The STM Frame Formatter is the final portion of the SDH Framer design. Its main function is to add the RSOH, MSOH and AU Pointers to the SDH frame as well as calculating the BIP over the STM-16 frame and scrambling it on the way out. This final scrambling is to assure sufficient timing content in the signal.
The controller runs continuously and is implemented with four counters which keep track of: the row number in the current frame; the column group in the current frame; the byte in the column group in the current frame; and the bit in the byte in the column group in the current frame. This is depicted below. Thus, the STM Frame Formatter always knows what portion of the STM-16 frame it is constructing.
The STM-16 Frame Formatter works according to the state machine shown below.
It inserts the different pieces of overhead when necessary and controls when the mux sends the byte-interleaved VC-4's from the Virtual Container Processor. A conceptual diagram of the STM-16 Frame Formatter can be seen below.