The situation illustrated in Section 46.4.5, "Clock Generator and Front Reference" also applies to the OpenScape 4000 based on the cPCI controller with the following changes: The clock generators are now located on the RTM modules and can be reached externally via the MCM module. Consequently, there is now only one connector for the "external clock box", which can also be used to connect the "frontref". The input clocks are distributed from here to both clock generators or the output clock originates in the active (master) clock generator. The "external clock box" interface on the MCM is mechanically and electrically identical to that of the MTSCG.