The Value of OTN
The Optical Transport Network, OTN, started development in the ITU over 10 years ago starting with the development of standard G.872 - Architecture of optical transport networks followed up by G.709 - Interfaces for the Optical Transport Network. These standards provide the basis for a next generation optical network infrastructure which will subsume/replace SONET/SDH.
The flexibility of OTN derives from its role as a digital wrapper. It natively provides the ability to wrap and transport a variety of client signals, both synchronous (e.g. SONET) and asynchronous (e.g. Ethernet). G.709 defines three interfaces speeds which have been chosen so they can carry SONET/SDH signals. OTU1 is 2.666Gbps, which can carry an OC48/STM16 signal, OTU2 is 10.709Gbps, which can carry an OC192/STM64 signal, and OTU3 is 43.018Gbps, which can carry an OC768/STM256 signal).
OTN and SONET are, in many technical ways, opposites of each other. SONET is based on synchronous mapping of payloads and OTN is based on asynchronous mapping of payloads. SONET was originally specified to operate on a single 1310nm wavelength (although it has, occasionally gone beyond this) and OTN was designed with WDM/DWDM in mind. SONET uses a fixed frame rate and increases frame size as the speed increases and OTN uses a fixed frame size and increases the frame rate as the speed increases.
There are four primary benefits of OTN (when compared to SONET/SDH). First is stronger forward error correction (FEC). Roughly 6% of each OTN frame is dedicated to an error correcting code. This results in a roughly 6dB coding gain for the OTN signal resulting in (your choice of) lower error rates, lower transmission power, greater transport distance.
The second benefit is more levels of tandem connection monitoring. As signals have to traverse more and more administrative domains it is necessary to maintain visibility at each level in order to ensure the appropriate signal quality. SONET only provides the ability to traverse a single tandem connection and OTN allows up to six.
The third benefit is the bit transparent transport of client signals. As noted above, OTN can be used to transparently wrap synchronous signals such as SONET/SDH (and provides pointer adjustment capability in order to transport the synchronous signal) as well as asynchronous signals such as Ethernet. In the case of Ethernet, transporting it wrapped in OTN can improve its fault detection capabilities and improve reliability.
Finally, OTN provides much more scalable switching capability. SONET/SDH defined only two levels of switching granuality (low-order and high-order). This created problems as the line rate increased resulting in additional mechanisms such as virtual concatenation. OTN does not impose any restrictions on the switching bit rates and different switching granularities can be defined as the line rate increases.
The Optical Transport Network is the next generation of transport infrastructure of carrier and private optical networks and provides the structure and infrastructure necessary to support a wide range of applications and services.
References and Sources
- ITU-T recommendation G.709- Interfaces for the Optical Transport Network
- ITU-T Optical Transport Network Tutorial. Timothy Walker, AMCC
- A G.709 Optical Transport Network Tutorial, Guylain Barlow, Innocor Ltd.
