Can real-time local area network protocols be made robust?

This paper proposes a formal definition of the robustness property which, we believe, developers of future real-time (RT) local area network (LAN) protocols must aim to realize. As a demonstration of the utility of this definition, the carrier sense multiple access with deterministic collision resolution (CSMA/DCR) protocol is then analyzed for its robustness and an extension of the CSMA/DCR protocol that is highly attractive for use in RT, fault-tolerant, bus-connected LAN systems is presented. CSMA/DCR is one of the few major RT bus LAN protocols, striking a good balance between the CSMA/CD protocol with weakly predictable timing behavior, and the TDMA protocol that has a tightly bounded worst-case delay but is inflexible. We show that the ordinary CSMA/DCR and several other LAN protocols are not very robust in that the protocols lose their real-time delivery properties when various types of faults occur. The extension to the CSMA/DCR protocol that we present in this paper maintains real-time delivery properties in the presence of multiple faults. Since robustness is implemented at the medium access control (PAAC) layer level, this version of CSMA/DCR is capable of delivering frames in bounded time when permanent and transient faults occur, thereby providing to the upper logical link control (LLC) layer a reliable RT service.