An evaluation of switched ethernet and linux traffic control for real-time transmission

Switched Ethernet networks are spreading to industrial environments more are more. The current trend is using them at all levels of a factory, replacing this way field-buses and other industrial networks. Switched Ethernet lacks the drawback of the non-deterministic collision resolution of coax cabling. However there are still some sources of indeterminism, mostly due to contention problems in message queues at switches and network interfaces. These problems can be dealt with using traffic control mechanisms for packet prioritisation and scheduling. These features have been largely inaccessible in Ethernet for a long time but, nowadays, they are widely available in industrial switches and also in the Linux TC (Traffic Control) mechanism. This paper presents an evaluation of traffic control mechanisms in a scenario where real-time periodic messages are transmitted along with some other bandwidth-consuming workload like video streams. The evaluation focuses on the effect of two traffic control mechanisms: rate limiting and packet prioritisation. Results show that these mechanisms provide switched Ethernet with a reasonably good predictability, even with high bandwidth utilisations. Rate limiting mostly reduces the average delay while packet prioritisation reduces the jitter and the dispersion.