Achieving real-time communication over Ethernet with adaptive traffic smoothing

Ethernet 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 are transported over an ordinary Ethernet, RT packets from a node may experience a large delay due to: contention with non-RT packets in the originating node; and collision with RT and non-RT packets from the other nodes. To resolve this problem, we designed, implemented, and evaluated adaptive traffic smoothing. Specifically, a traffic smoother is installed between the UDP or TCP/IP layer and the Ethernet MAC layer, 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.