ARINC 429 buses [ARI (2001)] can’t cope with the communication needs of modern aircraft. Indeed, ARINC 429 is a single-emitter bus with limited bandwidth and a huge number of buses would be required. Clearly, this is unacceptable in terms of weight and complexity. In order to cope with this problem, the AFDX (Avionics Full DupleX Switched Ethernet) [ARI (2002-2005)] was defined and has become the reference communication technology in the context of avionics. AFDX is a full duplex switched Ethernet network to which new mechanisms have been added in order to guarantee the determinism of avionic communications. This determinism has to be proved for certification reasons and an important challenge is to demonstrate that an upper bound can be determined for end-toend communication delays. An important assumption is that all the avionics communication needs can be statically described: asynchronous multicast communication flows are identified and quantified. All these flows can be statically mapped on the network of AFDX switches. For a given flow, the end-to-end communication delay of a frame can be described as the sum of transmission delays on links and latencies in switches. Thanks to full duplex links characteristics, no collision can occur on links and transmission delays on links depend solely on bandwidth and frame length. But, as confluent asynchronous flows compete, on each switch output port, highly variable latencies can occur when a frame crosses a switch. Thus it is necessary to analyze these latencies in order to determine the upper bounds on end-to-end communication delays for each flow. At least three approaches have been proposed in order to compute a worst-case bound for each communication flow of the avionic applications on an AFDX network configuration. They are based on network calculus, trajectories and model checking. Such a worst-case communication delay analysis allows the comparison between the computed upper bounds and the constraints on the communication delays of each flow. Moreover it allows the scaling of the switches memory buffers in order to avoid buffer overflow and frame losses. However, communication delays measured on a real configuration are much lower than the computed upper bound. This is mainly due to the fact that rare events are difficult to observe on a real configuration in a reasonable time.