Just look at these three pictures and you can understand the OTN layered structure

OTN (Optical Transport Network) is a hierarchical network in which service signals are transmitted between different layers.

Figure 1: Internationally defined 7-layer OTN structure

According to the ITU-T International Telecommunication Union, OTN is divided into 7 layers:

• Client signal layer: refers to the service signals to be carried by the OTN network, including IP, Ethernet, SDH, etc.
• OPU (Optical Channel Payload Unit): used to adapt service signals to make them suitable for transmission on optical channels.
• ODU (Optical Channel Data Unit): With OPU as the net load, it adds corresponding overhead and provides end-to-end optical channel performance monitoring, realizing end-to-end transmission of service signals in the OTN network.
• OTU (Optical Channel Transport Unit): With ODU as the net load, it adds corresponding overhead, provides FEC function and performance monitoring of OTU segment, and enables service signal transmission between 3R regeneration points in OTN network.
• OCH (Optical Channel layer): provides end-to-end networking functions for service signals. Each optical channel OCH occupies an optical wavelength to realize service signal transmission between access points.
• OMS (Optical Multiplex Section layer): provides networking functions for multi-wavelength signals that have undergone wavelength division multiplexing, and realizes the transmission of optical channels between access points.
• OTS (Optical Transmission Section layer): provides the function of transmitting optical signals on optical fibers and realizes the transmission of optical multiplexing sections between access points.

Figure 2: Analogous 7-layer structure of OTN

The hierarchical structure of OTN is analogized by express delivery.

• Customer signal layer: Business signals are like various goods sent by express delivery.
• OPU: used to adapt business signals, such as cartons for packing goods.
• ODU: Adds overhead for performance monitoring, such as sticking the approximate mailing address of the courier on the package box (such as xx district, xx city, xx province to xx district, xx city, xx province).
• OTU: Adding overhead for performance monitoring is like sticking the detailed mailing address of the courier on the package box (such as xx floor, xx community, xx street to xx floor, xx community, xx street).
• OCH: provides end-to-end transmission for business signals, such as express lines (such as express delivery from Beijing xx community to Guangzhou xx community).
• OMS: Provides end-to-end transmission for wavelength division multiplexed signals. For example, the regional transfer center between Beijing and Guangzhou. Beijing's community express delivery is first concentrated in the regional transfer center, and then the regional transfer center uniformly distributes and packages them and sends them to the Guangzhou regional transfer center (such as the Beijing regional transfer center to the Guangzhou regional transfer center).
• OTS: The function of transmitting optical signals on optical fibers is like the cities that an express route passes through (for example, Beijing to Zhengzhou is an OTS, Zhengzhou to Changsha is an OTS, and Changsha to Guangzhou is an OTS).

Figure 3: 7-layer structure of actual network OTN

To help you understand the OTN layered structure, let's look at the 7-layer structure in conjunction with actual OTN network equipment.

1. At the sending end, the SDH/Ethernet service signal is accessed by the client board and converted into the OPU, which is then processed by the board and output as the ODU.

2. ODU is accessed through the line board and converted into OTU. Multiple OTU optical signals are multiplexed together through the combiner and transmitted on the same optical fiber.

3. The optical signal is regenerated and amplified through the OA optical amplifier on the optical fiber transmission path to ensure that the optical signal can be transmitted over long distances.

4. At the receiving end, the optical signal is demultiplexed through a wavelength splitter to separate multiple optical signals. After being processed by the line board and the client board, the service signal SDH/Ethernet is output.

• Client signal layer: SDH, Ethernet, IP, etc.
• OPU: The connection point is usually located on the client side of the client board.
• ODU: The connection point is usually located on the line side of the customer board.
• OTU: The connection point is usually located on the line side of the line board.
• OCH: The connection point is usually located on the line side of the line board.
• OMS: The connection point is usually located at the location of the combiner/splitter board.
• OTS: The connection point is usually located on the OA board.

At this point, the 7-layer structure of OTN has been introduced. At this point, someone will definitely ask what the role of the OTN layered structure is? The OTN layered structure is the core foundation of functions such as overhead, alarm, performance, and protection. It is an important means of network management and is essential knowledge for our daily operation and maintenance.

So, after reading these three pictures, do you understand the OTN layered structure?