Overhead bytes
SDH provides layered precise monitoring and management.
SDH monitoring functions are implemented using overhead bytes. SDH provides monitoring at section and channel levels.
Sections are subdivided into regenerator and multiplex sections.
Channels are subdivided into higher-order and lower-order paths.
SDH provides a variety of overhead bytes, but only those involved in CPOS configuration are discussed in this section.
SOH
The section overhead (SOH) is further classified into the regenerator section overhead (RSOH) and the multiplex section overhead (MSOH).
The J0 regenerator section trace byte is included in RSOH to repeatedly send the section access point identifier. The receiver uses this identifier to make sure it is in continuous connection with the sender. This byte can be any character in the network of the same carrier. If the networks of two carriers are involved, the sending and receiving devices at network borders must use the same J0 byte. With the J0 byte, carriers can detect and troubleshoot faults in advance or use less time to recover networks.
POH
The payload of an STM-N frame includes the path overhead (POH), which monitors low-speed tributary signals.
The SOH monitors the section layer, and the POH monitors the path layer. The POH is divided into the higher-order path overhead and the lower-order path overhead.
Higher-order path overhead monitors paths at the VC-4/VC-3 level.
Similar to the J0 byte, the higher-order VC-N path trace byte J1 is included in the higher-order path overhead to repeatedly send the higher-order path access point identifier. The receiving end of the path uses this identifier to make sure it is in continuous connection with the specified sender. The sender and the receiver must use the same J1 byte.
In addition, the path signal label byte C2 is included in the higher-order path overhead to indicate the multiplexing structure of VC frames and the properties of payload, including the following:
Whether the path is carrying traffic.
What type of traffic is carried.
How the VC frames are mapped.
The sender and receiver must use the same C2 byte.