The CLOCK BOX (see Section 46.5, "Selection of Clock Generators") and the "front reference" or "frontref" (external clocking source directly on the clock generator) are independent of each other and fully integrated into the security equipment.

The CLOCK BOX must be configured as AECB (also applies for AICB, SASE, etc.) with the AMO REFTA, i.e. the required clock generator class can be selected. The clock generator of the simple MTSCG is in standard class 38 by default. If a working AECB is connected, class 68 can be selected for the networking clock generator (like NCG-X). If a CLOCK BOX with high free-run exactness (better than 1 x 10 to the power of minus eight) over a longer period of time is used, class 78 can be set. This class always narrows the trimming range to 0,1 ppm absolute. Setting the class does not effect or require a reset of the MTSCG board. Therefore, no duplex switch is necessary. Changeover of the class during operation results in a brief switch-off of a configured (line) reference.

If a CLOCK BOX is used, the state of the clock cable to the CLOCK BOX and the status of the background reference (OCXO) are also monitored. All changes are reported to the security equipment and are available for the NMS.

The "frontref" is part of the reference clock table. Its priority can therefore be freely selected. The frontref status of a system is continuously monitored, regardless of whether it is used for synchronization. This applies for the A and B controllers independently of each other.