Old Standard New Topic – Optical Transport Network

Posted on by FS.COM

The SONET/SDH network has grown to be the backbone of most of the modern telecommunications network that was originally designed for optical interfaces that used a single wavelength per fiber. As optical component technology has advanced, it has become more economical to transmit multiple SONET/SDH signals over the same fiber using wavelength division multiplexing (WDM) instead of going to a higher rate SONET/SDH signal. Based on experience with the SONET/SDH networks, the ITU-T defined the optical transport network (OTN), which was optimized for cost-effective transparent transport of a variety of client signals over WDM networks. This article may introduce some knowledge of OTN to you.

What Is OTN?
OTN is a standard for optical transport (G.709) developed by the ITU-T standards body, and is sometimes also called a “digital wrapper.” While OTN isn’t necessarily new as a protocol, it is new as a topic in the optical industry for many people. OTN adds operations, administration and maintenance (OAM) functionality to optical carriers, specifically in a multi-wavelength system such as dense wavelength division multiplexing (DWDM). It provides the network management functionality of SDH and SONET, but on a wavelength basis. The OTN is flexible in terms of frame size and allows multiple existing frames of data to be wrapped together into a single entity that can be more efficiently managed through a lesser amount of overhead in a multi-wavelength system.

Optical Transport Network

How Does OTN Work?
The OTN frame is very similar to a SONET frame in its structure and format. There are three overhead areas in an OTN frame: the Optical Payload Unit (OPU), the Optical Data Unit (ODU), and the Optical Transport Unit (OTU). One additional feature is the inclusion of a Forward Error Correction (FEC) function for each frame. The FEC improves the Optical Signal-to-Noise Ratio (OSNR) by 4 to 6 dB, resulting in longer spans and fewer regeneration requirements. A client signal is mapped into the OPU payload, with the OPU overhead providing information on the type of signal mapped into the payload and the mapping structure. The ODU overhead adds optical path-level monitoring, alarm indication signals, automatic protection switching bytes, and embedded data communications channels (GCC1/GCC2). The ODU is the basic payload that is electronically groomed and switched within an OTN network. The OTU overhead adds bytes to provide optical section layer PM, alarm indication, and the GCC0 data communications channel. The OTU represents a physical optical interface or port, such as an OTU2 (10 Gbps), OTU3 (40 Gbps) and OTU4 (100 Gbps).

OTN Optical Transport Network

OTN Application Migration
Originally, the G.709 digital wrapper was primarily used for transporting 10 Gbps wavelengths, enabling improved performance due to the FEC and improved OAM due to the OTN overhead bytes and standard frame structure. Lower rate signals, such as 4 x OC-48 and 8 x GbE were simply multiplexed into 10 Gbps payloads and then encapsulated into the OTN frame. Since each vendor had their own method of multiplexing lower-rate signals into 10 Gbps wavelengths, there was no way to share these aggregate 10 Gpbs wavelengths in large multivendor networks. This lack of common underlying mapping structures forced carriers to demultiplex each 10 Gbps or 40 Gbps wavelength at every core aggregation node and at every network boundary, which was very inefficient and costly. OTN standards evolved to include a standard multiplexing hierarchy, defining exactly how the lower rate signals map into the higher-rate payloads. This allows any OTN switch and any WDM platform to electronically groom and switch lower-rate services within 10 Gbps, 40 Gbps, or 100 Gbps wavelengths, without the need for external wavelength demultiplexing and manual interconnects. Below is a simplified OTN mapping diagram. A 2.5 Gbps signal (OC-48) is mapped into an OTU1 frame. Four of these 2.5 Gbps signals can be mapped into an OTU2 frame.


OTN has evolved over the last few years to be the preferred technology for building DWDM back-bone and long distance optical networks for carriers. It offers unified optical encapsulation layer, OTU2, into which all the common 10G interfaces are mapped into. In addition, the embedded FEC is defined by the OTN layer improves the link budget and OSNR in long distance optical network frequently based on optical amplifiers (EDFAs). The demand for high speed data services continues to be rising, as carriers and service providers try to accommodate customers’ demands for high throughput broadband services, therefore WDM based on OTN will be more and more widely deployed. FS.COM provides a full range of WDM mux/demux solutions to help you build cost-effective and reliable optical transport network.

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