The automation systems (AS) communicate with one another and with the engineering system and the operator systems (server/single stations) via the plant bus. This can be implemented in a similar way to the terminal bus, using SIMATIC NET standard components such as switches, network cards, communication modules and cables. In small plants with up to eight standard automation systems per operator system, you can also operate single stations and servers cost-effectively with "Basic Communication Ethernet" (BCE) and FastEthernet network cards on the plant bus. As far as availability is concerned, ring structures are always the first choice for the plant bus. Where there is particularly high demand for availability, the plant bus can also be configured as a redundant double ring (two CPs per AS CPU and OS server).