Industrial IEEE 802.3 networks with short delivery time for urgent messages

Two solutions are investigated for introducing priority mechanisms in the CSMA/CD (carrier-sense multiple access with collision detection) protocol defined in the IEEE 802.3 standard to provide short delivery times for urgent messages even when the overall traffic on the channel is heavy. Thus the CSMA/CD protocol, attractive because of its low cost, also becomes suitable for a class of industrial control applications. The proposed schemes can easily be implemented by using already-available chips and boards because they are based on the dynamic management of a transmission parameter, the slot time; this feature is already present in the standard IEEE 802.3, where it is set statically when the network is configured. The first configuration considered implements a message priority scheme, where the priority level for the station can be updated at each new message transmission, varying the slot time according to the message characteristics. The second configuration introduces a fixed-station-priority mechanism which allows each node in the network to be assigned to a specific priority class