Serial Data Gateway
WARNING: This page is about a different car, the 2005 Chevrolet Malibu. However, it is still accessible from the selected car via links, so may be relevant.
The body control module (BCM) in this vehicle functions as a translator between the High and Low speed GMLAN busses. The 2 communication protocols use the BCM to translate communicated serial data messages between the control modules over the 2 serial data busses.