OTN frame structure with details

 The **Optical Transport Network (OTN)** framework is defined by the ITU-T G.709 standard, which specifies the structure and functionality of the OTN. OTN is designed to provide a standardized way to transport and manage high-speed data over optical networks. Below is an overview of the OTN frame structure:


 **OTN Frame Structure**

The OTN frame consists of several layers and overheads, which are organized into a hierarchical structure. The key components of the OTN frame include:


1. **OTUk (Optical Transport Unit, level k):**

   - The OTUk is the basic frame structure used for transporting client signals.

   - It includes:

     - **Payload Area:** Carries the client data.

     - **OTUk Overhead:** Provides management and monitoring functions.

     - **OTUk FEC (Forward Error Correction):** Used for error detection and correction.


2. **ODUk (Optical Data Unit, level k):**

   - The ODUk is a sub-layer within the OTUk and is used to carry client signals.

   - It includes:

     - **OPUk (Optical Payload Unit, level k):** Carries the client payload.

     - **ODUk Overhead:** Provides additional management and monitoring functions.


3. **OPUk (Optical Payload Unit, level k):**

   - The OPUk is the payload area of the ODUk and carries the actual client data.

   - It includes:

     - **Client Signal:** The data being transported (e.g., Ethernet, SDH/SONET, etc.).

     - **OPUk Overhead:** Provides payload-specific management functions.


 **OTN Frame Hierarchy**

The OTN frame hierarchy is organized as follows:


1. **OTUk Frame:**

   - Contains the ODUk and OTUk overhead.

   - Includes FEC for error correction.


2. **ODUk Frame:**

   - Contains the OPUk and ODUk overhead.

   - Provides end-to-end monitoring and management.


3. **OPUk Frame:**

   - Contains the client payload and OPUk overhead.

   - Adapts the client signal to the OTN frame structure.


**OTN Frame Size**

The OTN frame size depends on the data rate (e.g., OTU1, OTU2, OTU3, OTU4). For example:

- **OTU1:** 2.66 Gbps

- **OTU2:** 10.7 Gbps

- **OTU3:** 43 Gbps

- **OTU4:** 112 Gbps


Each OTUk frame consists of 4 rows and 4080 columns, with additional columns for FEC.


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 **Key Overheads**

1. **OTUk Overhead:**

   - Includes fields for section monitoring (SM), trail trace identifier (TTI), and backward error indication (BEI).


2. **ODUk Overhead:**

   - Includes fields for tandem connection monitoring (TCM), path monitoring (PM), and automatic protection switching (APS).


3. **OPUk Overhead:**

   - Includes fields for payload structure identification (PSI) and payload-specific adjustments.

 **FEC (Forward Error Correction)**

- The FEC field is used to detect and correct errors in the transmitted data.

- It is located at the end of the OTUk frame.


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