Timed token protocols have increased in importance with the growth of local area network (LAN) communications. One of the most important timed token protocols is certainly IEEE 802.4, a standard given the name ‘token bus’, which represents one of the most interesting solutions to the problem of handling, on the same physical channel, the transmission of messages belonging to multiple priority classes. In the paper, the authors study some time characteristics of the IEEE 802.4 protocol, giving bounds to the mean and the maximum duration of a complete token cycle. It is shown how these results can be conveniently used by a network designer to tune the parameters that characterise an IEEE 802.4 local area network.