B0 byte

B0 byte of the current frame contains the BIP-8 value of the previous frame. BIP refers to Bit Interleaved Parity. BIP-8 thus means 8-bit interleaved parity.

How is it calculated?

Consider a STM-0 frame. It has 90 x 9 bytes in it. The BIP-8 value of this frame can be calculated as follows

Say, we want even parity (The value will be set if even number of 1s are there). Lets find the BIP-8  even parity for A1 and A2 byte of STM-0 frame.

A1 --> F6 --> 1111 0110
                       --EX-OR--  
A2 --> 28 --> 0010 1000
                       | | | |   | | | |
BIP-8-------> 1101 1110


The bit wise EX-ORing of every byte (8-bit) gives the value of BIP-8. The value of B0 is obtained by EX-ORing every byte of the STM-n frame.

B0 byte finds the parity for all sections of the frame namely Regenerative Section Overhead(RSOH), Multiplexed Section Overhead(MSOH) and the payload section of the frame.

This byte is used for detecting error in a frame at the receiver side and at intermediate nodes.