In a Token ring system, allocation is made sequentially to each node on the bus, regardless of whether the node needs the bus at the time of allocation. If the node misses its allocated time slot it must wait for it to come round again. This can lead to long and undesirable periods of inactivity on the bus. Ethernet systems are different again. Simultaneous bus access by more than one node in an Ethernet system causes ALL transmissions to be aborted.
ALL transmitters then attempt to re-transmit their respective messages.
In worst case situations the bus can be inactive for substantial periods of time as nodes continue to fight for access. Response times can become excessive. The bus may even lock up. With only the maximum capacity of the bus as a speed limiting factor, CAN will not collapse or lock up. Outstanding transmission requests are dealt with, in their order of priority, with minimum delay, and with maximum possible utilisation of the available capacity of the bus.