Main Board Unit (MBU)
The Main Board Unit (MBU) controls communication between the peripheral interfaces, supervises all resources in the system, controls the gain adjustment of the PCM signal, generates the System tones, and manages System call processing. The MBU2 is the same as the MBU except it is intended for use in North America only. The MBU supports 3 CO Lines, 1 DKT and 7 hybrid ports.