Optical Transport Network:

A set of Optical Network Elements connected by optical fiber links, able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals.

OTN is used because of following reasons;

• Operators’ need to use existing fiber to satisfy Growing demand for services and bandwidth.

• DWDM increased fiber bandwidth, but lacked protection and management capabilities.

• OTN combines the benefits of SONET/SDH with DWDM.

Figure shows the format of the synchronous OTN frame.

All types of frames are to be transported transparently in the OTM frame, without them having to be modified. A field is expected to add an FEC (Forward Error Correction) to perform the necessary corrections to achieve an error rate determined.

OTN interface consists of several levels. Starting from the optical fiber with the following layers are:

1. OTS (Optical Transmission Section), which supports signal transmission optical checking its integrity.

2. OMS (Optical Multiplex Section), which supports features to provide a wavelength-division multiplexing.

3. OCH (Optical Channel), which is the level of one end of the optical signal. This level allows modification of the connection and rerouting, and maintenance functions to the connection.

4. In order to get access for end-to-end networking of optical channels to transparently convey user information, the OCh layer is employed in the OTN structure. It also ensures connection rearrangement for flexible routing. Either full or reduced optical channel availability can be employed as required.

5. DW (Digital Wrapper), which corresponds to the digital envelope. The level Digital Wrapper itself is broken down into three levels:

6. OTU (Optical Transport Unit), which gives the possibility of adopting a correction using FEC.

7. The OTU layer gives additional functionality by adding forward error correction to the network elements and by allowing carrier operators to reduce the number of optical devices and switches used in the network (i.e. amplifiers, multiplexers, 3R regenerators). It also encapsulates two additional layers, the optical channel data unit (ODU) and the optical channel payload unit (OPU).

8. ODU (Optical Data Unit), which manages connectivity regardless of clients and offers protection and management of this connectivity. The ODU layer provides client-independent connectivity, connection protection and monitoring whereas the OPU enables access to the payload information of SDH/SONET signals.

9. OPU (Optical Payload Unit), which indicates a match between the signal and the type of client. The overall architecture of OTN is shown in Figure.

1. The OTN hierarchy to support these four layers may also include three sections as, which are: the optical transmission section (OTS); the optical multiplexing section (OMS); the optical physical section (OPS).

2. Different physical sections of the OTN are interfaced based on the information obtained from OTS, OMS and OPS, respectively.

3. OTS provides the required information about optical transmission functionality for optical signals to travel through optical fibers and the necessary switching operation at fiber end points.

4. The optical multiplexing section (OMS) gives the means of networking for a multiwavelength optical signal, whereas the OPS describe the optical characteristics of the physical section in order to provide reduced functionality excluding those sections which are not required to establish an interface between two domains.

Advantages:

• Protocol transparency, Integrity of the client signal is maintained, end user views exactly what was transmitted.

• Backward compatibility for existing protocols.

• Better switching scalability, OTN is able to switch at a higher bit rates than SDH.

• Better Forward Error Correction (FEC).