The pointer is used for alignment and indicates the start position of VC-n in the corresponding AU or TU frame. In this way, the receive end can correctly remove the corresponding VC from the STM-N, and then split the VC and C encapsulation to separate low-rate signals, such as PDH. That is, low-rate tributary signals can be directly dropped from the STM-N signal.

When the ACT indicator of an SDH board is off, it indicates that this board has a hardware fault and cannot be used,

According to ITU-T recommendations, the requirement that the MSP switching time should be within 50 ms is conditional. For example, the number of NEs on an entire ring network is less than 16.