Segment overhead function

1. Regenerator Section Overhead Functions

(1) Frame Alignment Bytes A1, A2A1 and A2 are used to identify the start position of an STM-N frame. A1 is 11110110 (F6), and A2 is 00101000 (28).

(2) Regenerator Section Trace Byte J0The J0 byte repeatedly transmits a marker representing an access point, enabling the receiving end of the regenerator section to confirm whether it maintains a continuous connection with the intended transmitting end. The J0 bytes in 16 consecutive frames form a 16-byte frame to transmit the access point identifier. Within the network of the same operator, this byte can be any character; however, at the network boundary between different operators, the J0 bytes at both the receiving and transmitting ends of the equipment must be the same. Operators can detect and resolve faults in advance and shorten network recovery time through the J0 byte.

(3) STM-1 Identifier C1In the original CCITT recommendation, the C1 byte was arranged in the position of J0, which is used to indicate the position of STM-1 in the higher-order STM-N. When an old device using the C1 byte interoperates with a new device using the J0 byte, the new device sets J0 to "00000001" to indicate "regenerator section trace not specified."

(4) Regenerator Section Error Monitoring Byte B1The B1 byte is used for online error monitoring of the regenerator section. It adopts the even-parity bit-interleaved parity 8-bit code (referred to as BIP-8). BIP-8 divides the monitored part into groups of 8 bits, then calculates the parity (odd or even) of the number of "1" bits in each column. If the number is odd, the corresponding bit in BIP-8 is set to "1"; if even, it is set to "0". That is, after adding the BIP-8 bits, the number of "1" bits in each column becomes even. For example, for the short sequence "11010100011100111010101010111010", the BIP-8 calculation is as follows:

In an STM-N frame, BIP-8 operation is performed on all bits of the previous STM-N frame after scrambling, and the result is placed in the B1 position of the current frame before scrambling. The receiving end compares the BIP-8 value calculated from all bits of the previous frame before descrambling with the B1 of the current frame after descrambling. If any bit is inconsistent, it indicates that the "block" monitored by this BIP-8 has an error during transmission. By detecting the number of inconsistencies between the BIP-8 calculated by the receiving end and the received B1, the number of error "blocks" (i.e., the number of error items) during signal transmission can be obtained, thereby realizing online error monitoring of the regenerator section.

(5) Regenerator Section Service Communication Byte E1E1 is used for regenerator section service communication, providing a 64 kbit/s path, which can be accessed or dropped at the repeater.

(6) User Channel Byte F1It provides a 64 kbit/s path for network operators, serving as a temporary data/voice channel for special maintenance purposes.

(7) Regenerator Section Data Communication Channel Bytes (D1, D2, D3)D1, D2, and D3 are used to transmit the operation, administration, and maintenance (OAM) information of the regenerator in the regenerator section, providing a channel with a rate of up to 192 kbit/s (3×64 kbit/s).

2. Multiplex Section Overhead

(1) Multiplex Section Error Monitoring Byte B2It is used for online error monitoring of the multiplex section. Three B2 bytes total 24 bits, performing bit-interleaved parity check. Previously, it was BIP-24 check, and later improved to 24×BIP-1. Its calculation method is similar to BIP-8, except that here the bits are grouped into 24-bit groups.The method of generating the B2 byte is: perform BIP operation on all bits of the previous scrambled STM frame except the regenerator section overhead, and place the result in the B2 byte position of the current STM frame before scrambling. The receiving end calculates the BIP value of the received previous frame, then XORs it with the B2 of the current frame to get the number of error blocks.

(2) Data Communication Channel Bytes D4～D12They form a transmission channel for operation, administration, and maintenance (OAM) information between multiplex sections of the management network, providing a channel with a rate of up to 576 kbit/s (9×64 kbit/s).

(3) Multiplex Section Service Communication Byte E2It is used for multiplex section service communication, and can only be accessed or dropped at devices containing the Multiplex Section Termination (MST) function block, providing a 64 kbit/s path.

(4) Automatic Protection Switching Channel Bytes K1, K2 (b1～b5)K1 and K2 are used to transmit the Multiplex Section Protection Switching (APS) protocol. They ensure automatic switching when equipment fails, enabling network self-healing, which is used in the self-healing scenario of multiplex section protection switching.The bit allocation and bit-oriented protocol of the two bytes are specified in Annex A of ITU-T Recommendation G.783. K1 (b1～b4) indicates the reason for the switching request, K1 (b5～b8) indicates the sequence number of the working system that initiates the switching request, and K2 (b1～b5) indicates the sequence number of the working system to which the protection system switching switch on the multiplex section receiving side is bridged.

(5) Multiplex Section Remote Defect Indication Byte K2 (b6～b8)It is used to send back the status indication signal of the receiving end to the transmitting end of the multiplex section, notifying the transmitting end that the receiving end has detected an upstream fault or received the Multiplex Section Alarm Indication Signal (MS-AIS). When there is a defect, the "110" code is inserted into K2 (b6～b8) to indicate Multiplex Section Remote Defect Indication (MS-RDI).

(6) Synchronization Status Byte S1 (b5～b8)Bits b5～b8 of the S1 byte are used to transmit synchronization status information, that is, the synchronization status of the upstream station is transmitted to the downstream station through S1 (b5～b8). The arrangement of S1 is shown in Table 1-3.

(7) Multiplex Section Remote Error Indication Byte M1M1 is used to send back the number of errors detected by the receiving end of the multiplex section to the transmitting end. The error information of the receiving end (remote end) is obtained by comparing the 24×BIP-1 calculated by the receiving end with the received B2. The number of error bits corresponds to the number of error blocks, and then the number of errors is represented in binary and placed in the M1 position, as shown in Table 1-4, Table 1-5, and Table 1-6.

(8) Bytes Reserved for Future International StandardsThe blank bytes in Figure 1-9 with unspecified purposes are reserved for future international standard use. Currently, some of these bytes are allowed to be used for related communications.

The SOH function of SDH is quite complete, but not all bytes are indispensable in all cases. Simplifying the interface according to actual conditions and omitting some non-essential bytes can reduce equipment costs. Only the A1, A2, B2, and K2 bytes are indispensable.

The selection of SOH bytes for the simplified interface is shown in Table 1-7. This simplified interface is only an option provided for manufacturers and network operators, and can be used according to actual conditions in practical applications.