What is function of AU-PTR & TU-PTR in SDH?

Hi there!

It is used to locate low-rate signals in an STM-N frame (payload), that is, to make the position of the low-rate signals predictable.

When packing the signal into an STM-N payload, the transmit end adds AU-PTR to indicate the position of the signal package in the payload. This is, the goods package to be loaded into the carriage is given a position coordinate value.
The receive end splits the required low-rate tributary signal from the STM-N frame payload according to the AU pointer value. That is, according to the position coordinates of the goods package, the required package is directly obtained from the carriage.

The goods package in the carriage is placed in a certain rule, byte-interleaved multiplexing. Therefore, you only need to locate the first goods package in the carriage.

Tributary unit pointer (TU-PTR)
 

If the rate of the multiplexed low-speed signal is low, that is, the package after packing is too small, such as 2M and 34M, level-2 pointer positioning is required. A TU-PTR locates a small-sized package in a medium-sized package. An AU-PTR locates a medium-sized package in a large-sized package.