Analysis of the PROFIBUS Token Passing Protocol over Wireless Links
For the PROFIBUS, a standardized and well-known fieldbus system, it is attractive to use wireless media. A natural approach in creating such a system is to re-use as much existing technology as possible. In the area of wireless local area networks (WLANs) clearly the IEEE 802.11 standard is the leading technology. Hence, the question comes up, how this technology can be used in the creation of a wireless PROFIBUS. In this paper we consider the solution of running the existing PROFIBUS MAC and link-layer protocol directly on top of an IEEE 802.11 DSSS PHY, without the 802.11 MAC functionality. We show that the PROFIBUS MAC protocol (a token-passing scheme on top of a broadcast medium, stations organized in a logical ring) is vulnerable against bit errors and packet losses. Specifically, the membership of a station in the logical ring is harmed by loss of token frames. Ring membership is important, since only ring members are allowed to transmit data. Lost stations can’t do so, no matter how time-critical and important their data is. Their reinclusion into the ring takes some time. The problems are serious and we conclude that for creation of a wireless PROFIBUS alternative solutions for MAC- and link-layer protocols should be investigated.
