Tapping the applicable byte enables an automatic decode – a byte description including the Hexadecimal and Binary value is provided. For some bytes, an advanced
decode of the various bits is also available.
7.5.1.1 Section Overhead
The following is a partial list of SOH bytes and their corresponding functions:
Section Layer
Framing Bytes (A1/A2)
The A1/A2 bytes indicate the beginning of the STM-N frame and provide a frame alignment pattern
A1 is the 1st framing byte: 11110110 (Hex F6)
A2 is the 2nd framing byte: 00101000 (Hex 28)
Both A1 and A2 bytes are unscrambled
The Frame Alignment Word of a STM-N frame is composed of (3 x N) A1 bytes followed by (3 x N) A2 bytes
A1 [Framing] Byte
Path Trace Byte (J0)
Regenerator section trace
Used to transmit a 16-byte identifier (trace) (15-byte string and a CRC-7 byte) repeatedly so that all regenerators can verify their connection. 64-byte strings are also
available. It contains up to 62 characters plus carry return (CR) and line feed (LF), any unused characters are filled with blanks (00h)
Used for continuity testing between regenerators
B1 Byte (RS-BIP)
An 8-bit even parity code used to check for transmission errors over the regenerator section
Its value is calculated over all the bits of the STM-1 frame before scrambling
The checksum value is placed in the RS overhead of the following STM-1 before scrambling
Path Trace Byte (J0)
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