How OTN supercedes over SDH/SONET

Here we will discuss what are the advantages of OTN(Optical Transport Network) over SDH/SONET.

The OTN architecture concept was developed by the ITU-T initially a decade ago, to build upon the Synchronous Digital Hierarchy (SDH) and Dense Wavelength-Division Multiplexing (DWDM) experience and provide bit  rate efficiency,  resiliency and  management  at  high capacity.  OTN therefore looks a  lot like Synchronous Optical Networking (SONET) / SDH in structure, with less overhead and more management features.

It is a common misconception that OTN is just SDH with a few insignificant changes. Although the multiplexing structure and terminology look the same, the changes in OTN have a great impact on its use in, for example, a multi-vendor, multi-domain environment. OTN was created to be a carrier technology, which is why emphasis was put on enhancing transparency, reach, scalability and monitoring of signals carried over large distances and through several administrative and vendor domains.

The advantages of OTN compared to SDH are mainly related to the introduction of the following changes:

Transparent Client Signals:

In OTN the Optical Channel Payload Unit-k (OPUk) container is defined to include the entire SONET/SDH and Ethernet signal, including associated overhead bytes, which is why no modification of the overhead is required when transporting through OTN. This allows the end user to view exactly what was transmitted at the far end and decreases the complexity of troubleshooting as transport and client protocols aren’t the same technology.

OTN uses asynchronous mapping and demapping of client signals, which is another reason why OTN is timing transparent.

Better Forward Error Correction:

OTN has increased the number of bytes reserved for Forward Error Correction (FEC), allowing a theoretical improvement of the Signal-to-Noise Ratio (SNR) by 6.2 dB. This improvement can be used to enhance the optical systems in the following areas:

• Increase the reach of optical systems by increasing span length or increasing the number of spans.
• Increase the number of channels in the optical systems, as the required power theoretical has been lowered 6.2 dB, thus also reducing the non-        linear effects, which are dependent on the total power in the system.
• The increased power budget can ease the introduction of transparent optical network elements, which can’t be introduced without a penalty.    These elements include Optical Add-Drop Multiplexers (OADMs), Photonic Cross Connects (PXCs), splitters, etc., which are fundamental for the  evolution from point-to-point optical networks to meshed ones.
• The FEC part of OTN has been utilised on the line side of DWDM transponders for at least the last 5 years, allowing a significant increase in reach/capacity.