In order to detect the start of the 2 Mbit/s signal and thereby the start of the customers data, The V5 byte must be seen be the distant end. This is achieved by adding four overhead bytes to the multiframe, which together form a calculated byte count to the start of V5. This is called a pointer value and is known as the TU Pointer.
There are four pointer bytes called V1, V2, V3 and V4, which are used to calculate the location of V5.
Each VC-12 consists of 36 bytes of information and these 36 bytes fill up exactly 4 columns of the STM-1 frame.The 3 TU-12's will fit exactly into 12 columns of the STM-1 frame .
Mapping of a TUG-3 into a VC-4 signal:
These are the steps followed in multiplexing VC-12 into VC-4. In next post, we will read about VC-4 Path over head