OTH (Optical Transport Hierarchy)

The OTH (Optical Transport Hierarchy) was the initiative to bring intelligence and OAM&P features to WDM. OTH is the underlying optical multiplexing principles and techniques for aggregating several light wave signals onto a single multiplexed signal, which is then transported as a single signal across a single optical fiber. WDM's operation is independent of the original signals format, frames or bit-rate so it can handle any signal type. However, there are some physical restrictions in the WDM mode of operations.

1.       All the client wavelengths must be different to avoid interference.

2.       All the wavelengths must be sufficiently separated by a distance, a guard gap, to avoid signal cross talk.

WDM's advantage though is that it can take many different wavelengths (client signals) and multiplex them onto a single fiber, which had previously been only capable of handling one wavelength (client signal). This efficiency of resource usage is just one of WDM's beneficial features another is that it can carry transparently any client signal type. For instance client inputs can be from SDH, TDM, IP, ATM or any data stream as it is all transparent to WDM, it concerns itself only with carrying wavelengths.

At this point, it will be good to introduce the term Lambdas as this term is often used to describe a single wavelength. Consequently, the words wavelength and lambdas will be used inter-changeably in this book as the terms are synonymous.  On a similar note, the term OTH (Optical Transport Hierarchy) is used to describe the efforts to the OTH party to standardize the OTN network. However it can also be considered to be interchangeable with the term G.709, which was the resultant standard.

The OTH goal was to apply the principles of OTH design, learned from SDH, with the technology of WDM to provide the basis for a manageable fully meshed optical transport network, the OTN.