Its like workgroup computing with each control module containing all the software it needs to perform its function and make choices based on the information it gets from other control units compared with the decision tree stored in its program.
The SAM's (Signal Acquisition Modules) perform various functions such as reading in fuel tank level sensor information and placing that data on the CAN or activating the headlights when the "lights on" message is received so for the most part they are switching modules.
The car does not have a server and client based environment but a set of equal modules all transmitting and receiving information that is required for the correct functioning of themselves and neighboring units, a small catch is some modules are more "equal" than others for instance if the rain/light sensor goes offline your only going to get a warning on the dash and the inconvenience of having to manually activate the affected systems but if the EIS dies your going to have a big problem not because its in control of the rest of the car but because the other control units are waiting on the messages generated by it to activate.
A power roof control module for example wont function if the EIS fails simply because it wants the key to be in the correct position before it will operate and if the EIS is not reporting that it wont run the pump or the ECU wont release the engine for starting because its not received the data from the EIS telling it the correct key has been inserted.
Its supposed to be a fault tolerant network thats why it uses a star topology a switched environment like whats used in LAN's would allow for a better network for obvious reasons but if the switch where to fail everything would fail, in the star setup the failure of any one module should not bring the whole network down with certain exceptions of course, the only switching that occurs in the cars is what takes place in the gateways connecting the busses and to a point they can also fail if the AGW was to go down no big deal you would lose the entertainment system but thats not exactly critical if the ZGS was to fail however you would have a more serious problem because its one of the main connections between the networks.
