OTN Technology Demystified

Optical Transport Network(OTN) is a modern optical transmission network technology, which is based on a transparent optical transmission platform to provide users with optical transmission service of high bandwidth, high reliability, and high security. The following is a detailed introduction to OTN technology:

Overview of OTN

OTN is an optical transmission network technology, which uses transparent optical signals to transmit and manage optical signals in the network, and supports various types of optical signals, including digital signals, analog signals and baseband signals. OTN network is composed of optical transmission equipment, optical control equipment, interface equipment and management system.

OTN Hierarchy

• Customer signal layer refers to the service signals carried by the OTN network, including IP, Ethernet, and SDH.

• The Optical Channel Payload Unit(OPU) is used to adapt service signals and make them suitable for transmission on optical channels.

• The Optical Channel Data Unit(ODU), with OPU as the net load, adds corresponding overhead and provides end-to-end optical channel performance monitoring to realize the end-to-end transmission of service signals in OTN network.

• The Optical Channel Transport Unit(OTU), with ODU as the net load, adds corresponding overhead and provides the FEC function and OTU performance monitoring. The service signal is transmitted between 3R regeneration points in OTN network.

• The Optical Channel layer(OCH) provides end-to-end networking for service signals. Each optical channel occupies one optical wavelength and transmits service signals between access points.

• The Optical Multiplex Section layer(OMS) provides networking functions for multi-wavelength signals of wavelength division multiplexing to transmit optical channels between access points.

• The Optical Transmission Section layer(OTS) provides the function of transmitting optical signals over optical fibers, enabling the transmission of optical multiplex section between access points.

An Analogy to OTN's 7-Layer Structure

Here is an analogy between OTN's hierarchy and express delivery.

OTN's 7-Layer Structure in Practical Application

• At the sending end, the SDH/ Ethernet service signals are accessed by the customer board and converted into an OPU. The OPU is processed by the board and then outputs the ODU.

• The ODU is accessed by the line board and converted into an OTU. Multiple OTU optical signals are multiplexed together through an optical multiplexer for transmission in the same optical fiber.

• The OA is used to regenerate and amplify the optical signal on the fiber transmission path to ensure that the optical signal can be transported over long distances.

• At the receiving end, the optical signal is demultiplexed by an optical demultiplexer, and multiple optical signals are separated. After processing by the line board and the customer board, the service signal SDH/ Ethernet is output.

Signal Structure of OTN Technology

Optical Transport Network(OTN) is an optical fiber transmission system whose frame structure is composed of OPU/ODU/OTU layers.

The Optical Channel Payload Unit(OPU) is responsible for accepting customer signals (STM-N, ATM, IP, MPLS, etc.). Firstly it adjusts the code speed of customer signals, so that the rate of customer signals is consistent with the specified OPU speed, and adds redundant bits when inconsistent, which is called mapping, and then adds customer signal identification and other information.

The Optical Channel Data Unit(ODU) is the protection of the information in the OPU. It provides monitoring and fault location bytes for OPU's adding to the end-to-end channel and the transmission performance of the cross-carrier, so as to verify the integrity of the ODU link, evaluate the transmission quality, indicate and detect transmission defects. It also adds control bytes, to make the OTN system automatically switch in trouble.

The Optical Channel Transport Unit(OTU) is used for framing. It adds frame positioning bytes before ODU (frame header) to facilitate frame synchronization at the receiving end, adds forward error correction bytes after ODU (frame tail) to ensure the correctness of the entire OTU data, and adds some management bytes.