Its basically a computer to computer communication link with the electrical systems basically broken down into functional units.
Lets take side lights for example.
You want to turn on the lights? In the old days 12v would be provided to your light switch, your switch would then switch this 12v through to the front and rear side lights (possibly through a relay) You have 4 main power wires to the 4 side lights.
These days the lighting controller takes charge of the light. The light switch is just connecting one of the control pins of the lighting controller to earth. You press the button and signal the controller to turn on the lights. The lighting controller will tell the rear lighting controller (over the can bus) to apply power to the appropriate rear lighting cluster taking power from the single 12V rear power ring main.
This of course allows the system to be operated by the environmental light sensors, to turn on lights automatically when its dark. The lighting control is also talking over the can bus to the system that controls the wipers, so if the wipers are on full, it could turn on the lights.
This of course could be linked to the intelligent battery charging system telling it to put more power into the battery, and of course signal the engine controller to up the idle speed a tad due to the increased electrical load.
Basically its to reduce all the thick current carrying cables down to a minimum, replace them with a single signal cable, and be able to control all aspects of what the car is doing.
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