Detailed study of multiplexing process in SDH - Interview notes

       Let us study the overview of the process followed by a 2 Mbit/s PDH input signal until it becomes part of an STM-1 frame. You can compare the details of each individual stage to SDH multiplexing structure provided in previous post.

Mapping of a 2 Mbit/s PDH signal into a C-12:

      The 2 Mbit/s PDH input signal is mapped into a Container 12 (C-12). The input frame consists of 32 bytes of information and this fits directly into the C-12 as shown

Mapping of a C-12 into a VC-12:

 At the time of mapping a C-12 into a VC-12, we need to add four bytes of overhead control information. But we can add only one byte per frame of customers' data So, this process takes place over 4 consecutive frames & described below: -

Frame number One has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. One byte of overhead control information added to the start. This byte of over head is called the V5 byte and is known as the VC-12 Path OverHead (POH).

Two more important features of the V5 byte are:

BIP-2 is Bit Interleaved Parity Check-2. This looks at the data in the C-12. It counts all of the binary one's that it sees in the odd bit positions (i.e. bits 1,3,5,7 etc) and then it counts all of the binary one's that it sees in the even bit positions (i.e. bits 2,4,6,8 etc). This BIP-2 is then recalculated at the distant end. If the count is different, then some bit corruption has occurred.

 FEBE is Far End Bit errors. This bit is set correspondingly to the result of the BIP-2 check. If errors are received at the distant end then there needs to be a mechanism for informing the sender of the problem.

Frame number Two has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. It then has one byte of overhead control information added to the start. This control byte in frame 2 is the Lower Order Path Trace or J2 byte. J2 is used to check continuity of a 2 Mbit/s path.

Frame number Three has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. One byte of overhead control information added to the start. This control byte N2, in frame 3 is called the Network Operator or Tandem Control byte. N2 is used to transmit performance-monitoring information where the circuit spans differing vendors networks. 

Frame number Four has one byte of fixed stuffing added to the end. It also has one byte of variable stuffing added to the start. One byte of overhead control information added to the start. Control byte in frame 4 is called K4 and it is used for 2 Mbit/s Automatic Protection Switching or APS. APS is used to automatically switch a single 2 Mbit/s circuit to its alternate path if a fault condition occurs.

In the next post we will learn about : 

 Mapping of a VC-12 into a TU-12 signal.