J0 byte in STM frame

The byte J0 in STM frame is used for path tracing / section tracing. In simpler terms, this byte will contain a 8-bit word with which the receiver checks the connectivity with the transmitting node. The same word will be sent continuously for transmitting one particular information using more than a frame.

Consider an example of a a node which transfers a lengthy message using more than one frame (say 3). The transmitter will assign the same value to the J0 byte of all three frames so that the receiver can check the continuity of the frames from a transmitter.

Thus, the J0 byte can have any values ranging from 0x00 to 0xFF.

If this section tracing or path tracing function is not needed, it can be disabled by setting the value of J0 byte as 0x01 according to the standards specified.

Thus, to be short, J0 byte acts as an identifier which helps in checking the continuous connectivity with the transmitting node. This feature can also be disabled by setting the value of J0 as 0x01.