The relief byte is used for ring protection
7.4.1.2 Path Overhead Layer (POH)
POH
The following is a partial list of POH bytes and their corresponding functions:
J1 byte (STS Path Trace)
High Order STS-N path trace byte
An unique message is assigned to each path in a SONET network – therefore the path trace can be used to check continuity between any location on a
transmission path and the path source
This user programmable byte repeatedly transmits a 15-byte string plus 1 (CRC-7) byte so that a receiver can continually verify its connection with the
transmitter
A 64-byte free-format string is also permitted. It contains up to 62 characters plus carry return (CR) and line feed (LF), any unused characters are filled with
blanks (00h)
The message is transmitted one byte per STS-3 frame
J1 Byte
B3 byte
Even code parity which determines if a transmission error has occurred over a path
Its value is calculated over all the bits of the previous STS-3
The computed checksum is placed in the B3 byte before scrambling
B3 Byte (STS Path BIP)
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