Achieving real-time communication over Ethernet with adaptive traffic smoothing

Ether-net continues to be one of the most popular LAN technologies. Due to the low price and robustness resulting from its wide acceptance and deployment, there has been an attempt to build Ethernet-based real-time control networks for manufacturing automation. However it is difficult to build a real-time control network using the standard UDP or TCP/IP and Ethernet, because the Ethernet MAC protocol, the 1-persistent CSMA/CD protocol, has unpredictable delay characteristics. When both real-time (RT) and non-real-time packets al-e transported over an ordinary Ethernet, RT packets from a node may, experience a large delay due to (i) contention with non-RT packets in the originating node and (ii) collision with RT and non-RT packets from the other nodes. To resolve this problem, Lye designed, implemented and evaluated adaptive traffic smoothing. Specifically?: a traffic smoother is installed between the UDP or TCP/IP layer and the Ethernet MAC layers and,works as an interface between them. The traffic smoother first gives RT packets priority over non-RT packets in order to eliminate contention within each local node. Second, it smooths a non-RT stream so as to reduce collision with RT packets from the other nodes. This traffic smoothing can dramatically decrease the packet-collision ratio on the network. The traffic smoother; installed at each node, regulates the node's outgoing non-RT stream to maintain a certain traffic-generation rate. In order to provide a reasonable non-RT throughput, the traffic-generation rate is allowed to adapt itself to the underlying network load condition. This traffic smoother requires only a minimal change in the OS kernel without any modification to the current standard of Ethernet MAC protocol or the TCP or UDP/IP stack. We have implemented the traffic smoother on both the Linux and the Windows NT platforms, demonstrating significant reduction of the RT packet deadline-miss ratio when both RT and non-RT packers are transported or er the same Ethernet. More precisely: installation of the proposed traffic smoother on every node is shown ro reduce the RT message deadline-miss ratio by two orders of magnitude under a heavily-loaded condition, while lowering the non-RT throughout only by half.