What Is the ODUk-LCK Maintenance Signal?
LCK is an abbreviation for Locked Indicator.
OTN Network Equipment will often transmit the ODUk-LCK (Locked) maintenance signal to indicate that this particular OTN interface has been administratively locked-out and is now unavailable to user traffic.
In other words, if the system operator decides to lock out (or prevent OTN traffic from flowing through) a given OTN interface, then the system will transmit the ODUk-LCK maintenance signal as a replacement signal through that OTN interface.
Whenever an OTN Network Equipment (NE) transmits an ODUk-LCK maintenance signal, it generates and transmits a framed repeating “0101 0101” pattern within the entire ODUk signal.
If we were to map the ODUk-LCK Maintenance signal into an OTU data stream, the Source STE would transmit a series of OTUk frames in which the FAS, MFAS, and OTUk Overhead fields are all valid. The rest of the OTUk frame (e.g., the ODUk/OPUk portion of the frame) will consist of a repeating 0101 0101 pattern.
The Source STE will compute the FEC field based on the contents within these OTUk frames.
Figure 1 shows a drawing of an OTUk frame transporting the ODUk-LCK maintenance signal.
Figure 1, Illustration of an ODUk-LCK signal within an OTUk frame
What are the timing/frequency requirements for the ODUk-LCK Maintenance signal?
The Source STE will need to transmit this indicator at the same nominal bit rate for an ordinary OTUk signal.
Like any OTUk signal, the Source STE will need to transmit this data at the nominal bit-rate ±20ppm.
Table 1 presents the nominal bit-rates for the OTUk signals (and, in turn, for the OTUk signal, whenever it is transporting the ODUk-LCK indicator) for each value of k.
Table 1, Required Bit Rates for the OTUk Signal – when transporting the ODUk-LCK signal.
When would OTN Network Equipment transmit/generate the ODUk-LCK Maintenance signal?
Earlier, I mentioned that the Network Equipment would transmit the ODUk-LCK signal via an OTN Interface signal when the system operator has locked-out system users’ OTN traffic from using this particular OTN interface.
The ODUk-LCK maintenance signal will replace OTN traffic, which the system operator has locked out intentionally from this particular OTN interface.
However, the NE will transmit the ODUk-OCI indicator whenever the OTN traffic is missing because the system configuration has removed the OTN traffic upstream.
The NE will transmit the ODUk-AIS indicator whenever the intended OTN traffic is missing due to a service-affecting defect upstream.
The NE will transmit the ODUk-LCK indicator via an OTN interface whenever the OTN traffic is missing. The system operator has locked out that particular OTN interface from all other OTN traffic.
One practical example of an OTN NE transmitting the ODUk-LCK indicator would be when the system operator has configured a particular port (or OTN interface) to operate in some diagnostic or loopback mode.
How does a Receive Terminal detect and declare the dLCK (ODUk-LCK) defect condition?
The Receiving NE (or Sink PTE) that is receiving the ODUk-LCK maintenance signal will declare the dLCK defect condition whenever it gets a STAT field value of [1, 0, 1] within three (3) consecutive OTUk/ODUk frames.
NOTE: The STAT field is a 3-bit field that resides within the PM (Path Monitor) byte-field in the ODUk overhead.
This 3-bit field will have the value [1, 0, 1] because the NE overwrites the ODUk overhead with the repeating “0101 0101” pattern whenever it transmits the ODUk-LCK maintenance signal.
Please see the ODUk Frame post for more information about the STAT field.
How does a Sink PTE clear the dLCK defect condition?
The Sink PTE will clear the dLCK defect condition whenever it has accepted a STAT field value of something other than “[1, 0, 1]”.
NOTE: The Sink PTE should accept a new STAT field value if it receives at least three (3) consecutive ODUk frames that contain a consistent STAT field value.