Considerations for multicast scalability of fieldbus protocols on IP/Ethernet

PID (proportional integral derivative) control is a feedback mechanism widely used in industrial control systems, and publisher/subscriber is an asynchronous one-to-many messaging mechanism commonly used in fieldbus protocols. Some fieldbus protocols are extended with IP/Ethernet, and publisher/subscriber messages are mapped to IP multicasts when extending these protocols. Some such protocols map each publisher/subscriber message to an individual IP multicast Group ID. This one-to-one mapping requires a large number of Group IDs if a system has a large number of devices, causing scalability issues on the Ethernet and IP layers. A single large Ethernet has to be divided by IP networks to resolve related scalability issues. However, this causes scalability issues related to IP multicasts. The purpose of this paper is to clarify these issues from the viewpoint of a network layer model. A further study item is to discuss solutions in depth; here, a tunneling technique is shown as a solution. The analysis presented in the paper is applicable to any fieldbus protocol with a one-to-one mapping model between publisher/subscriber messages and IP multicast Group IDs.