Supporting firm real-time traffic in fault-tolerant real-time systems based on cyclic scheduling — The WICKPro protocol

Industrial control network research has become increasingly interested in wireless multi-hop networks over the last years. These wireless networks present several advantages over wired networks, but unfortunately they are usually error-prone. Although packet losses can be counterbalanced by retransmissions, this can jeopardize the deadlines of the supported traffic. For this reason, a fault-tolerant design which carries out a feasibility analysis is essential to provide probabilistic scheduling guarantees. In this paper, we analyze and enhance the wireless chain network protocol (WICKPro) from a fault-tolerant realtime system perspective. WICKPro is a medium access control and routing protocol designed for wireless mesh networks with chain topologies which supports firm real-time traffic and is implemented over 802.11. It uses a token-passing scheme and is based on both an off-line and an on-line packet scheduler. The offline cyclic packet scheduler reserves time for packet transmissions and retransmissions, whereas the on-line packet scheduler drops packets when losses are higher than those reserved for a specific period of time. WICKPro features are evaluated in laboratory and field experiments.