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This post continues to share SDH principles. In the Overhead and Pointer section, we need to know the functions of common bytes in overhead.
Part 3. Overhead and Pointer
1. Functions of bytes in overheads
A1, A2：frame alignment byte.
J0：regenerator section trace byte.
B1：bit interleaved parity-8 (BIP-8). This byte is used to monitor bit errors at the regenerator section layer.
E1, E2：orderwire bytes. These bytes are used to provide a voice channel for official contact.
D1~D12：data communication channel (DDC) bytes for OAM message transmission.
B2：bit interleaved parity check Nx24 (BIP-N $) byte, used for monitoring bit errors at the multiplex section layer.
K1, K2(b1-b5): automatic protection switching (APS) channel bytes. These bytes are used to transmit APS signaling.
K2(b6-b8): The MS-RDI byte is sent back by the receiving end (sink) to the transmit end (sink) to indicate that the receiving end detects a fault or receives an MS alarm indication signal.
The default J1 byte value of Huawei SDH equipment is Huawei SBS.
The B3 byte is used to monitor the bit error performance of VC-4 transmission in STM-N frames
C2 is the signal label byte, which is used to indicate the multiplexing structure of the VC frames and the payload property. The transmitted C2 byte must match the C2 byte to be received.
G1：Path status byte, which transmits some alarms and performance events from the information receiving end to the information sending end.
2. The pointer is used to indicate the start position of VC-n in the corresponding AU and TU. In this way, the receiving end can correctly separate the corresponding VC and C low-speed signals from the STM-n. such as a PDH signal. The STM-64 has 64 AU pointers.
3. The difference between the AU pointer and the TU pointer is as follows: The range of the pointer is different, the byte for storing the pointer value, and the adjustment unit are different.
4. The interval between two adjacent pointer alignments must be at least three frames.
5. The K1 and K2 bytes can send the APS switching instruction or indicate the failure of the multiplex section signal.
6. According to the ITU-T recommendation, the K1 and K2 bytes are used to transmit APS protocol information. There are only four bits to indicate the address of the source node and the address of the destination node. Therefore, the number of nodes in an MSP ring cannot exceed 16.
7. The signal identifier byte C2 is used to indicate the multiplexing structure and payload attributes of the VC frame. To transmit the C2 byte, the C2 byte must match the C2 byte to be received. When the value of C2 is 00H, the VC-4 is not loaded with services.
8. M1: MS-REI byte. It represents the number of bit errors detected by the receiver after receiving the MS signal from the receiver to the source.
9. In the SDH optical synchronous transmission system，the S1 byte is used to transmit the quality and usage information of the clock source.
10. The J1 byte needs to be scrambled before SDH signals are transmitted through optical fibers.
11. The E1 byte in SDH is used for orderwire communication of the regenerator section. Applicable to orderwire telephones between relay stations (REG) stations. The E2 byte is used for orderwire communication of the multiplex section and is applicable to orderwire calls between TM and ADM sites，not to the REG station.
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