The D2B (Digital Data Bus) ring is simply the fiber optic network cables between the audio system components. All communication between components takes place via this network. Fiber optics are used because of the large amount of data that can be handled and the speed at which it works. On MB vehicles, the COMAND/Audio head unit is the ring "master". Different vehicles have different ring configurations based on the equipment installed. The most basic system is merely the head unit and CD changer. A fully equipped S-Class, depending on model year, may have a CD changer, Sound Amplifier, Tele-Aid, Voice Control, and C-Tel Interface on the ring. For the system to work properly the ring "master" needs to know which components are installed, and in what order. The head unit is responsible for "waking up" the components as well as monitoring for malfunctions. It can also assist in diagnosis. If a particular component malfunctions, the head unit can store a fault code. All in all it's a pretty good system, although it was phased out beginning in 2003. I hope this answers your question.