The LPT function transfers the link state information through certain OTN overhead bytes.
In the case of the WDM equipment, the LPT function enables transparent transmission of information about the faults of the service access point or service network. Hence, the LPT function ensures consistency of the states of the physical links between client equipment. That is, if a link becomes faulty, both the upstream equipment and downstream equipment can switch services to the backup network.
Trigger Conditions
When the WDM equipment accesses data services, the LPT function can be triggered on the following conditions:- Faults at the access point: R_LOS or LINK_ERR occurs on the client-side optical interface.
- Faults on the service network:
- The line-side signals are OTN frames, including R_LOS, OTUk_AIS, OTUk_LOF, ODUk_PM_AIS, ODUk_PM_LCK, ODUk_PM_OCI, ODUk_TCMn_AIS, ODUk_TCMn_LCK, ODUk_TCMn_OCI.
- The line-side signals are SDH frames, including R_LOS and R_LOF.
Working Principle
Figure 1 shows the application of the LPT function.
When access point 1 fails, RS-A detects the local link fault, switches to the backup network, and attempts to communicate with RS-B through the backup network. In this case, RS-B cannot obtain the link fault information about RS-A and thus still attempts to communicate with RS-A through the original WDM network. As a result, the communication between RS-A and RS-B is interrupted.
When the LPT function is enabled, the local transport equipment, that is, NE1, detects the fault of the link between NE1 and RS-A, and sends the LPT packets to the opposite transport equipment, that is, NE2. Then, NE2 automatically disables its interface (Link Down) through which NE2 is connected to RS-B. As a result, the interface on the RS-B detects the link fault. In this manner, RS-B also switches to the backup network and sets up a connection with RS-A through the backup network.