Ring stability of the PROFIBUS token-passing protocol over error-prone links

The PROFIBUS is a well-known and widely used fieldbus. On the medium access control layer, it employs a token-passing protocol where all active statio ns form a logical ring on top of a broadcast medium. This protocol is desig ned to deliver real-time data transmission services in harsh industrial env ironments. A necessary prerequisite for timeliness and quality of service i s the ring membership stability of the logical ring in the presence of tran smission errors, since only ring members are allowed to transmit data. In t his paper, the ring membership stability under high error rates and using d ifferent error models is analyzed. The choice of the error behavior is in t urn inspired by properties of possible future transmission technologies, e. g., wireless LANs. It is shown that the protocol has serious stability prob lems. To attack these problems, two changes to the protocol and its paramet ers are proposed, which can be implemented in a purely local manner. We tly show that they significantly improve ring stability.