Optical Channel Layer

The Optical Channel Layer is the highest level layer and interacts directly with the clients system interface. It also provides the mechanism for transporting the client's digital signal through into the OTH compliant network. The optical channel trail from source to sink composes of two distinct logical signals:  

1.The optical client payload 

2.The  optical channel overhead ( the information added by OTH) 

To provide for end-to-end networking the following capabilities are included:  

•Optical Channel connection rearrangement 

•Processes for ensuring the integrity of the information added by the optical channel – This involves the introduction by OTH standards of bit-error rate evaluation and FEC (forward error correction) 

•Supervisory features for allowing network level management and control – this function involves the introduction of OTH standards for provisioning of connections, quality of service parameter exchange, and link protection schemes for network survivability  

The process required to create an optical channel from client data signal is:  

1.To create the optical channel from the client’s signal through interfacing with a transponder/transceiver in order to generate a continuous data stream that can be modulated onto a carrier wavelength 

2.The generation of management/maintenance signals for the OS overhead 

The OTH Goals

Some of the design goals of the OTH initiative were to provide the following:  

1.Develop a holistic system to handle rapid traffic growth 

2.Develop a system to handle transparently diverse signal types, frames, bit-rates and protocols 

3.To develop management OAM&P features through a supervisory information channel that did not negatively impact payload bandwidth 

4.Preserve the natural transparency and protocol independence of WDM and its inherent advantages 

5.To develop OTN standards – for optical termination, regeneration and cross-connects 

6.To work towards standards that would make full mesh OTN network practical 

Optical Transport Hierarchy

Up until the advent of the OTN initiative, the optical layer had been used simply as a physical medium over which to transmit data. This was primarily because of the impossibility to process optical data without first handling it over to the electrical domain. The OTH principle was seen by many as the first step in rectifying this shortcoming and in attempting to deliver some 'intelligence' to a passive system. Now, the OTH does not rewrite the laws of physics and it is still impossible to process data optically. However not to be deterred by such detail they opted for reserving a specific wavelength in order to use as a management or supervisor channel on which they could transport management information between equipment. Designers deemed this out-of-band supervisory channel a good solution as it did not interfere or unduly affect 

The payload of the underlying system was in contrast to SDH's overhead bytes. The OTH out-of-band supervisor channel would carry the same management information as SDH frames carried as overhead, but without using up precious cargo space in the frames. 

The OTH however would have to face come challenges as WDM differed quite considerable from the SDH framed protocol. For instance, there was no defined frame, DWM was light waves, without any frame structure or protocol and that was one of its strengths. Secondly, SDH handled one signal trail per path whereas WDM would be carrying several client signals per path and each would have to have its own individual management information with regards its own client’s state and fault. 

However, the challenge to delivering the project was easily outweighed by the potential benefits. If the principles of the OTH initiative could be applied to a working WDM out-of-band optical supervisory channel (OSC) then the potential was massive. Optical networks would be able to take on new topologies, such as partial and complete mesh networks using optical add/drop multiplexors, cross connects and optical switches in practical real world deployments. This would be due to the underlying management information being carried across the supervisory channel providing insight into the state of the network at every optical termination point, just like SDH. 

Furthermore, it would be possible to even implement protection and link fail-over as a network reliability measure, again due to having insight as to the link state between each optic termination link. However, the comparisons with SDH don't end there the OSC could also make provisioning of services possible, as it provides the ability to create, delete, and manage a service remotely, as can be done with SDH. 

In short, the OSC would do away with the WDM black-box approach to service management and provide all the SDH style visibility, manageability, flexibility and those precious OAM&P features for which everyone was clamoring.

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.