Overhead Bytes
SDH provides monitoring and management in layers. Monitoring is classified into section monitoring and path monitoring. Section monitoring is classified into regenerator section monitoring and multiplex section monitoring. Path monitoring is classified into higher-order path monitoring and lower-order path monitoring. Different overhead bytes help to implement the monitoring functions.
This section describes only some SDH overhead bytes used in configuration. For details, refer to a book about the particular topic.
SOH
SOH consists of RSOH and MSOH.
The Payload of an STM-N frame contains the path overhead (POH) that monitors low-speed tributary signals.
J0, the regeneration section trace message is contained in RSOH. This byte is used to transmit the Section Access Point Identifiers (SAPIs) repeatedly to check the connection between the receiver and the transmitter. The byte can be any character in the networks of a carrier, whereas the J0 byte of the receiver and the transmitter must match each other at the border of networks between two carriers. With the J0 byte, a carrier can locate and rectify faults in advance to speed up the network recovery.
POH
SOH monitors section layers, whereas POH monitors path layers. POH is classified into lower-order path overhead and higher-order path overhead.
The higher-order path overhead monitors the paths at VC-4 and VC-3 levels.
J1, the higher-order VC-N path trace byte, is contained in the higher-order path overhead. Similar to j0, J1 is used to transmit SAPIs repeatedly to check the connection between the receiver and the transmitter. J1 bytes of the receiver and transmitter must match each other.
C2, the path signal label byte, is contained in higher-order path overhead. C2 is used to specify the multiplexing structure and the attributes of the information payload in a VC frame, including whether the path is loaded with services, service types, and the mapping mode. C2 bytes of the receiver and transmitter must match each other.