Optical Channel (OCh, OChr)

Now that the system has created all the information required and prepared it in a container of appropriate size for it to cross an OTH network and if necessary, for it to be regenerated, it is a requirement to provide an additional transparent network connection. The requirement for this transparent connection is due to the fact that to be able to direct and process client information - which is inside an OTUk remember – the system must be able to access the individual wavelength state without performing a full optical to electrical conversion. 

The way the OTH standard handles this is to use the OCh OH but instead of inserting it into the client carrier wavelength they utilize another wavelength channel altogether and this is the purpose of the OSC (optical supervisor channel) that has been mentioned in passing on several occasions. 

The OSC is carried on a frequency out with the carrier wavelength spectrum so that it does not interfere with the carrier wavelengths. Furthermore all vendor active network equipment has electrical access to this OSC channel. The OSC is the only signal, which all OTH compliant equipment can access.

Consequently if any piece of equipment on the optical path detects a client signal failure (loss of signal) it will process the corresponding OCh OH to inform the other system equipment about the state of the client wavelength. 

Note: There are two versions of the optical channel OCh & OChr.  The non-standard version is called OChr, where 'r' represents the reduced version that does not insert an OCh OH into the OSC wavelength.